Re: water ballast
- Nels, consider, in an inverted craft with water ballast tanks awash
while it is certainly true that the ballast is contributing little or
nothing towards righting the craft, it is also certain that it is
contributing nothing towards actually sinking the craft either.
If, however, the same weight of water ballast was replaced with lead,
and considering that the craft is barely afloat in the first place,
the craft would now sink. The denser lead would displace less water,
and down she would go.
If a designer didn't want that scenario to happen then the designer
would have to find space for flotation inside the hull anyway - and
lots of it - to offset that lead. Whereas the water-ballasted design
is already not sinking, and only needs enough flotation high up to
make the boat unstable when inverted.
So, using lead as ballast because it takes up less space is a great
argument if you don't mind sinking. But personally, afloat in the big
wide water with all that can go wrong I'd rather have a boat that
will stay with me when disaster strikes, no matter how crippled she
may be, than a SINKER that will give up the ghost, abandon me, and
disappear under the waves....
Which reminds me of a report I saw recently, in Latitude 38, of a
pinky schooner that did just that recently in the South Seas. Filled
with lead ballast down she went. No storm. No reef. No collision. No
floating containers in mid-ocean. Just a light fog (as I recall).
No, this sinker caught her big keel in a fisherman's trawling net and
got pulled sideways enough to flood. It took two or three minutes,
and she was gone. Her skipper, sailing solo, had just enough time to
take to the inflatable....
--- In email@example.com, "Nels" <arvent@h...> wrote:
> There is another disadvantage of a boat with water ballest tanks.
> it becomes inverted and fills with water, it can be almost
> for the occupants to return it upright as they would have to be
> strong enough to lift the weight of the water filled ballest tank
> above the water and over 180 degrees to get the boat back on it's
> feet. Not easy to do with 500 pounds of water while you are
> in the water. Your only option is likely to drain the tank which
> be impossible when outside the boat and in the water. It happened
> one of Bolger's designs apparently.
> "A couple years ago, one of my water-ballested designs met with an
> accident that completely flooded her. The wood structure had
> bouyancy, so she didn't sink, but she floated bottom up with the
> outside ballest tanks awash. Some foam high up in the hull would
> righted her and saved some inconvenience,even danger in cold
> BWAOM page 245
> So in this case I would rather be in a boat without water ballest
> tanks. Also without the tanks, it would allow more options to load
> down with a more effective ballest than water. i.e. lead, rocks,
> guys filled with beer.
- 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 firstname.lastname@example.org, "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.