Re: [Paddlewise] Weatherhelming versus Weathercocking Usage

Concerning the cause of weathercocking

James wrote:
>The
> reason being that the bow is cleaving "still" water and, thus, wants to
> go straight whereas the stern is moving through perturbed water (the
> wake) and, therefore, is easily moved to the side.

and GaryJ wrote
>>>>>as a result of the fact that the bow
makes waves, and to turn the bow youhave to lift it over or through those
waves.
  The stern on the other hand is ell between the bow waves and can move back
and
forth more easily. <<<<<

The cause of weather helm is not the bow wave or the turbulence. These are
both rather side  effects (literally) of the cause. The cause is the
pressure difference between the front and back of a moving hull. A
stationary hull has equal pressure all around but move that hull in any
direction and the pressure will be greater on the part that is in the
direction of the movement (usually, but not necessarily, towards the bow).
Add a side wind to this moving hull and it will blow the part of the hull
where there is less pressure resisting the side wind further in a given time
than the part with the greater pressure. While both ends are moving downwind
the stern is moving faster downwind than the bow so one has the impression
that they are turning into the wind.

Craig wrote a long and nearly completely accurate description but I was
frustrated that he seemed to ignore the pressure difference aspect created
by a moving hull but rather treated the subject more from a static point of
view. To quote him:

>>>>>>The sum of all the wind forces acting on all the parts of the boat
that are
above the water is a point known as the "center of effort".
The sum of all the water forces acting on all the parts of the hull that are
below the water is known as the "center of lateral resistance".
On any boat, any time the center of effort is aft of the center of lateral
resistance that boat, left to its own devices, will rotate into the wind
(weathercock) and any time it's forward of the center of lateral resistance
the boat will rotate downwind (leecock).<<<<<<<

Absolutely true, but until you understand that it is very hard to create a
lee helm in a kayak because of the natural weather helm created by the
pressure differences you aren't getting a real understanding. The very fact
of moving the kayak forward moves the center of lateral resistance forward
as well. Moving the kayak backwards moves the center of lateral resistance
aft. While I agree with everything Craig says I think a couple of the things
may be misleading. For example Craig wrote:


>>>>>>>Take a look at your kayak from the side. Chances are that the cockpit
is
slightly aft of the middle of the boat. In this case your kayak will almost
certainly have a tendency to weathercock or turn into the wind. If moving
your seating position to maximum doesn't change the way your kayak handles
in wind then you have to make some adjustments.<<<<<<<<

>From the above true statement one is likely to draw exactly the wrong
conclusions (and I think Craig is misinterpreting the reasons for weather
helm--although his statement is still true because of the predominance of
weather helm). Since almost all kayaks have a tendency to weathercock the
statement is true but it is not due to the cockpit being further to the rear
than center as is implied here. If one moves the cockpit it will either
change the hull's trim or the shape of the hull design underwater will have
to be changed in order to keep the trim level. If you let the trim change by
just moving the paddler further back you will find that the kayak will
weather helm less (not more as is implied here). This is because water is a
lot denser than air. The change in the wind water couple from sinking the
stern deeper and raising the bow more out of the water (both moving the
center of lateral resistance aft and the kayak's center of windage forward)
will therefore do much more than compensate for the shift in the paddlers
windage aft. Note also that the paddler's windage is close to the balance
(or pivot) point so is not given much of a lever to act compared with the
bow and stern changes due to change in trim. In our (Mariner) kayaks with
instantly adjustable sliding seats one shifts the seat back to reduce a
weather helm and it is quite effective. If you compensate for a weight shift
(caused by moving the paddler to the rear) by making the kayak hull design
more Swede-form (finer bow and fuller stern) to maintain a level trim you
are also changing the areas of the moving hull that are under increased and
decreased pressure as well as changing the pressure itself. In so doing you
are also moving the balance point (center of lateral resistance) of that
moving hull. Even if the hull is static you are likely to find that the
windage from the paddler remains close to the neutral point because of the
changes you had to make to keep the hull floating level and their effect on
its center of lateral resistance.

Finally, I think leecocking is a misnomer but is an understandable one (and
I have no objections to its use). If weather cocking refers the rooster
shape of a traditional weathervane then maybe we should use "asscocking",
"cockbutting", or the more genteel "tailfeathering" to describe this
cockamamie behavior in a kayak ;-)

Matt Broze
www.marinerkayaks.com
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