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

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

>>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

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

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


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