Water Ballast

David Kautz (david_kautz@hp.com)
Tue, 16 Mar 1999 14:39:49 -0800


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West Wight Potter Website at URL
http://www.lesbois.com/wwpotter/
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I want to put on my "Dr. Science" hat and rant and rave for a moment, if
you folks will indulge me, please.

There was a comment posted earlier by a happy new P-19 owner (good
choice, by the way) who remarked, as many people do, that water ballast
is not effective until it comes up above the waterline. I've heard this
so many times now - I've seen it in an article on a MacGregor focussed
website - and I can't stand it any longer, I've got to speak up. Whew!

Anyone who has tried to sail, or even tow, a swamped boat will tell you
that water below the waterline definitly does not become weightless.
Righting moment (the boat's desire to float with it's mast pointing
straight up) is all about the relationship between the center of
buoyancy and the center of mass (also know as center of gravity or
"cg"). As a boat heels, the center of buoyancy moves to leeward and,
hopfully, the center of mass moves to windward (on a P-15, the occupants
are most of the ballast, hence the "hopefully"). Most water ballast
boats flood a compartment beneath the floor and it has the same effect
as if you put similar weight of concrete or lead there.

One of the disadvantages of any kind of inside ballast, including water,
is that the distance from the center of buoyancy is short and there
isn't much "leverage" to right the boat. To make is effective, you've
got to use LOTS of weight since it doesn't have good mechanical
advantage. Picture this: A boat (kinda like a P-19) knocked down on it's
side with 400 lbs. of ballast 3 ft. away horizontally from the center of
buoyancy will have a righting moment of 1200 ft-lbs trying to turn it
vertical again. If you were using interior ballast, your ballast might
only be 1 ft. away from the center of buoyancy, you would then need 1200
lbs of ballast to get the same righting moment as our previous example.

The disadvantage to this is that you carry that all that ballast with
you on every point of sail - it can make the boat smoother in chop but
less responsive and slower in light winds. The increase in weight
increases displacement and wetted area slowing the boat down. The boats
still feel tender because designers try to keep the weight as low as
they can get away with for decent performance and still maintain
self-righting. And since water is not particularly dense, and you need
so much weight, it takes up a lot of room. The headroom in a fixed keel
Catalina 250 is substantially greater than that of the water ballasted
C250.

The advantage of water ballast is that you get to leave all this weight
at the ramp. A larger boat becomes practical to trailer since it won't
have the weight of swing keel, ballasted daggerboard or shoal draft
keel. The swing keel ALONE in a Catalina 22 weighs 75 lbs. MORE than an
entire Potter 15.

In summary, unless your hull has a hole in it, the water outside knows
nothing about the water inside. The problem with water ballast is not so
much WHAT it is but WHERE it is.

End of tirade - Thanks for listening, I feel better now......

Dave Kautz
P-15 #1632 Tilly Lucy
Palo Alto, CA