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post #518 of Old 10-03-2012
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Re: Full or fin keel?

Originally Posted by Jeff_H View Post
Mitigating against the harshness of the motion, dampening from the keel and rig can greatly improve rotational motion comfort, slowing changes in direction without adding the kind of inertia, which tends to make the boat more dramatically over shoot the wave face and so experience grater impact forces.

And this is where the specifics of the design come into play. Too much form stability and the boat will throw its occupants around mercilessly. Too little form stability and the boat will roll and pitch the occupants to death. Which is why I personally advocate a more moderate design philosophy than seems to be the case with the more extreme current design practices.

Coming back to the Full keel vs Fin topic, this another area where the specifics of the design come into play. A full keel would tend to have a comparatively large area which helps to increase dampening force thereby slowing roll rates, but full keels a shallower draft which works against creating as large a dampening moment. So depending on the design, a deep fin, although generating less sideforce, may in fact generate an equal or greater dampening force.

Jeff, you are thinking way too simplistically. We had a short discussion earlier in this thread. A significant problem with the long keel is that the force generated by movement of the keel stops being a damping component, but instead changes the dynamic center.

So you cannot say that the damping increases as some power, because at some point it is no longer damping. In large waves, the long keel on a boat with large hull moment can actually cause the boat to heel more because the dynamic center may drop below the CG of the boat.

So invariably, the shallow long keel often provides the better damping for a heavy boat. When I create my thread this will become clear.

There is no boat with wide beam and long keel will result in a good rough water boat. This has been pounded into yacht designers since time begin. Unless you put a coresponding heavy ballast to counteract the hull moment. But then it simply changes the mass moment to hull moment ratio which is what I suggest using as a rough water indicator; for lack of a better term, Bryces rough water coefficient.


Last edited by BryceGTX; 10-04-2012 at 12:20 AM.
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