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post #8 of Old 02-01-2013
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I'm new to all this, but I enjoy learning, so when I bought my Catalina 25 I started reading up on this a bit. I'm not an expert, but the following is my understanding. Like Ward said, each form of movable keel has its advantage and disadvantage. A swing keel, like on the Catalina 25, carries a good bit of the balast of the boat in the part that swings. In the Catalina 25, that's something like 1200 or 1500 lbs of balast that hangs down. When the keel is down, you move 1500 lbs (or some portion thereof) up to 5 feet from the boat. This significantly improves the boat's righting motion, and gives her characteristics more similar to a fin keel. Don't get me wrong, I'm not saying that it's "just like" a fin, but the boat tends to behave more like a fin than a shallow draft full keel. The down side is that you have to raise and lower 1500 lbs. And, if the winch, cable, turning ball, swage fittings, pivot bolt, or other part of the lifting mechanism breaks, you have 1200/1500 lbs of steel that will come crashing down and smack into the keel trunk, which in many cases results in the boat sinking, or at least rapidly taking on water. It is rare to see this happen, but it is an important to know about and monitor this as a failure mode.

With boats like the O'Day, you have a very light weight centerboard that swings. 65 lbs crashing into the keel trunk is going to do a lot less damage than 1200-1500 lbs. Of course, the down side is that the balast is moved much higher up in the boat compared to a fin or swing keel. If you think back to high school physics classes where you talked about leverage, moving the ballast closer to the center of rotation/center of gravity on the boat means it takes more balast to counteract the rotational forces imposed by the sails - that is, you need more balast to improve the boat's righting ability if you're not going to put the weight on a long fin keel. The swing centerboard still improves the boat's pointing ability, so it isn't a useless part of the boat, and the mechanisms to control the centerboard are simpler and eaiser to inspect and maintain.

Some boats use dagger boards. Though you typically see these on smaller boats like Sunfish, there are some very large boats that use these. The problem with a dagger is that it won't "kick up" when you run aground, where the swing keel and swing centerboard will do that.

Hope this helps!

- Jim
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