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Shortening Keels
Obviously. it really varies with the particular boat and the amount of the keel being altered. In theory if you are only shortening the keel a little and the bulb is designed to maintain so that the keel maintains the same center of gravity, then it should not affect the structure of the boat or its seaworthiness. It just won''t go to windward as well.
The problem is in the details. First of all a boat is generally designed with a certain amount of keel area that contributes to lift. When you cut a keel down you are not only reducing the area of the keel by what you cut off but also by the area of the keel covered by the bulb. You are also adding the additional drag of the bulb. All of that is bad medicine for upwind or downwind performance.
Then there are construction issues. Modern bolt on keels are constructed with ''L'' hooks in the end of the bolts. These are cast into the lead and are ''nested'' at different depths in the lead so that they do not interfere with each other''s bond to the lead. If you cut too much of the keel off, or accidently drill through one of these bolts when you drill to attach the bulb, you have reduced the strength of the connection.
With encapsulated keels the problem gets worse as you are cutting through the bottom of the encapsulation which is what holds in the ballast and then depending on secondary bonds to support your keel. You also potentially disturb the bond between the ballast keel and keel encapsulation which is quite difficult to repair.
In terms of seaworthiness, even if the boat ended up with the same stability, it would roll at a faster rate (smaller moment of inertia) and through a wider angle (because it looses the dampening affect of a deeper depth foil) with the keel depth reduced .
One minor point, asked if adding a bulb will make a boat too stiff. Stiffness generally refers to form stability rather than ballast stability and so adding a bulb may make a boat more stabile but not more stiff.
Regards
Jeff
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