Re: [Paddlewise] Weatherhelm

-----Original Message-----
From: 735769 <735769_at_ican.net>


>I wrote;
>
>>Okay, here is what I know about that. I was at the towing tank the day
>>before observing the tests and trying to talk the testers to run a rudder
>>down test (and also to try an old scratched up Boat which I had vs. a
brand
>>new one with a shiny bottom which they had already tested).
>
>(Large SNIP)
>
>John Winters wrote:
Sounds interesting. I can see why the results puzzled the tank people. If
>you can get it Horner's "Fluid Dynamics" might supply the information
>needed. I copied some parts of the book but not the ones needed for this.
>Maybe someone with an affliation with a university can get a copy.
>
>The presence of cavitation on rudders and skegs interests me most. I have
>read cavitation defined as "the process of formation of the vapour phase of
>a liquid when it is subjected to reduced pressure at constant ambient
>temperature" . I interpret this as meaning that, since water cannot support
>tension the flow breaks down with the formation of bubbles and cavities
>(I.E. it vaporizes) .  It seems like one needs pretty low pressures to do
>this. If cavitation does exist then that may well explain the higher drag
>since the creation of such low pressures  might involve a lot of energy.



Actually, I was a little loose in my wording. I believe John's definition of
cavitation is correct and that there isn't low enough pressure around a
rudder blade to create it. Basically the pressure has to get low enough that
water boils at the waters temperature creating water vapor. This is a
serious problem for high speed propellers as not only is efficiency lost but
the propellers can be literaly eaten away by it.  True cavitation not likely
to happen to rudders and skegs. I was using the term more loosely to mean
the rudder or skeg developed a resonance and whipped back and forth at
several  cycles per second due to its shape, flexibility, or play in the
system. I suspect this does cause more drag but I have no data to back this
up. I have noticed though that the skegs that get into resonace are the
shaped ones. I suspect that the lifting shape flies it of to one side until
it reaches the limits of its motion due to stiffnes or a stop (of its
looseness) and then as it rebounds it generates lift in the other direction
ad infinitum..
There is another way a sterm mounted rudder may have increased drag and that
is though ventilation. Lower pressures around the blade allow air to
infilterate around the rudder from where it pierces the surface.
I wish I had known that Sea Kayaker would do the rudder test they told me
they weren't going to do. I might have stayed another night in Vancouver and
videod one of the runs with the rudder down from the underwater viewing
window of the test tank. Air around the rudder should have been easy to see
from underwater.
copyright 1999  (Does anybody know how to easily make the circle C copyright
symbol in HTML?)
Matt Broze
http://www.marinerkayaks.com


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