Re: [Paddlewise] Pro's and Con's of the "Swede Form"

-----Original Message-----
From: Nick Schade <schade_at_guillemot-kayaks.com>
To: paddlewise_at_lists.intelenet.net <paddlewise_at_lists.intelenet.net>
Date: Tuesday, May 25, 1999 8:39 AM
Subject: Re: [Paddlewise] Pro's and Con's of the "Swede Form"


>At 8:55 AM -0400 5/24/99, John Winters wrote:
><snip A whole bunch of stuff>
>>
>>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

>
>Nick
>
>
>
>Nick Schade
>Guillemot Kayaks
>10 Ash Swamp Rd
>Glastonbury, CT 06033
>(860) 659-8847
>
>Schade_at_guillemot-kayaks.com
>http://www.guillemot-kayaks.com/
>
>>>>>"It's not just Art, It's a Craft!"<<<<
>
>
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