Re: [Paddlewise] Stability

Merijin wrote:

> The theory says that the local gravity acts perpendicular to the surface
of the water, ad said previously in
> this discussion.
> If this is correct, then how is it possible to surf on a non-broken wave?
My idea of a surfing situation has
> always been that there does exist a gravity component in the direction of
movement of the boat. This
> gravity component should accelerate the boat.
> Or is it just that the rotation movement if the water results in a kind of
"local flow" on the waves surface that
> diminishes the relative velocity of the boat with respect to the water?
> I am puzzled....

Understandable.
Doesn't seem intuitive.

In the discussion on stability we confined ourselves to boats traveling
across the face of the wave and not with it hence the concern over
athwartships righting arms and moments. When  a boat travels with the wave
we must add the forward speed of the boat and its related forces. The net
effects of dynamic lift, buoyancy, drag (as the boat moves through the
water)  produces a force tangential to the face of the wave. If the
resulting equilibrium speed is the same as that of the wave the surfing boat
remains on the wave but still moves fast relative to the water molecules in
the wave. "Weight" always acts "down" and the resultant of weight and
centripetal force act towards the center of the non breaking wave.

Maintaining equilibrium speed generally requires using a fair amount of
energy and one has to paddle pretty hard to surf non breaking waves. If you
do not paddle you cannot surf non breaking waves.

Breaking waves present a different problem and one has to work to avoid
being swept ahead at the translational speed of the breaking wave crest
which exceed twice the wave speed (or, if into these things, work to
maintain an ideal position on the breaking wave face.

You may have  noticed how short surfing lasts in deep water waves. Because
waves come in different sizes moving at different speeds the large surfable
waves resulting from the merging of two or more waves disappear as the
faster wave moves past and the wave size diminishes right out from under
you.

Cheers,

John Winters
Web site address http://home.ican.net/~735769




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