Swing Keel vs Swing Centerboard

Moderator: GreenLake

Swing Keel vs Swing Centerboard

Postby Breakin Wind » Fri Mar 15, 2013 11:13 pm

Greetings all,

I was looking on the web for something entirely different, but as happens sometimes I got sidetracked by a photo of a sailboat with a swing keel. I've seen pictures of keelboats, and from those I assume a keelboat to be a bluewater craft with a rigid deep keel, commonly weighted at the bottom?

So from that, I sort of extrapolated that a swing keel would also have a weighted base, but somehow it was articulated or levered to rotate or pivot as the keel swung up to make a more shallow draft. The picture I saw looked quite a bit like a Daysailer centerboard, maybe just bigger but it was a bigger boat.

So what defines a swing keel as opposed to a swing centerboard? or maybe I've been operating under a misimpression all along that there is a difference, other than the size of the boat?

Just curious... Thanks,
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Re: Swing Keel vs Swing Centerboard

Postby 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 ~ ~
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Re: Swing Keel vs Swing Centerboard

Postby kokko » Tue Mar 19, 2013 6:42 pm

I'm surprised to see you ask this question after I had you out on my Catalina
Surprise has a swing keel. 700 lbs that is raised by the winch just inside the cabin.
DS1 Truelove
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Re: Swing Keel vs Swing Centerboard

Postby GreenLake » Tue Mar 19, 2013 7:13 pm

Perhaps because he didn't see it on a photo?


:D :D
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Re: Swing Keel vs Swing Centerboard

Postby brucybaby » Wed Mar 20, 2013 12:15 pm

Image

The swing keel (hanging) of my 'former' Chrysler C-20. 'Twas a wonderous thing. 400 Lbs of iron (plus another 400 Lbs of lead in the shoal) kept this boat virtually capsize proof. But!!!, if the cable or the pin (that holds the 400LB hunk of iron) snaps, the iron slab bashes through the front of the shoal keel .... and you sink! And some of us might think that DS2 centerboards are a PIA to work on!!!! P'shaw, these are a whole 'nother ball game.
Bruce
'71 Oday DS2-Dashaway: Hull# 25873 Class# 4842
Ray Twp., MI
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