Date: Fri, 21 May 1999 23:01:36 +0200
From: Hal Christiansen 
Subject: [Paddlewise] Pro's and Con's of the "Swede Form"

As previously mentioned I am in the process of selecting my first kayak.

I am hoping to entice several of you more experienced paddlers to 
'wax philosophical' on the pro's and con's of the Swede form boats.


THANKS
- -Hal



Date: Fri, 21 May 1999 23:27:46 -0700
From: "Matt Broze" 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

Well I'm not named John so this may not be "the gospel" but this subject is
also pet pieve of mine so you sucked me in to giving a response.
The Swede-form shape (greater underwater volume aft of the midpoint) has
less resistance moving at the water's surface than either a fish-form shape
(its opposite) or a symmetrical hull. The finer bow more gently parts the
water for less wave-making resistance. The longer run of positive pressure
in the forebody also can result in a longer area of the hull being in
laminar flow (laminar flow over a surface creates about four times less drag
than turbulent flow).
Note: A fish-form shape has less resistance underwater or in the air (where
there is no wave drag). This has confused some designers who have consulted
hydrodynamic texts, but not gotten the full picture of what happens at the
water’s surface. Fast ships, canoes and kayaks are Swede-form. Fast
submarines and fast fish are fish-form.)

For a kayak a Swede-form hull has many other advantages over fish-form. They
include:
1)Less pounding in head seas because they are narrower in the area where
pounding occurs (but, bottom shape is a bigger factor in pounding so some
V-bottomed fishforms will be softer than a flat bottomed Swedeform shape),
2)Easier and quicker turning (turns are enhanced by the greater curve to the
stern--and leaning makes this effect even more pronounced),
3)Less weatherhelm (more windage and a longer lever arm in front of the
paddler and less behind)
4)More of its volume is usable storage space behind the paddler and less of
the volume is in the wasted space around the legs.
5)A narrower beam where the paddle enters the water means easier more
efficient paddling (less boat to reach over) and less turning moment
produced with each (less off-center) stroke.

Fishform advocates correctly point out that Swedeform is less directionally
stable (other things being equal). One of them used to even say fish-form
was self-correcting (when he really meant self-stabilizing).  I see this as
a disadvantage in a kayak. Directional stability just means you work harder
for each degree you turn rather than being able to translate momentum into
turning by leaning. Course keeping is actually harder with fish-form because
correcting your course after a wave has altered it is more difficult with a
kayak that resists turning. Fishform will be harder to turn and leaning
doesn't help turning nearly as much as with Swedeform. The less
directionally stable Swedeform shape will track just fine if some keel is
added in the rear, but when leaned it will still retain its superior turning
ability. The best of both, tracks straight and turns readily with a slight
lean.

As far as choosing a kayak goes, trying to figure out what is going to be
best for you intellectually is a hopeless task, you will just get bogged
down in controversy and verbiage. Paddle kayaks that are recommended to you,
and designed for what you want to do. You will feel what you like. Few can
translate the verbiage,  dimensions or even the shape of a kayak into the
"feel" that will result so why not go straight for the feel.
Matt Broze
www.marinerkayaks.com



Date: Sat, 22 May 1999 09:42:33 -0400
From: "John Winters" 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"


>I am hoping to entice several of you more experienced paddlers to
>'wax philosophical' on the pro's and con's of the Swede form boats.


Not much philosophy to it unfortunately :-)

The issue has lots of complications but here goes.

From a wave making standpoint in smooth water it does not matter which form
you use so long as the longitudinal center of buoyancy (LCB) lies within
the appropriate range (roughly 48 to 55% aft of forward waterline ending.
This surprises most people who look upon the waterline shape as cutting
through the water and the water being pushed to the side. The water
actually dives under the boat and travels roughly along the buttock lines.
If you can get hold of Taylor's "Speed and Power of Ships"  you can see
some fascinating drawings of the water flow around a wide variety of hull
forms.

This problem with smooth versus rough water led to the debacle with the
English America's Cup challenger Sceptre. They tested only in smooth water
an the results looked great. Unfortunately they had to race in rough water
where the finer American boat that had more resistance in smooth water won.

The location of the LCB does make a difference aft where the fuller stern
provides virtual lengthening and fills in the boundary layer aft. Like
anything, you can carry this too far and cause separation drag aft (see
above).

Another difference has to due with performance in rough water. Finer ends
with flared topsides have less energy loss than fish form  boats and also
tend to pitch less.

One of the more interesting aspects of this has to do with "how" a boat has
its form. For instance, some boats have swede form waterlines but the LCB
still lies forward of amidships. The opposite can occur as well. Many
people look at the waterline and say "fish form" when in reality the boat
has a Swede form hull. Some of the Inuit Greeenlandic boats have this shape
I.E. fine waterlines forward coupled with a full underbody forward. All
very confusing but fun if you enjoy that kind of thing.

The control aspects of form get even more complex. If one maintains the
same generic shape and shifts the LCB fore and aft you get all kinds of
wonderful results. Smart designers don't work that way. They treat the boat
as a system where every part interacts with every other part. In this way
you can often create a boat that "looks" all wrong according to the common
wisdom but actually works quite nicely.

I recommend that you not get caught up  in the x VS y stuff. Usually the
reputation of a boat tells you more than anything else. I hasten to add
ETC. to all this. Whole books get written on just one or two aspects of
this topic.

I am sure Matt Brose will have some comments on this as might Nick Schade.
Their opinions can  help a lot as both have done a lot of paddling and
study of boats.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/




Date: Sun, 23 May 1999 00:51:36 -0700
From: "Matt Broze"
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

From: John Winters 

>From a wave making standpoint in smooth water it does not matter which form
>you use so long as the longitudinal center of buoyancy (LCB) lies within
>the appropriate range (roughly 48 to 55% aft of forward waterline ending.
>This surprises most people who look upon the waterline shape as cutting
>through the water and the water being pushed to the side. The water
>actually dives under the boat and travels roughly along the buttock lines.

Why do you think it appears to do that? Are these hulls pushing hull speed
and the trough that follows the bow wave is actually what is "diving under".
It sure seems to me that the net effect is that the (incompressible) water
had to be displaced to the side and upward first and then is falling back to
below grade as the hull is passing through.

>If you can get hold of Taylor's "Speed and Power of Ships"  you can see
>some fascinating drawings of the water flow around a wide variety of hull
>forms.

I don't have a copy but confirmed this with some other texts. Were any of
the tests pictured done at very low speeds? I have trouble believing that
the flow along the hulls surface will follow the buttock lines at low
speeds.

>Another difference has to due with performance in rough water. Finer ends
>with flared topsides have less energy loss than fish form  boats and also
>tend to pitch less.

I forgot to mention that one. I'll have to remember to add it to my list.
I'd like to learn more about this before I make that claim though. Do you
have a source that explains why there is less energy loss. Is it just the
loss due to pitching more or is it also the greater pressure exerted on the
blunter ends by the waves?

>One of the more interesting aspects of this has to do with "how" a boat has
>its form. For instance, some boats have swede form waterlines but the LCB
>still lies forward of amidships. The opposite can occur as well. Many
>people look at the waterline and say "fish form" when in reality the boat
>has a Swede form hull. Some of the Inuit Greeenlandic boats have this shape
>I.E. fine waterlines forward coupled with a full underbody forward. All
>very confusing but fun if you enjoy that kind of thing.

Most people aren't looking at the waterline at all but at the seam line (top
view) when deciding if a boat is Swedeform or fishform and that can be even
more misleading than just looking at the waterline. For instance, our Escape
looked way more Swedeform than it actually was because there was
considerable flair behind the cockpit (to get more secondary stability and
storage room) but at the same time the flare was reduced markedly to near
vertical sides just in front of the cockpit (in order to make it narrow in
the paddling area). It then flared out again further forward into a ship
like bow. It looked radically Swedeform but it was only moderately so.
Matt Broze
www.marinerkayaks.com



Date: Sun, 23 May 1999 09:18:16 -0400
From: "John Winters" 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

(SNIP{ about flow)

>Why do you think it appears to do that? Are these hulls pushing hull speed
>and the trough that follows the bow wave is actually what is "diving under".
>It sure seems to me that the net effect is that the (incompressible) water
>had to be displaced to the side and upward first and then is falling back to
>below grade as the hull is passing through.

I suspect that to most people it  "appears" to be pushed to the side mostly
because one can so easily see the divergent wave system and assumes that
the water gets pushed to the side. Actually the bow wave doesn't go
anywhere, it remains at the bow. The transverse system is created
continually and the flow continues as the trough (and then later to more
crests and troughs depending on speed).

If we watch the crests and troughs along the hull we have to explain why
the water appears to flow aft instead of back and forth. Tough to do if the
water flows in and out rather than just accelerating and decelerating fore
and aft.

The flow under the hull exist regardless of speed. Another interesting
group of diagrams in Taylor shows the net sinkage of the hull due to the
increased flow velocity under the boat (Bernoulli's theorem at work). Many
people make the same assumption that the water is pushed to the side and
flows back at the stern and some of it does. The flow, however is three
dimensional not two dimensional.

The amount of flow in any direction depends upon the hull shape and, more
importantly, the ratio of beam to length. At one extreme, a flat plate like
a rudder,  the flow goes around, at the other, a surfboard, most of the
flow goes under. Sea kayaks, being relatively wide and short , have
considerable flow under the boat. Fine ends don't seem to have any effect in
that eventually the flow has to get under the boat.

(SNIP)

>I don't have a copy but confirmed this with some other texts. Were any of
>the tests pictured done at very low speeds? I have trouble believing that
>the flow along the hulls surface will follow the buttock lines at low
>speeds.

Not sure about very low speeds but Taylor tested as low as S/L 0.6 which is
getting down there where wavemaking doesn't have much effect. I would guess
that, at speeds below wavemaking the water still travels along the hull
longitudinally rather than axially since if it didn't our use of friction
formulas would not apply since the "length" of the surface would across the
hull surface not along the hull surface. This problem of flow bothered me
when developing the hull length correction I.E. whether to use just the
waterline shape or to include the change in volume at the ends. I ended up
using both (the ultimate compromise :-))

To quote the Admiral, " Perhaps their most notable feature (flow lines) is
the strong tendency of water to to dive under the forebody, as it were."


>I forgot to mention that one. I'll have to remember to add it to my list.
>I'd like to learn more about this before I make that claim though. Do you
>have a source that explains why there is less energy loss. Is it just the
>loss due to pitching more or is it also the greater pressure exerted on the
>blunter ends by the waves?

We can see the visual evidence in the spray created. It takes energy to
splatter water all over the place. Also, one can easily imagine the energy
lost when a boat slams. The best treatment of this I have found is in Kents
"Ships in Rough Water". In simple terms, the boat creates waves that
radiate away from the boat as it plunges into the waves. The larger the
waves, the more energy used to create them. I like to use diving as an
example of what happens. A good clean dive makes small waves and the diver
plunges deeply into the water. Do the cannonball and you make huge waves
and don't go very deep.

Havelock used this formula to obtain the mean wave pressure on the hull:

R= 1/2gp^2Bsin^2a

where
g = acceleration of gravity
p = density of water
B = beam
a =equals waterline the half angle forward

Obviously much too simple :-) but it hints at  the importance of hull
fineness forward.


(SNIP)

>Most people aren't looking at the waterline at all but at the seam line (top
>view) when deciding if a boat is Swede form  or fishform and that can be even
>more misleading than just looking at the waterline. For instance, our Escape
>looked way more Swedeform than it actually was because there was
>considerable flair behind the cockpit (to get more secondary stability and
>storage room) but at the same time the flare was reduced markedly to near
>vertical sides just in front of the cockpit (in order to make it narrow in
>the paddling area). It then flared out again further forward into a ship
>like bow. It looked radically Swedeform but it was only moderately so.

Good point. I get so absorbed with what I see under the water that I
completely ignored that part of it.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/



Date: Mon, 24 May 1999 00:16:54 -0700
From: "Matt Broze"
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

From: John Winters 

>If we watch the crests and troughs along the hull we have to explain why
>the water appears to flow aft instead of back and forth. Tough to do if the
>water flows in and out rather than just accelerating and decelerating fore
>and aft.

I think we are looking at this wrong. I don't think the water is doing
anything but getting out of the way. The waves on the surface aren't causing
any acceleration or deceleration of the "flow" back and forth. The hull is
moving at a relatively constant speed in a given direction and what we have
been calling flow is just the indicators of the direction the boat is moving
as it scrapes through the water. (your surfboard and rudder examples helped
me see this). No water goes down under the surfboard without also displacing
some other water out of the way and since water is not comressible the total
of all these nudges between molecules is upward (making a wave). The
indicators are on the hull so they mostly show which way the boat is moving.
Whether the shape is a rudder blade, surfboard or something in between it
will displace either the water it is moving through (or in the case of the
board at speed it itself will be displaced upwards--planing). If every
molecule of water in the vicinity was tagged and tracked we would see some
draged forward by the hull, others scraped from the surface and flung
forward, much nudged to the side and in so doing nudging others upward to
make room. The net effect is visible at the surface in the form of waves.
No water (other than a few molecules being dragged along) actually dove
under the boat rather the hull bottom in the forebody is contacting water
that hasn't been displaced yet and that still water is holding the little
tufts of yarn in position while the other ends attached to the hull are
pulled forward by it so they pretty much pull straight back at least until
they run into something that causes a major back eddy (separation of flow)

>To quote the Admiral, " Perhaps their most notable feature (flow lines) is
>the strong tendency of water to to dive under the forebody, as it were."

I'm not sure I want to argue with the Admiral but imagine a flat bottomed
ship built like a cow-catcher on a train. It should be clear that most of
the water displaced would be lifted and deposited to the side by the
cow-catcher bow yet if we looked at the little telltales we attached on the
bottom of the hull they would point in the opposite direction of the ships
motion even though no water would have gone downward under the hull at all
to make them do that. Why then are we or the good Admiral assuming some
downward flow based on this "flow" direction evidence?



Date: Mon, 24 May 1999 08:55:44 -0400
From: "John Winters" 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

(SNIP)

>I think we are looking at this wrong. I don't think the water is doing
>anything but getting out of the way. The waves on the surface aren't causing
>any acceleration or deceleration of the "flow" back and forth.

Waves are symptoms of what happens not the cause. By examining the symptoms
one can get at the cause. The wave system (and particularly the transverse
wave system) reveals the energy expended in pushing the water out of the
way. Crests indicate lower velocity and troughs indicate higher velocity
along the hull. By measuring the pressure along the hull one can determine
the local velocity. (Eggert, did this in 1939).

The water pushed out of the way by the boats passage piles up in the
transverse wave system and we can see the effects of displacement by
observing the size of these waves as Dr. Inui did in his research. From
this Inui developed ships forms that had almost no wave making resistance.

From this we pass on to the other phenomenon that occurs, notably the
sinkage of the hull. As velocity increases the hull sinks (most tank test
series measure sinkage) due to the increased velocity under the hull.  Now
we must ask, why? If no increase in flow occurs under the hull, why does
the boat sink? Clearly increased flow does occur under the hull that will
cause an increase in kinetic energy over potential energy thus reducing the
hydrostatic forces supporting the hull. Where does this water come from?
Does it enter in from the sides?" Not if it gets pushed away to the side.
Does it come form the stern? Not if the boat moves ahead. Clearly it must
come from ahead. If we have increased velocity we must also have increased
volume.

Now I have made this rather simple and I hope I have not left out some
important point.

> The hull is
>moving at a relatively constant speed in a given direction and what we have
>been calling flow is just the indicators of the direction the boat is moving
>as it scrapes through the water. (your surfboard and rudder examples helped
>me see this). No water goes down under the surfboard without also displacing
>some other water out of the way and since water is not comressible the total
>of all these nudges between molecules is upward (making a wave).

Yes, the water passing under the boat pushes water to the side but the flow
remains critical. As pointed out previously, that flow causes the hull
sinkage. How one looks at this depends upon how one asks the question. If
we ask, "Does the water get pushed to the side by the boat?" and answer,
"Yes it does." then we can get led to the erroneous conclusion that the
flow under the boat has relatively little significance or that it does not
dive under the boat. If we ask, how does the water flow around the boat and
get the answer described by observing the flow we get led to a different
conclusion that the water flowing under the boat displaces the water to the
side.

How does this affect design? Consider the distance the water travels. A
poorly shaped hull might have a greater effective length than one that
ignores this information for one example. Also, if the water only gets
pushed aside you might be tempted to design boats like the old plank on
edge sailboats of the 19th century.

>The
>indicators are on the hull so they mostly show which way the boat is moving.

One hopes ;-). At the stern the flow actually can travel the opposite
direction so one has to be careful about telling direction of travel from
tell tales. from Of course, not all flow test have indicators lying on the
hull. Some curious types had indicators lying off the hull surface. The
sailing types did this kind of thing because they had to study the effects
of leeway.

(SNIP)
>If every
>molecule of water in the vicinity was tagged and tracked we would see some
>draged forward by the hull, others scraped from the surface and flung
>forward, much nudged to the side and in so doing nudging others upward to
>make room.

Have you switched to discussing the boundary layer here as opposed to the
general flow? Keep in mind the boundary layer gets carried along with the
hull and the phenomena inside the boundary layer don't necessarily reflect
what happens to the general flow.


>The net effect is visible at the surface in the form of waves.
>No water (other than a few molecules being dragged along) actually dove
>under the boat rather the hull bottom in the forebody is contacting water
>that hasn't been displaced yet and that still water is holding the little
>tufts of yarn in position while the other ends attached to the hull are
>pulled forward by it so they pretty much pull straight back at least until
>they run into something that causes a major back eddy (separation of flow)

I think you may not be aware of how these test are done. Perhaps a reading
of Taylor will clear this up and save me writing a lot about it here but I
will describe it if anyone has an interest. If Taylor is not handy you can
read the aero texts that show the direction of flow at a distance from the
body. The Marin tank does its studies with yarn and the yarn is held off
the hull surface to allow the yarn free movement.

I don't understand what you have said here so can't comment much but
sailors who use yarn telltales no that the yarn reflects the flow over the
sail and  if they have done any of the "fishing rod tests" no that the flow
adjacent the sail reflects the flow at a distance from the sail.

>I'm not sure I want to argue with the Admiral but imagine a flat bottomed
>ship built like a cow-catcher on a train. It should be clear that most of
>the water displaced would be lifted and deposited to the side by the
>cow-catcher bow yet if we looked at the little telltales we attached on the
>bottom of the hull they would point in the opposite direction of the ships
>motion even though no water would have gone downward under the hull at all
>to make them do that. Why then are we or the good Admiral assuming some
>downward flow based on this "flow" direction evidence?

Well, I can't argue this without seeing what kind of shape Matt proposes.
The cowcatcher example Matt provides may not have much to do with ship
shapes. It may have some value in studying crab scrapes or snowplows
though. We have to choose our examples carefully. Would we assume we know a
lot about the flow around ships by studying the flow of water around a farm
plow? Can we study the flow around a wing by testing a house in a wind
tunnel?

Nevertheless, there are ships with "cowcatcherlike" bows (mostly bulbs and
such) and they have the same flow Taylor describes. Taylor's ships had bulb
bows.

The flow indicates where the water goes. To assume that it doesn't go where
it is going kind of boggles.

If this is getting too anal for readers just speak up and Matt and I can
carry this on off-list.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/



Date: Mon, 24 May 1999 08:48:37 -0500
From: "Larry Koenig" 
Subject: [Paddlewise] P&C of the Swedeform

>If this is getting too anal for readers just speak up and Matt and I can
>carry this on off-list.

I for one am fascinated by the discussion so far.  I'd never much considered
what is going on around my hull in this kind of detail and would love to see
this sorting out ongoing.
Larry Koenig


Date: Tue, 25 May 1999 07:36:02 -0400
From: "John Winters" 
Subject: [Paddlewise] flow

A couple of people have asked about how flow gets determined in the tank so
I may as well post it for everyone.

The most common method uses yarn or plastic streamers. In some cases these
get taped right to the hull and in others they get suspended away from the
hull on small wires.

Another clever method uses finely ground aluminum particles suspended in
the water. This works best in observing the flow form overhead.

Another method uses dye that is injected in front of the boat's path. This
works OK but the dye soon dissipates.

In wind tunnels smoke provides visual evidence of flow.

Taylor  coated the models with sesquichloride of iron mixed with glue. Then
he injected pyrogalic acid through a small hole resulting in a smear of ink
that revealed the flow. He would then drill a new hole where the ink smear
lost definition until he eventually worked his way to the stern. For flow
away from the he used meshes of fine string or wire coated with
sesquichloride of iron  and injected pyrogalic acid at known points. Taylor
says," The relative flow indicated in the immediate vicinity of the model
is found as regards to type quite a distance from the skin, so as regards
motion near the hull we need consider only the disturbance close to the
bottom, or the lines of flow as they may be called."

These days I think this gets done mathematically since one doesn't mess up
the tank or models.

One of the things that impress me has to do with the similarity in flow
lines for widely divergent hull shapes. The flow lines (crudely done with
yarn I admit) I got on my sprint boats looked remarkably like those I got
for my canoes and they looked remarkably like the ones Taylor got on his
ship models.

Matt asked why I thought the flow would dive under the hull and I must
answer "I don't know" but the evidence points to something other than water
being pushed only to the side.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/



Date: Tue, 25 May 1999 10:56:21 -0400
From: Nick Schade 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

At 8:55 AM -0400 5/24/99, John Winters wrote:

>
>The flow indicates where the water goes. To assume that it doesn't go where
>it is going kind of boggles.


One demonstration of water diving below the boat can be seen in shallow
water. Most people have probably experienced the big wake and slow progress
you get when paddling in water 1 foot deep or so. The easy explanation for
this is waves like deep water so the stack up like surf at a beach. But,
your boat is not effected by the size of the wave once the wave leaves the
boat. You don't go slower just because your wake encounters shallow water.
There must be some other explanation.

Water flowing under the boat must be displaced and in turn displace other
water farther down and out. This chain of displacement eventually will push
out to the surface. But this is pushing fairly slowly, albiet a large mass.
When you get into shallow water, the water has fewer choices on where it
can go so it must move faster. This is made worse by the Bernoulli effect
which sucks the stern down even further making it even harder for the water
to get out of the way. Even though it may be pushing less water it is
pushing it out of the way at a higher velocity. The energy in the water due
to the motion is KE=1/2 m v^2. Moving a large mass slowly tends to be more
efficient than moving a small mass rapidly.

The result is when the energy is released at the water surface, larger
waves are made. If the water was being pushed purely to the side, you would
not see such a dramatic change in shallow water. The typical wake of a
kayak is pretty small. Probably small enough to travel pretty easily in 1
foot of water. If the water was just being pushed to the side, not much
would change in shallow water, but because the water must find a fast way
to get out of the way, shallow water slows you down.

Nick



Nick Schade
Guillemot Kayaks
10 Ash Swamp Rd
Glastonbury, CT 06033
(860) 659-8847

http://www.guillemot-kayaks.com/

>>>>"It's not just Art, It's a Craft!"<<<<



Date: Tue, 25 May 1999 10:33:33 -0500
From: (Chuck Holst)
Subject: [Paddlewise] FW: Pro's and Con's of the

>>
The typical wake of a kayak is pretty small. Probably small
enough to travel pretty easily in 1 foot of water. If the
water was just being pushed to the side, not much would
change in shallow water, but because the water must find a
fast way to get out of the way, shallow water slows you down.

Nick
>>

I have experienced severe drag in a Perception Sea Lion in
one foot of water. I haven't noticed it in my Romany, but that
might be because I haven't paddled it in similar water.

Chuck Holst  



Date: Tue, 25 May 1999 12:03:26 -0700 (PDT)
From: "K. Whilden" 
Subject: Re: [Paddlewise] flow

Matt and John are discussing whether water is displaced sideways (Matt's
idea) or vertically (John's idea). To them I ask, why one or the other,
and not a combination of both? My intuition tells me that water is
displaced normal to the local surface of the hull, and probably in a
uniformly thick layer. At least for a smoothly varying hull. If each
parcel of water is not displaced in the normal direction, then water
pressure anomallies are created as the parcel collides with an adjacent
parcel in one direction, and leaves a void in the other direction. I would
further imagine that this effect is what creates extra drag and
initiates turbulence. A cylindrical hull would minimize this effect, and
so to those who know, I ask could this be why the fastest hulls are
more cyclindrical in cross-section? Of course sea kayaks aren't just
interested in pure speed and therefore are willing to sacrifice a little
speed for better handling. 

Another interesting question is what happens to the displacement direction
of water at a hard chine?

Cheers,
kevin
	 ___________________                                                            
	/   Kevin Whilden   \
       |Dept. of Geosciences ___
       |University of Washington \
        ________________________/                       

On Tue, 25 May 1999, John Winters wrote:

> These days I think this gets done mathematically since one doesn't mess up
> the tank or models.

Can this really be done mathematically? I thought that calculations of
flow for any object that moves on the interface to two different fluids
was extraordinarily difficult. If it can be done... how?



Date: Wed, 26 May 1999 00:00:06 -0400
From: Andy Knapp 
Subject: [Paddlewise] Pros and Cons...

Chuck writes:
"I have experienced severe drag in a Perception Sea Lion in
one foot of water. I haven't noticed it in my Romany, but that
might be because I haven't paddled it in similar water."

Romanys and other British kayaks have features that make them immune to the
laws of physics. Maybe Dr. Inverbon can elaborate.

- -Andy Knapp
Minneapolis
Rain's over. Now it's too windy.



Date: Wed, 26 May 1999 00:53:28 -0700
From: "Matt Broze" 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

From: Nick Schade 

>At 8:55 AM -0400 5/24/99, John Winters wrote:
>
>>
>>The flow indicates where the water goes. To assume that it doesn't go where
>>it is going kind of boggles.
>>


The flow over the hull is the resultant of the motion of the hull and the
direction the water is actually moving. If the water moves 2" down while the
boat moves 200" forward the flow past the hull would appear to be at a
slight downward angle but the water would have moved only 2" downward (and
slightly forward due to the friction with the hull). So the water is mostly
moving up or down (and out to the side during a crest and back in the
trough) with the waves lifting it up and down and being pulled along at some
speed between as fast as the boat (or even faster for spray flung forward at
the bow) and not forward at all some distance from the boat (the area where
water is being set into forward motion gets further from the hull (in all
liquid directions) as the boat moves past resulting in the wake behind the
boat.

>One demonstration of water diving below the boat can be seen in shallow
>water. Most people have probably experienced the big wake and slow progress
>you get when paddling in water 1 foot deep or so. The easy explanation for
>this is waves like deep water so the stack up like surf at a beach. But,
>your boat is not effected by the size of the wave once the wave leaves the
>boat. You don't go slower just because your wake encounters shallow water.
>There must be some other explanation.

I disagree. Your boat is affected by both the size and the length of the
wave created at the bow. My undersanding is that as the wavelength is
shortened due to the shallow water effect the boats hull speed is now lower
(it gets trapped in a now shorter slower moving wave). For a fast moving
kayak you first feel this in four or five feet of water. Some Canoe racers
can actually plane their canoe if they can keep their speed up (and break
the wave barrier) on suddenly hitting an area of shallow water.

>Water flowing under the boat must be displaced and in turn displace other
>water farther down and out. This chain of displacement eventually will push
>out to the surface.

Since water is incompressible the first effect of disturbing it can only be
upwards, hence the bow wave an upward motion of the water.

>But this is pushing fairly slowly, albiet a large mass.
>When you get into shallow water, the water has fewer choices on where it
>can go so it must move faster. This is made worse by the Bernoulli effect
>which sucks the stern down even further making it even harder for the water
>to get out of the way.

Maybe but, once you are in water less than one foot deep the turbulence near
the stern of the hull--the wake (water which the hull is dragging along a
little in the direction the hull is moving is now also dragging against the
fixed ocean floor and this really slows you down. At least that's how I
understand it. There used to be a kayak race around Jetty Island which had
extensive shallows around it. The trick was to get far enough away from the
island to be in deep enough water to have enough extra speed to make up for
the extra distance you had to travel by staying farther away from the island
as you circled it. A real excercise in frustration if you got it wrong and
those slower paddlers you had left behind earlier started passing you up.

>Even though it may be pushing less water it is
>pushing it out of the way at a higher velocity.

Why would it be pushing less water?

>The energy in the water due
>to the motion is KE=1/2 m v^2. Moving a large mass slowly tends to be more
>efficient than moving a small mass rapidly.
>
>The result is when the energy is released at the water surface, larger
>waves are made. If the water was being pushed purely to the side, you would
>not see such a dramatic change in shallow water. The typical wake of a
>kayak is pretty small. Probably small enough to travel pretty easily in 1
>foot of water.

A wave begins to "feel" the bottom in water about 1/2 as deep as the
wavelength(the waves orbital motion below the surface is touching the bottom
which slows it down--compressing its energy into a shorter space thereby
making the bow wave higher, steeper and harder to climb). At 5.4 knots the
wavelength created in deep water is about 16 feet long--which means it
begins to feel bottom in 8 feet of water. [At 3 knots the wavelength is
about 5 feet long so the wave-drag effect starts (but is probably not
noticible yet in 2.5 foot deep water)]. In four feet of water the 16 foot
wave is slowed a little to 5.2 knots. In 2 foot water that wave slows to
about 4.4 knots and that sure slows your racing speed. In 1 foot deep water
that waves speed is down to 3.3 knots and so is the kayaks hull speed unless
it can break the wave barrier and start to plane. In 1/2 foot deep water
wave speed is slowed to 2.37 knots. Remember that in water less than about a
foot deep bottom drag due to turbulence drags one down even more than this.

>If the water was just being pushed to the side, not much
>would change in shallow water, but because the water must find a fast way
>to get out of the way, shallow water slows you down.

Sure it would.

Water does speed up in a constriction--the Venturi effect (a special case of
the Bernoulli effect)--which if under the boat would lower the boat further
than normal and this would bring the stern turbulence even more into contact
with the bottom. I should point out here that when two ships pass near each
other or a ship passes near a wall the same effect moves them closer
together (a real danger for ships). Two kayaks coasting along less than a
foot apart will demonstrate this sucking together effect as well. This would
seem to indicate that water is also moving around the hull (I'm applying the
same arguments you are using to convince us that flow is going under a
hull).

It is easy to see in very shallow water why it would be impossible for much
water to go down, it would just bump into the bottom. Due to the
incomressability of water even in the deepest ocean no water can go down
without moving some other water upwards and since the water can't go upwards
though the hull it must go first to the side or raise up in front of the bow
(due to the pressure in front of the moving hull).
Here is another thought. In areas of lower pressure the flow will be moving
back downward and in this area the flow would be diving under the hull as
the flow past the hull moves parallel to the waters wave angle at the
surface. It would seem the flow line on the hull would move back upward with
the next wave crest but if that wave were well back on the hull the flow
might keep moving down to fill in the area of low pressure behind the moving
hull. I don't know if this is right just some thoughts I'm having in trying
to apply basic principles to the flow question.
Matt Broze
www.marinerkayaks.com



Date: Wed, 26 May 1999 01:45:09 -0700
From: Dave Kruger 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

Matt Broze wrote:



> A wave begins to "feel" the bottom in water about 1/2 as deep as the
> wavelength(the waves orbital motion below the surface is touching the bottom
> which slows it down--compressing its energy into a shorter space thereby
> making the bow wave higher, steeper and harder to climb). At 5.4 knots the
> wavelength created in deep water is about 16 feet long--which means it
> begins to feel bottom in 8 feet of water. [At 3 knots the wavelength is
> about 5 feet long so the wave-drag effect starts (but is probably not
> noticible yet) in 2.5 foot deep water]. In four feet of water the 16 foot
> wave is slowed a little to 5.2 knots. In 2 foot water that wave slows to
> about 4.4 knots and that sure slows your racing speed. In 1 foot deep water
> that wave's speed is down to 3.3 knots and so is the kayaks hull speed unless
> it can break the wave barrier and start to plane. In 1/2 foot deep water
> wave speed is slowed to 2.37 knots. Remember that in water less than about a
> foot deep bottom drag due to turbulence drags one down even more than this.

Thanks for this, Matt -- never had anything quantitative before, though I sure
noticed the "shallow water" effect.  Feels like paddling in mud.

> Water does speed up in a constriction--the Venturi effect (a special case of
> the Bernoulli effect)[snip]
> I should point out here that when two ships pass near each
> other or a ship passes near a wall the same effect moves them closer
> together (a real danger for ships).

Local examples:  

1. There is a very narrow place in the shipping channel where it is jammed up
against the WA side of the Columbia River -- about 3 to 4 miles downriver from
Skamokawa.  It is notorious for this "Venturi" effect on freighters passing in
opposition, and the River pilots slow way down there.  

2. There was a glancing bow-to-bow collision a year or so ago off Hammond, OR
(close to the River mouth) in which a downbound freighter tagged an
upriver-bound Coast Guard buoy tender (150 foot ship?) pretty good.  No
serious injuries, but probably a lot of soiled underwear .  Night time, but
good visibility.  Scuttlebutt is that neither skipper thought they were "too
close."  Guess they were.

- -- 
Dave Kruger
Astoria, OR



Date: Wed, 26 May 1999 02:12:30 -0700
From: "Matt Broze" 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

From: John Winters

>(SNIP)
>
>>
>>I think we are looking at this wrong. I don't think the water is doing
>>anything but getting out of the way. The waves on the surface aren't
>causing
>>any acceleration or deceleration of the "flow" back and forth.
>
>Waves are symptoms of what happens not the cause. By examining the symptoms
>one can get at the cause. The wave system (and particularly the transverse
>wave system) reveals the energy expended in pushing the water out of the
>way. Crests indicate lower velocity and troughs indicate higher velocity
>along the hull. By measuring the pressure along the hull one can determine
>the local velocity. (Eggert, did this in 1939).

You are right.  I didn't think that one trough very well.

>The water pushed out of the way by the boats passage piles up in the
>transverse wave system and we can see the effects of displacement by
>observing the size of these waves as Dr. Inui did in his research. From
>this Inui developed ships forms that had almost no wave making resistance.
>
>From this we pass on to the other phenomenon that occurs, notably the
>sinkage of the hull. As velocity increases the hull sinks (most tank test
>series measure sinkage) due to the increased velocity under the hull.  Now
>we must ask, why? If no increase in flow occurs under the hull, why does
>the boat sink? Clearly increased flow does occur under the hull that will
>cause an increase in kinetic energy over potential energy thus reducing the
>hydrostatic forces supporting the hull. Where does this water come from?
>Does it enter in from the sides?" Not if it gets pushed away to the side.
>Does it come form the stern? Not if the boat moves ahead. Clearly it must
>come from ahead. If we have increased velocity we must also have increased
>volume.

It seems to me that any speed increase in the flow over the boat speed could
be due to the orbital motion of the wave trough and any extra sinkage of the
hull (beyond that caused by its own speed) reflects that the boat is in the
trough. The boat speed itself should cause some sinkage  (the fact there is
flow and that it is under and to the sides of the hull (but not on top)
would cause the Bernoulli effect to be balanced to each side but the
unbalance effect below the hull would make the hull ride lower. I don't see
how increasing the speed of the flow implies the need for a greater volume
of water from anywhere but above (dropping down to make a wave trough).

>> The hull is
>>moving at a relatively constant speed in a given direction and what we have
>>been calling flow is just the indicators of the direction the boat is moving
>>as it scrapes through the water. (your surfboard and rudder examples helped
>>me see this). No water goes down under the surfboard without also displacing
>>some other water out of the way and since water is not comressible the total
>>of all these nudges between molecules is upward (making a wave).
>
>Yes, the water passing under the boat pushes water to the side but the flow
>remains critical. As pointed out previously, that flow causes the hull
>sinkage. How one looks at this depends upon how one asks the question. If
>we ask, "Does the water get pushed to the side by the boat?" and answer,
>"Yes it does." then we can get led to the erroneous conclusion that the
>flow under the boat has relatively little significance or that it does not
>dive under the boat. If we ask, how does the water flow around the boat and
>get the answer described by observing the flow we get led to a different
>conclusion that the water flowing under the boat displaces the water to the
>side.
>
>How does this affect design? Consider the distance the water travels. A
>poorly shaped hull might have a greater effective length than one that
>ignores this information for one example. Also, if the water only gets
>pushed aside you might be tempted to design boats like the old plank on
>edge sailboats of the 19th century.

Do you think a Hobiecat's narrow hull is ineffecient? My understanding is
that it is so fast because rather than having to climb over the bow wave it
simply cuts through it . Being so narrow and moving water only a little to
the side and hardly downward it wouldn't suffer much from the Bernoulli
effect either.


I think our differences here are because you are looking at what the
apparent flow over the hull is as it moves through the water and I am trying
to understand what is actually happening to the water as the hull goes by.
You are riding on the boat and I am swimming in the water. I realize that
skin friction and separation drag is going to be determined by how the water
appears to flow past the hull. I also realize it would make little or no
difference in the physical effects if the water flowed by a teathered hull
or the hull moved through still water.  I'm just trying to point out that
the telltales are going to mostly point in the direction of travel and if
the hull is in the way they will point as closely as they can to the
direction of travel.

>>I'm not sure I want to argue with the Admiral but imagine a flat bottomed
>>ship built like a cow-catcher on a train. It should be clear that most of
>>the water displaced would be lifted and deposited to the side by the
>>cow-catcher bow yet if we looked at the little telltales we attached on the
>>bottom of the hull they would point in the opposite direction of the ships
>>motion even though no water would have gone downward under the hull at all
>>to make them do that. Why then are we or the good Admiral assuming some
>>downward flow based on this "flow" direction evidence?
>
>Well, I can't argue this without seeing what kind of shape Matt proposes.
>The cowcatcher example Matt provides may not have much to do with ship
>shapes. It may have some value in studying crab scrapes or snowplows
>though. We have to choose our examples carefully. Would we assume we know a
>lot about the flow around ships by studying the flow of water around a farm
>plow? Can we study the flow around a wing by testing a house in a wind
>tunnel?
>
>Nevertheless, there are ships with "cowcatcherlike" bows (mostly bulbs and
>such) and they have the same flow Taylor describes. Taylor's ships had bulb
>bows.

I was merely proposing a shape that would show a flow sraight back on the
bottom that clearly did not require a downward flow yet exhibited the same
behavior of the telltales as was being claimed as evidence of a downward
flow of water.

>The flow indicates where the water goes. To assume that it doesn't go where
>it is going kind of boggles.

Again you are an observer riding the boat.

The flow lines over the hull are the resultant of the motion of the hull and
the direction the water is actually moving. If the water moves 2" down while
the boat moves 200" forward the flow would appear to be at a slight downward
angle but the water itself would have actually have moved 2" downward and
slightly forward (due to the friction with the hull). So the water is mostly
moving up or down with the waves and being pulled along at some speed
between as fast as the boat (or even faster for spray flung forward at the
bow) and not forward at all some distance from the boat (which gets wider as
the boat moves past resulting in the wake behind the boat.
I don't know why the flow lines around a moving hull are as they are (and
I'm not disputing the direction of the lines just maybe the expanations
given--that I don't see clearly)
I'm trying to understand why they are that way in fairly simple terms and
basic principles that even I can understand.

It is getting into our busy season and I am spending way too much time in
front of this computer so I'm going to try to taper off my responses, but if
I can't control myself more I'll have to quit Paddlewise cold turkey. It has
been fun.

Matt Broze
www.marinerkayaks.com



Date: Wed, 26 May 1999 06:32:44 -0400
From: "John Winters" 
Subject: Re: [Paddlewise] flow

Kevin wrote;

>Matt and John are discussing whether water is displaced sideways (Matt's
>idea) or vertically (John's idea).

I think I may not have made myself clear. I said that the flow dives under
the hull and follows the buttuck lines.  As the water dives under the hull
it displaces water to the side. This is established by flow tests in tanks
(not my idea although I wish I had thought of it first). The result
(Admiral Taylor appears to be the first to show a proof) has had  repeated
support in other tanks and tests.

>To them I ask, why one or the other,
>and not a combination of both? My intuition tells me that water is
>displaced normal to the local surface of the hull, and probably in a
>uniformly thick layer.

This sounds so logical that one likes to accept it and I once thought the
same thing (as did a lot of sailsboat designers). How disappointing to
discover that water didn't see things our way. :-(

>Another interesting question is what happens to the displacement direction
>of water at a hard chine?

The chine tends to redirect the flow much as fins or keels etc do.

Nick provides a good example of the effect of flow under the boat. I will
ask Professor Inverbon about the Brit boats.

David wrote;

>I have observed bulbous bows on commercial vessels. I recall reading that
>they are used to increase fuel efficiency. I have wondered what effect one
>might have when used on a kayak. (Other than being an efficient kelp catcher
>that is.)

Bulbs have positive effects on resistance through wave cancellation. They
only work when the bow is fully immersed and kayaks rarely spend much time
in conditions where the bow can stay immersed. Also, their draft works
against them in this respect.  Followers of Baidarkas will have read
Dyson's comments on this as an explanation for the bifid bows on some
baidarkas. Regrettably, videos show the bow emerging from the water. I say
regrettably because one would like them to work.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/




Date: Wed, 26 May 1999 07:10:16 -0400
From: "John Winters" 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

Matt wrote;


>The flow over the hull is the resultant of the motion of the hull and the
>direction the water is actually moving. If the water moves 2" down while the
>boat moves 200" forward the flow past the hull would appear to be at a
>slight downward angle but the water would have moved only 2" downward (and
>slightly forward due to the friction with the hull). So the water is mostly
>moving up or down (and out to the side during a crest and back in the
>trough)

This just doesn't follow. If the water moved out to the side in the crest
and then back in the trough how does one explain the progressive widening
of the transverse wave system?


(SNIP)

(SNIP of Nick's comments) >

>I disagree. Your boat is affected by both the size and the length of the
>wave created at the bow. My undersanding is that as the wavelength is
>shortened due to the shallow water effect the boats hull speed is now lower
>(it gets trapped in a now shorter slower moving wave). For a fast moving
>kayak you first feel this in four or five feet of water. Some Canoe racers
>can actually plane their canoe if they can keep their speed up (and break
>the wave barrier) on suddenly hitting an area of shallow water.

The hull speed doesn't get lower (Always Froude 0.40) the resistance
increases due to the shallow water effects on the wave system. The effect
of higher speed in shallow water has nothing to do with planing. Planing is
caused by dynamic lift acting on the hull. The effect in shallow water
comes from the change in wave pattern at the critical speed V=(gh)^.5 where
h = water depth.  Below this critical speed the boat generates two waves
(Bow and stern). Above this speed the wave pattern for the pressure point
(bow) created by the boat increases in angle until the critical speed gets
reached when the wave angle approaches approximately 80 degrees and only one
wave gets created. At this point the all wave making energy is transmitted
to this single wave called the wave of translation. With added speed the
angle of the wave begins to decrease and the wave system consists only of
diverging waves with lower wavemaking resistance.

In short, the boat does not plane it simply enters a new wave making
regime.


Nick wrote;
>>Water flowing under the boat must be displaced and in turn displace other
>>water farther down and out. This chain of displacement eventually will push
>>out to the surface.

Matt wrote;
>Since water is incompressible the first effect of disturbing it can only be
>upwards, hence the bow wave an upward motion of the water.


I think I can now see where Matt comes from. I think he missed the
Bernoulli effect and that has caused him to go astray on this.


(SNIP)


>>The energy in the water due
>>to the motion is KE=1/2 m v^2. Moving a large mass slowly tends to be more
>>efficient than moving a small mass rapidly.
>>
>>The result is when the energy is released at the water surface, larger
>>waves are made. If the water was being pushed purely to the side, you would
>>not see such a dramatic change in shallow water. The typical wake of a
>>kayak is pretty small. Probably small enough to travel pretty easily in 1
>>foot of water.
>
>A wave begins to "feel" the bottom in water about 1/2 as deep as the
>wavelength(the waves orbital motion below the surface is touching the bottom
>which slows it down--compressing its energy into a shorter space thereby
>making the bow wave higher, steeper and harder to climb).


No one climbs their bow wave. It is a bit like lifting yourself by your
bootstraps.


(SNIP)

>It is easy to see in very shallow water why it would be impossible for much
>water to go down, it would just bump into the bottom. Due to the
>incomressability of water even in the deepest ocean no water can go down
>without moving some other water upwards and since the water can't go upwards
>though the hull it must go first to the side or raise up in front of the bow
>(due to the pressure in front of the moving hull).

The water need not go down (or upwards) if it increases its velocity. As
Nick and I have pointed out the water eventually does rise but it is pushed
out of the way by the water passing under the boat (verified by tank
testing and mathematical methods), .

>Here is another thought. In areas of lower pressure the flow will be moving
>back downward and in this area the flow would be diving under the hull as
>the flow past the hull moves parallel to the waters wave angle at the
>surface. It would seem the flow line on the hull would move back upward with
>the next wave crest but if that wave were well back on the hull the flow
>might keep moving down to fill in the area of low pressure behind the moving
>hull. I don't know if this is right just some thoughts I'm having in trying
>to apply basic principles to the flow question.

The flow tests do not support this. The flow quite clearly dives under the
hull and follows the buttocks. I would be interested in seeing  flow
studies that show otherwise.


Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/



Date: Wed, 26 May 1999 08:02:39 -0400
From: "John Winters"
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

(SNIP)


>It seems to me that any speed increase in the flow over the boat speed could
>be due to the orbital motion of the wave trough and any extra sinkage of the
>hull (beyond that caused by its own speed) reflects that the boat is in the
>trough. The boat speed itself should cause some sinkage  (the fact there is
>flow and that it is under and to the sides of the hull (but not on top)
>would cause the Bernoulli effect to be balanced to each side but the
>unbalance effect below the hull would make the hull ride lower. I don't see
>how increasing the speed of the flow implies the need for a greater volume
>of water from anywhere but above (dropping down to make a wave trough).

The flow diagrams do not show flow from the surface to the bottom. The
"source" of the flow comes from ahead not the side.

(SNIP)

>Do you think a Hobiecat's narrow hull is inefficient? My understanding is
>that it is so fast because rather than having to climb over the bow wave it
>simply cuts through it . Being so narrow and moving water only a little to
>the side and hardly downward it wouldn't suffer much from the Bernoulli
>effect either.

Actually the Hobie cat hull has very poor efficiency at kayaking speeds.
Used to kick their butts all the time in my Thistle. Of course, when the
speed got up to the point where I had to plane and they didn't, then they
kicked my butt. Boats with high L/B ratios don't experience the same
problems that boats with low L/B ratios experience. To assume, however,
that there would be no difference would lead one to erroneous conclusions.
One cannot assume that the flow regimes for a scow will be identical to
than of a SWATH ship. The principles remain the same but their degree
varies. In this way, the flow along the bottom of a wide shallow boat
varies from the flow along the narrow bottom of a deep narrow one in
degree. We cannot assume that, because a deep narrow boat has so little
flow along its narrow or nonexistent bottom that there would be no flow on
any other hull form. In the same way we cannot assume that, the flow on the
bottom of a wide boat has to be duplicated on a narrow one. What we do is
study the boats we deal with. Thus we are careful not to apply the design
principles of a submarine to a sea kayak and in so doing avoid cowcatcher
bows.

Boats do not climb their bow waves nor do they cut through them. Boats
create their bow wave. Regardless of where it lies on the boat the first
wave created (called a pressure disturbance) is the bow wave. I know I
sound like a broken record on this but ................


>I think our differences here are because you are looking at what the
>apparent flow over the hull is as it moves through the water and I am trying
>to understand what is actually happening to the water as the hull goes by.
>You are riding on the boat and I am swimming in the water. I realize that
>skin friction and separation drag is going to be determined by how the water
>appears to flow past the hull. I also realize it would make little or no
>difference in the physical effects if the water flowed by a teathered hull
>or the hull moved through still water.  I'm just trying to point out that
>the telltales are going to mostly point in the direction of travel and if
>the hull is in the way they will point as closely as they can to the
>direction of travel.

I don't get the point. The flow markers (whatever they are) will point in
the direction of flow and that isn't  always in the direction of travel.
The flow is what actually happens to the water. I cannot debate the
philosophical aspects of sitting or swimming. They would not let me swim in
the test tank possibly because they saw how much coffee I drink. :-)


>I was merely proposing a shape that would show a flow sraight back on the
>bottom that clearly did not require a downward flow yet exhibited the same
>behavior of the telltales as was being claimed as evidence of a downward
>flow of water.

I recognize what you were doing. Imagine a ski gliding across the surface
of the snow. Clearly the ski glides across the surface of the snow. Suppose
we turn the tip down instead of up. Then the ski would plow under the snow.
Should we conclude from this that skis do not glide over the snow because
we can create a ski that does not? One should not apply the design
principles of a snowplow to a Ferrari.


>>The flow indicates where the water goes. To assume that it doesn't go where
>>it is going kind of boggles.
>
>Again you are an observer riding the boat.

?????

>The flow lines over the hull are the resultant of the motion of the hull and
>the direction the water is actually moving.

Yes.

>If the water moves 2" down while
>the boat moves 200" forward the flow would appear to be at a slight downward
>angle but the water itself would have actually have moved 2" downward and
>slightly forward (due to the friction with the hull).

The water moves with the hull in the boundary layer not in the free stream.
This discussion has to do with the free stream flow.

>So the water is mostly
>moving up or down with the waves and being pulled along at some speed
>between as fast as the boat (or even faster for spray flung forward at the
>bow) and not forward at all some distance from the boat (which gets wider as
>the boat moves past resulting in the wake behind the boat.

The waves are continuously created by the energy expended by the boat.
These waves get left behind as can be seen in the wave train where the energy
dissipates.

>I don't know why the flow lines around a moving hull are as they are (and
>I'm not disputing the direction of the lines just maybe the expanations
>given--that I don't see clearly)
>I'm trying to understand why they are that way in fairly simple terms and
>basic principles that even I can understand.

Well, I never said I was a teacher. I think the best thing would be to read
some of the better texts. Since Taylor isn't easily found you might try
"Principles of Naval Architecture" by the Society of Naval Architects and
Marine Engineers. Sometimes heavy going but gets you there.

>It is getting into our busy season and I am spending way too much time in
>front of this computer so I'm going to try to taper off my responses, but if
>I can't control myself more I'll have to quit Paddlewise cold turkey. It has
>been fun.


Making a living always comes first. Will miss your valuable input.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/



Date: Wed, 26 May 1999 09:45:08 -0400
From: Nick Schade 
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"

At 12:53 AM -0700 5/26/99, Matt Broze wrote:


>>Even though it may be pushing less water it is
>>pushing it out of the way at a higher velocity.
>
>Why would it be pushing less water?

In deep water the motion can be distributed over a larger volume. In
shallow water it must displace the water available which is not as much as
available in deep water.

>
>>The energy in the water due
>>to the motion is KE=1/2 m v^2. Moving a large mass slowly tends to be more
>>efficient than moving a small mass rapidly.
>>
>>The result is when the energy is released at the water surface, larger
>>waves are made. If the water was being pushed purely to the side, you would
>>not see such a dramatic change in shallow water. The typical wake of a
>>kayak is pretty small. Probably small enough to travel pretty easily in 1
>>foot of water.
>
>A wave begins to "feel" the bottom in water about 1/2 as deep as the
>wavelength(the waves orbital motion below the surface is touching the bottom
>which slows it down--compressing its energy into a shorter space thereby
>making the bow wave higher, steeper and harder to climb). At 5.4 knots the
>wavelength created in deep water is about 16 feet long--which means it
>begins to feel bottom in 8 feet of water. [At 3 knots the wavelength is
>about 5 feet long so the wave-drag effect starts (but is probably not
>noticible yet in 2.5 foot deep water)]. In four feet of water the 16 foot
>wave is slowed a little to 5.2 knots. In 2 foot water that wave slows to
>about 4.4 knots and that sure slows your racing speed. In 1 foot deep water
>that waves speed is down to 3.3 knots and so is the kayaks hull speed unless
>it can break the wave barrier and start to plane. In 1/2 foot deep water
>wave speed is slowed to 2.37 knots. Remember that in water less than about a
>foot deep bottom drag due to turbulence drags one down even more than this.

The reason the wave "feels" the bottom is due to water moving down. If
there was no downward motion the wave would never care about the bottom.

The fact that a boat feels a lot of drag when moving in shallow water is
well estabilished. This drag is due to the difficulty of a wave of a given
wavelength to propogate in shallow water. But why can't the wave propogate?

Because the water is shallow there is not as much mass available to deal
with the energy in the wave so the water has to move faster. The wave
propogation speed can't increase because the water is not deep enough to
sustain a the required longer wavelength. Instead the waves increase in
height and eventually break, thus disipating the energy in friction.

However, once the wave is generated it does not have much further effect on
the boat. The wave can go its seperate way and disipate its energy anyway
it finds convenient without further effect on the boat.

Waves are just a symptom of drag. The bow wave and resulting trough at the
stern when paddling in shallow water are a by-product of the interaction of
the bottom of the boat and the bottom of the sea through the medium of the
water. There would be no interaction without the water being moved downward
under the boat.

>>If the water was just being pushed to the side, not much
>>would change in shallow water, but because the water must find a fast way
>>to get out of the way, shallow water slows you down.
>
>Sure it would.
>
>Water does speed up in a constriction--the Venturi effect (a special case of
>the Bernoulli effect)--which if under the boat would lower the boat further
>than normal and this would bring the stern turbulence even more into contact
>with the bottom. I should point out here that when two ships pass near each
>other or a ship passes near a wall the same effect moves them closer
>together (a real danger for ships). Two kayaks coasting along less than a
>foot apart will demonstrate this sucking together effect as well. This would
>seem to indicate that water is also moving around the hull (I'm applying the
>same arguments you are using to convince us that flow is going under a
>hull).

Kayaks generally have a greater beam than draft. I will not try to argue
that no water moves sideways. John says empirical data shows that it tends
to follow buttocks lines and I am prepared, to a certain extent, to accept
that. Imagine a perfectly flat bottomed boat with perfectly vertical sides
and a nice waterline shape. You would think the water hitting the sides and
moving back along the length of the boat would move straight out and then
straight back in. However, the bottom gets larger towards the middle. This
larger area needs to get water to cover it from somewhere, so it draws down
water from the sides. Obviously, there is some friction involved so the
water does not move perfectly along the buttocks lines, but it does its
best.

So, in deep water, some water does get pushed out to the side, but alot of
it goes down to the bottom to fill the large bottom area.

The water near the surface which gets pushed out to the side creates enough
Bernoulli effect to draw together two kayaks paddled side-by-side. But the
effect between the boat and a shallow bottom is much greater.

>It is easy to see in very shallow water why it would be impossible for much
>water to go down, it would just bump into the bottom. Due to the
>incomressability of water even in the deepest ocean no water can go down
>without moving some other water upwards and since the water can't go upwards
>though the hull it must go first to the side or raise up in front of the bow
>(due to the pressure in front of the moving hull).

Yes, in shallow water, the water can not move down, but in deep water it
can. In shallows the water below the boat must be squeezed outward as you
would think. So yes, the water is being pushed sideways. But this causes
increased drag, because it needs to displace the small amount of water
_rapidly_ out of the way, instead of distribuiting this displacement over a
larger mass of water which it could move slowly by initially pushing it
downward.

The problem in shallow water is not so much that the water is being moved
sideways, which I will admit it has to do eventually in all cases, but that
it must move that water much quicker than if it could move a larger mass of
water downward before that motion is dissipated out to the side. Moving the
small amount of water fast manifests itself at the surface as higher energy
waves. Again, it is not the waves that are the problem, it is what causes
them that matters.

Nick



Nick Schade
Guillemot Kayaks
10 Ash Swamp Rd
Glastonbury, CT 06033
(860) 659-8847

http://www.guillemot-kayaks.com/

>>>>"It's not just Art, It's a Craft!"<<<<