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

At 9:31 PM -0700 5/27/99, Matt Broze wrote:
<snip>
>>(SNIP)
>John said:
>>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 ................
>
>
>Of course the boat creates the wave but that wave sure looks to me like a
>hill to climb that is bigger than the hill that would have to be climbed to
>reach planing speed if one started from a position where the wave hadn't
>already been formed. I am well aware that true planing is beyond human power
>except with the use of hydrofoils (at least at the present time and into the
>forseeable future).

I have used words to the effect of "climbing its own wave" to describe what
is happening at hull speed myself because, although I knew better, I did
not understand it well enough to come up with a better explaination. As
much for my own benefit as anyone elses, I am going to try again.

We tend to think of the waves as creating, or being the source of drag,
when they are really the visible evidence that drag has already occurred.
When a hull moving through the water experiences drag, it is due to the
water applying a force to the boat. This force is sapping energy from the
boat. For energy to be conserved, the water must increase in energy. In
other words the water is put into motion by the drag of the boat on the
water at the same time the boat is being slowed down by the drag of the
water on the boat.

The energy put into the water by the boat must go somewhere. Some of it is
lost in frictional heat in turbulence where the water molecules rub against
each other and heat the water slightly. The rest of it produces waves. If
there was some other way to disipate the energy the waves would not appear,
for example if fish were imparted kinetic energy and got squirted out of
the water, or if little propellors hooked to generators absorbed it all.
Even if the waves never appear, the drag still would have occurred. Once
you see the waves it is too late, the damage is done.

When John says "Boats create their bow wave.", it sounds a little obvious.
We didn't think evil demons were doing making them. But, what it means is
that the boat can not avoid the wave. If there is a lot of drag the wave
will be big. If the boat can power its way through the drag and get into
another drag regime, the wave will change. If the drag diminishes, the wave
will diminish. You can't climb over the wave because the wave is basically
innocent. The wave is a side effect, not a cause, of drag.

This is not completely true because the changing wave shape can change the
flow of water around the boat and this can change the drag, but it is not
really the wave itself creating the drag. Drag is from friction and
viscosity.


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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