Re: water ballast
- --- In firstname.lastname@example.org, "David Romasco" <dromasco@g...> wrote:
> Gee, Nels; it would seem obvious to the scientific mind that whatyou need
> is something that meets all your needs at once, like foam ballast!It'll
> also work as flotation, too.I've
> Y'know, I used to think I knew stuff about boats, but the things
> learned in this group amaze me more every day!Me too!
> David Romasco
I believe you are familiar with some of John Welsford's designs?
Which ones of his use water ballest? I recall many years ago that
there were a series of trailer sailers from NZ that used water
ballest and named "Hartley Trailer Sailers" Do you recall those
designs. They were multi chine stitch and glue constrution as well.
Very advanced for their time.
Also Chuck Leinweber has built a Michalak design with water ballest
tanks and loves it.
Lots of choices for everyone.
I plan to use water ballest and additional lead in LESTAT - in the
form of jugs of fresh water and lead acid batteries. As well as
additional floatation/ballest using an air mattress.
- 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.