59405Re: Water Ballasted Chebacco
- Jan 11, 2009Someone suggested that I try the water balast calculation with the
boat as-designed, that is without rounding off the bottom. After
finding this post from a respected NA, Tad Roberts, on the Woodenboat
Water ballast is useful and does work, but only in very specific
circumstances. In the typical lead mine sailboat (and your physics
experiments above) that Bob is referring to, it would be useless.
Mr. Bolger, Hunter, and I believe Macgregor all use water ballast to
good effect. These are all hulls that are shallow, wide bottomed (in
Bolger's case rectangular), and lightweight. The water ballast is
contained in tanks that are the full width of the boat, but not very
high vertically. The rectangular or almost rectangular midsection
means these boats rely on form stability at low heel angles. As the
boat heels, the water ballast rises above the waterline very quickly,
and becomes working ballast. The barge hull form runs out of form
stability quickly, and this is were the ballast becomes effective.
But only at fairly high heel angles, probably 20 degrees or more.
Most of us aren't used to sailing at those high angles any more.
It enables a hull of very light trailering weight to also be self
righting (if she has either floation or a watertight cabin) from a
complete knockdown. Water ballast will also smooth out motion and
give a very light hull some momentum. In the case of the Macgregor it
enables a useable (and reasonably safe) sailboat, to dump her ballast
and plane off under outboard power. Another excellent feature is that
if your water ballasted boat is knocked down and fills, she will not
sink. Coupled with extremely shallow draft on launching, which can be
very handy, it makes sense.
As always, horse's for course's.
All the best, Tad.
The Chebacco, as designed, is just the form Tad suggests would
benefit from water ballast. I revised my Freeship model to put 500#
of water ballast in the 19' flat bottom boat and...
ta daa - it is much stiffer than my round bottom idea. At a 20
degree angle of heel (sailing to windward in a fresh breeze, the flat
bottom water ballasted boat has a righting moment of 1880 ft-lbs, 30%
better than the round bottom boat and 80% more than the unballasted
boat. Evan at 5 and 10 degrees the water ballasted boat has almost
double the moment of the stock boat. The extra 500# causes the boat
to sink down just 1.5" deeper.
The ballast tank is essentially a false bottom parallel to the
waterline and 5" deeep at its deepest point in the middle of the
boat. Sounds like an even better idea than the original post.
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