Re: [Paddlewise] Excessive Weather Cocking (or is it Fairy gliding with the wind)

On Wednesday, December 11, 2002, at 10:13 AM, Peter Rathmann wrote:

> Matt Broze wrote:
>> [Lots of good information about hull speed and wave-making.] ...
>> Holding a heavy
>> boat up against gravity means that a lot of energy is being used to 
>> do so ...
>
> While I commend you on a good description of hull speed and the
> transition between displacement and planing regimes, I do have a 
> quibble
> with the above sentence.
> It takes energy to initially lift the boat against gravity when it
> starts to plane, but no further work is done against gravity once the
> boat is at a constant level while planing.  In the same way energy is
> required to lift your boat up onto some sawhorses, but the sawhorses do
> not expend any energy while holding the boat up at a constant height.
> The energy required to move a planing boat goes into wave formation,
> surface friction, air resistance, and some losses to turbulence at the
> propellor (or paddle) to water interface.  Adding up all of these will
> give the total energy with no additional amount needed to hold the boat
> up against gravity.
>

I agree with you. It does not take any energy to maintain a boat at a 
constant elevation on the face of a wave. But there is something called 
"slope drag". This is the force that cause a boat to surf when running 
with the waves. It does require a force to overcome the desire of the 
boat to slide backwards down its bow wave. Since this counteracting 
force is developed by the paddler sticking his paddle in the water and 
pulling, it does require an expenditure energy to fight gravity and 
stay on your own bow wake.

However, just because there is some energy required to stay on the 
wave, I don't think it is appropriate to say that at hull speed, you 
can't go any faster because your are "climbing" over your own wake. The 
wake is the effect of displacing the water around the hull. This 
displacement of water adds energy to the water. The energy in the water 
is dissipated by means of waves. The energy applied to the water 
increases with the square of your velocity. So the faster you go, the 
higher the energy you are applying to the water and the harder you have 
to paddle. Hull speed is when you reach a point on the velocity/energy 
curve where the energy required to go faster starts to climb steeply 
and your additional power does not add much additional speed.

The fact that you have to use additional power to stay up on your wake 
is a tertiary effect. The boat moving fast adds energy to the water, 
this energy makes waves and thus you need to stay on the wave. The 
primary reason you can't go faster is the kinetic energy you are 
imparting to the water is more than you can supply by paddling. The 
slope drag just makes it that much harder.

Nick Schade

Guillemot Kayaks
824 Thompson St
Glastonbury, CT 06033
USA
Ph/Fx: (860) 659-8847
http://www.guillemot-kayaks.com/

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