by GreenLake » Fri Mar 15, 2013 11:56 pm
Both a swinging keel and a centerboard can be realized as flat plates pivoting around some bolt or axle. Either one could also be profiled to some degree.
The CB is just what the name says, a board. If there's a weight in the CB it may mainly serve to keep the CB down, rather than to act as ballast.
A very heavy plate that substantially adds to the ballast of the boat would be something I'd call a swing keel. (A CB with moderate, but still substantial weight added might be known as a weighted CB rather than a swing keel.) For safety and other reasons, swing keels usually do not form the sole ballast for a boat. It may not be practical to make the full ballast movable (the task of raising the keel would become more difficult).
There are two main considerations behind a swing keel, one is the ease of trailering (lower CG and easier slipping) and other is the ability to sail in shallow waters including the ability to dry out at anchor. One common design for a swing keel is to have a shallow keel stump into which the swinging keel folds. That way, there's no CB trunk in the cabin, but the keel is limited to a narrow blade.
For reasons of geometry, having a keel bomb doesn't work well with swinging keels, however, it's certainly been tried for lifting keels - which are the parallel to a dagger board.
Now, there's a big difference between a swinging keel and a canting keel. The latter swings sideways, and usually does have a keel bomb. By swinging sideways, the canting keel can exert more righting moment with less heel, maximally so if the keel is swung to, or even just above the water's surface. At that point, there no longer is a contribution from the keel to prevent leeway, hence the need to add dagger boards....
~ green ~ lake ~ ~