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post #51 of Old 11-15-2013
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Re: Full Keel Vs. Fin or Modified Full Keel

I am not sure whether you are talking about rolling in a short chop taken abeam or larger breaking waves. In either case, it depends on the specific design of the boat, its loading, and the shape of the wave in question. In a chop abeam, there will be situations were each type would be superior but a long keel with cut away forefoot would generally be worse in all cases. Iin a large, breaking wave taken abeam, except a long keel with cut away forefoot, the reality is that each keel type has benefits and liabilities.

The case is often made that a full keel is safer in a roll situation since it is typically shallower than most other keel types and so experiences less wave surface sheer forces (the force that results from the difference in speed between the water on the surface of the wave and in the center of the wave). But full keels generally have a lot of side area and does not stall so that area tends to generate a lot more rolling force even if the wave sheer speed difference is smaller. Typically full keel boats have smaller form stability so they have less force trying to initially roll them into conformance with the wave face (good) but that same lack of form stability means that there is less force trying to right them in the trough (bad). Traditional full keel boats tend to have large roll moments of intertia which resists rolling the boat at the start of the incident (good), but which may tend to cause them to continue rolling at the trough (bad) and since the roll moment of interia often comes from heavy spars, that tendency gets amplified (bad).

Probably the best choice in a large breaking wave taken abeam would be a keel-centerboarder with the board up. It would have a shallow keel and so would experience the least wave surface sheer, and has a small side area so the combined side forces would be smaller as well.

The deeper fin typically has a lower vertical center of gravity and a smaller area working in its favor. But being deeper the fin keel is exposed to a greater speed difference between the wave face and bottom of the keel due to sheering of the wave face. Mitigating against that higher speed is the tendency of deep fins to stall at large angles of attack which by definition a wave abeam would be. Then there is role of roll moment of inertia. Fin keels especially with a bulb when combined with the typically taller rig on a modern design produces proportionately high roll moments of inertia which would initially minimize the induced roll angle.

Probably the worst option is a long keel with cut away forefoot and an attached rudder is the worst option being deeper than a full keel but unable to stall as easy as a deep fin, and with a higher vertical center of gravity. All bad...

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Last edited by Jeff_H; 11-15-2013 at 08:22 AM.
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