Re: [Paddlewise] Stability vs Down Wave Stability

Jerry wrote;


>I share your preference for a boat that provides some  resistance to
>capsize.  Not having wind directional stability, ie. strong
weathercocking,
>is something I dislike greatly.  However, it leads to extra work rather
than
>to upset.  Roll instability can lead to capsize, especially if accompanied
>by lack of attention or carelessness.  But down wave directional
instability
>can be scary.  I have been in some boats that felt truly fine in windy
>choppy conditions, until I turned down wind.
>
>I have been told that a "double ender", with fine entry and exit hull
shape,
>is a term for a sailboat that handles down weather easily.  Does the same
>apply to kayaks?

I believe this to be valid in comparison with wide sterned power and sail
boats. The wider sterns are more affected by the following seas and tend to
increase the chances of broaching. However, this "rule" commonly applied to
power and sail boats may not be strictly applicable to light boats like
kayaks that lack draft and can be accelerated so easily.

Kayaks and canoes use a superior steering system unhampered by being at the
stern and having the slow response of larger heavier boats. Of course, the
term "fine" can present problems. The Chinook that has very fine ends
(relative to many boats) seems to broach easily (my experience and that of
some others). The balance between fine and full presents a huge challenge.
When the designer gets it right he probably deserves sainthood.

On the negative side, kayaks operate in "heavy" seas most of the time
(relative to ships) and that means constant attention. When the wave length
equals (aprox.) the boat length and the midsection is in the trough the
narrower ends must support the boat and provide stability. Despite the
apparent gravity increase in the trough there can still be a loss of
righting arm and, consequently, righting moment.

The "on site" problem for paddlers is whether to slow down or to speed up
to avoid broaching.
Marchaj discusses this in his book "Seaworthiness: The Forgotten Factor and
I suspect his thoughts might apply to kayaks more than sailboats due to the
relatively heavy seas we paddle in.

Dave Kruger correctly mentioned the directional instability down wave as
being due to the water particle motions in the wave. The trick from a
design standpoint will be to minimise the relative effects. For instance.
One can increase buoyancy and reduce draft forward to reduce the effect of
plunging the bow into the back of the wave. As Dave points out, you can
also use the shape of the boat to control its direction while surfing. When
travelling slower than the wave in serious conditions a sea anchor can be
used to keep the boat normal to the waves.  For non traumatic conditions
the usual braces etc. work fine.

Cheers,
John Winters
Redwing Designs
Specialists in Human Powered Watercraft
http://home.ican.net/~735769/


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