Re: water ballast
- --- In firstname.lastname@example.org, "Gene T." <goldranger02-boats@y...>
>In the article where I read about this the water ballest tanks were
> This confuses me! Water trapped up at the highest
> point of the hull, where the water ballast tanks are
> in this inverted boat doesn't seem like an impediment
> to rolling back over. The "obvious" logic escapes me!
> Gene T.
divided into two areas - one in each bilge. The single crew could not
right the boat until he got into water shallow enough where he could
lift the weight of the near tank high enough out of the water to flip
it over. While in the water he could not get enough leverage to do so
no matter what he tried. The most effective way was bearing down on
the near leeboard - using it as a lever. If the tanks had been empty
he could have done it easily is what he claimed. The biggest drawback
in this particular design however, was deemed to be the large cockpit
that was not self-draining once it swamped. The water was coming in
faster than going out - so the weight of the water in the cockpit
plus the water in the ballest tanks and the force of the wind on the
furled sail, caused the boat to roll over. The guy was sleeping in
the cuddy and when he opened the hatch, it filled with water as well.
(The boat was designed as a "coastal cruiser" and got hit with a
thunderstorm while lying offshore to get away from the bugs and
At any rate, that convinced me to follow what Bolger recommends when
it comes to seaworthiness - and that is - a boat that can recover
from a knockdown on it's own.
I live where the water is cold most of the year, and we get some
pretty nasty thunderstorms.
- Free surface effect? I don't follow. Nor did I say anything about
water ballast 'sloshing around in the bilge,' because you're right,
sloshing water does nothing for righting moment - but neither does
water ballast in a bilge filled with sloshing water. picture those
plastic jugs of water 'ballast' tied with string to a frame member in
the bilge of a skiff half filled with water. The jugs meader around
at neutral bounancy (disregarding the jug itself) contributing zero
to stability until the boat is heeled enough to bring a jug out of
But in a dry bilge where the water in the jugs no longer has to
displace it's own volume in water before it acts to weigh down the
hull the jug is continually pressing against the bottom of the hull,
weighing the boat down, adding to stability.
I think your statement that it depends on how you "draw the envelope"
is dead on. In a given boat, whether ballast in put in the boat or
tacked on outside the boat has the potential to make a huge
difference depending on the density of the ballast, and can actually
contribute nothing to stability (water) or even give negative
stabilty (foam) because tacking on ballast outside a given hull in
effect adds to the overall volume of the hull. Placeing the same
ballast inside the hull makes the boat float deeper, but doesn't
change the shape of the boat - no matter what the displacement of the
ballast itself - thus potentially adding to stability depending where
it is placed.
On a boat STILL ON PAPER, the distinctions between outside and inside
water ballast can be less clear depending on the priorities of the
designer, because one way or the other room has to be found for the
ballast. If the boat has to be given more volume in order to add
ballast without compromising other priorities (like headroom) then
the distinction between 'inside' and 'outside' ballast - as far as
the designer is concerned - becomes fuzzy - because if to make room
for the ballast the designer has to deepen the keel or harden the
bilge - thus changing the overall volume - the designer is, in
effect, tacking on the ballast 'outside' despite the fact that the
builder will actually be placing the ballast 'inside.'
--- In email@example.com, "John Bell" <smallboatdesigner@m...>
> Free surface effect. Water sloshing around in the bilge doesnothing for
> righting moment. Bad example. ....and
> ....Basically it all depends on how you draw the envelope.
> ----- Original Message -----
> From: "oneillparker" <jboatguy@c...>
> | If you put the water ballast back in the boat, and then flood the
> | boat enough to cover the ballast, measure, then take the jugs out
> | measure again, the amount of force required to tip the boat agiven
> | amount will be equal.