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post #507 of Old 10-03-2012
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Re: Full or fin keel?

Originally Posted by skygazer View Post
I always enjoy Jeff's informative and thought provoking discussions.

If I'm understanding this correctly, the modern boat remains in sync with the wave surface by generating more righting force (due to it's flatter wider shape). The greater righting force is also generating more rapid and continuous (rotational) acceleration. This greater acceleration would be more exhausting to the human occupants.
First of all thank you for the kind words. To address your comment, to some extent you are correct that that a boat with greater form stability will stay in sync with the wave face, and in some conditions will be less comfortable and tiring for the occupants. But the the reality of whether the boat is more comfortable to the occupant is also dependent on other factors.

So for example, depending on wave size, steepness and frequency, on the boat with less form stability, the delay in the change in direction may be such that the boat actually continues moving past the point the wave shape changes at the crest or trough. In those conditions, being out of sync. the boat experiences a harder impact with the wave surface and so actually experiences a harsher deacceleration than the boat with more form stability, and so can be harder on the occupants.

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.


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