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

Weathercocking and a weather helm are two names for the same thing, a
tendency to turn up into the wind. Weather helm probably came from sailboats
which, for one thing, actually have a helm. Rotation due to waves in a
following sea is called broaching. I'm not sure what waves pushing back on a
bow angled into the waves is called. That would be the equivalent of
leecocking (or a lee helm) in the wind. A long object (with equal depth at
each end) will come to rest laying across the direction of the waves so
whichever end is pointed more into the waves gets the stronger push on it.
Gravity acting on the mass of the boat is balanced by the buoyancy in the
water that the hull sinks into (displaces).. Once a hull is moving about the
speed of a wave (1.34 times the square root of the wavelength) that is the
same length as the hull's waterline length the next wave behind the bow wave
no longer supports the stern of the hull as high as it had been when at rest
or at slower speeds. About that speed, known as "hull speed" the stern
starts to drop into the trough of the first wave and to go any faster the
hull must climb out of the hole in the water (the trough behind the first
wave) the hull is making. Therefore, gravity must be overcome to some extent
to go faster. A powerboat with a powerful engine climbs with its bow up
steeply for a while and then once it gets to a speed of about 2.5 times the
square root of the waterline length it again levels off and planes across
the surface. Its center of gravity is several inches higher than when the
hull was moving slowly. The drag does not increase at as fast a rate once
planing is established but it is still increasing rapidly. Holding a heavy
boat up against gravity means that a lot of energy is being used to do so
and this is why a displacement hull is a much more efficient way to move a
given weight over the water than is a planing hull. A human is not capable
of planing a kayak. Friction is increasing drag at about the 1.84th power
(nearly the 2nd power) at speeds below hull speed. Once into the area where
hull speed comes into play and becomes a major drag force the resistance is
going up at about the 4th power. That means to go twice as fast will
require16 times the horsepower. An explanation of how length and hull speed
relates to kayaks and the choice of length can be found at the start of the
FAQ section of our website.
Matt Broze
http://www.marinerkayaks.com

----- Original Message -----
From: "PeterO" <rebyl_kayak_at_iprimus.com.au>
To: "'Matt Broze'" <mkayaks_at_oz.net>; "'Paddlewise'"
<Paddlewise_at_paddlewise.net>
Sent: Tuesday, December 10, 2002 12:25 PM
Subject: RE: [Paddlewise] Excessive Weather Cocking (or is it Fairy gliding
with the wind)


> Matt wrote: -
> >comment on just that point. If something is
> >not clear to you and you want a clarification
> >of what I mean, please ask me.
>
> and in an earlier post: -
> >eventually as speed is increased you will bump
> >into "hull speed" beyond which you would be
> >using energy at an unsustainable rate (because
> >you are working against gravity
>
>
> G'Day Matt & Paddlewise,
>
> Your explanations confirmed and developed my understanding of previous
> posts. A couple of minor questions.
>
> What is the difference between "weatherhelm" and "weathercock"? Does one
> describe wind effects and the other wave effects on a kayak?
>
> Secondly you mentioned the effect of gravity on energy consumption. Is
this
> solely due to the increased cadence and therefore the increased frequency
of
> lifts for a paddle and associated water near the end of a stroke? This
> assumes there is no significant energy use against gravity in 'climbing'
> waves unless one is pushing hard into surf.
>
> All the best, PeterO
>
>
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