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posted to alt.sailing.asa
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Any thoughts onhow to make this boat better
Jax?
"Dave Doe" wrote in message
. nz...
In article ,
says...
Are you saying that the water ballast inside a boat hull
does not affect it's stability? That the water ballast
"weighs nothing" until it is above the waterline?
Dave Doe wrote:
Nope, never did. I'm saying it's stability is indeed
improved (more
mass to move)
That would be an issue of inertia... dynamic stability, if
you like the term.
ie stability.
... but question its righting ability vs lead keel - and
posed a question about that.
Well, I don't think that anybody has claimed that water
ballast provides equal righting moment to external lead
ballast. But it can provide significant righting moment,
especially if the boat is designed from the start to utilize
water ballast effectively.
Provide some evidence.
The problem is that water in the ballast has the same
density of the
water it is in
Well there you go again. The water ballast is not "in water"
it is in the boat.
Where is the boat?
Will no amount of repetition get this
point across? You used the example of an empty jug vs one
1/2 full of water... is the water in the jug "in water" or
is it in the jug?
Let me propose this example- a cooler full of ice & beer is
lighter than water (hence lighter than water ballast) yet
can be pretty heavy when you're carrying it down the dock.
If you put it in the lowest possible location in the boat,
right down against the hull, and tie it in securely (to the
handles, so you can still open the lid of course), will this
improve the boat's stability?
What is your point? Put feathers in the bottom of your boat?
Or put
water? Or go deeper 1and put lead? I think you need to
consider the
basics when considering a sailing vessel; water on or about the
centerline is a waste of potential that is even more easily
achieved
otherwise. It is the same density of the stuff you're moving
through -
it's a very major factor, as you're floating on it. Consider
the
obvious. When the boat is on an angle the only force the water
can
exert is on the air below it and not the water (it exerts no
effective
force on the water if you can get your head around that). And
that is
only because it is held there in it's ballast tank and not
allowed to
'seek it's natural place at the bottom' (it's heavier than the
surrounding air - but please don't dissregard the whole
equation - what
the boat is in).
Consider the absurd, a boat of no mass other than its water
ballast. It
will sit in the water, level with the ballast waterline. If you
are to
heel it - well you work it out. The maths is easy, consider
the water
ballast as a "solid" (as it cannot move).
Then consider the same mass many times denser at a point well
beyond the
fulcrum point the water ballast is on.
Also think of boat speed and drag (wetted area).
Short of moving magically moving the water ballast from one
side to the
other - same as the tack - water ballast is a crock.
It's a lot heavier than air, but look at "what you're doing"
and the
alternatives.
Come on - do my maths (example question posed already, no
answers yet)
--
Duncan
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