[Paddlewise] sp*ns*nsm, etc.:

> From: "Evan Dallas"
> Subject: [Paddlewise] Kayak speed vs length
>
> I've often heard (and intuitively accept) that all other factors being equal, a
> longer kayak will be faster than a shorter one.  I've got a friend who disagrees
> with this, but I've had a hard time explaining why this would be true.
>

A longer kayak isn't faster for a given energy input, but it has a
higher "hull speed"- that is, a speed that cannot be exceeded without
more adding energy than is available.

There's a nice description at
http://www.dt.navy.mil/ip/mfp/paper11.html, the guts of which is this:

As a boat travels through the water it generates waves. This is because
the boat displaces a volume of water and that water has to go somewhere.
At low speeds the boat spans over multiple waves. As the boat speeds up,
the length of the waves increases and the boat eventually finds itself
in a trough with a length equal to the boat's waterline. This happens at
a speed equal to (in knots) around 1.34x the waterline length (LWL) in
feet. At this speed the drag on the boat from these waves is minimal.

As the boat tries to go faster than this "hull speed",  the wavelength
doesn't increase. It can't because it can't get any longer than the LWL
of the boat. Instead, the boat tries to pass the wave, and to do so, it
has to climb the crest of the wave in front of it. This takes a lot  of
energy. (If the boat has enough energy to do so, it goes up on plane,
stops making waves and drag drops drastically)

So any relatively low powered craft, like a kayak, has a sort of
terminal velocity- a speed at which it takes more energy than a paddler
has available to exceed- and that speed is mainly a function of LWL.

-- mike
 -------------------------
 Michael Edelman
 mje_at_spamcop.net
 http://www.foldingkayaks.org
 http://www.findascope.com


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