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/




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