On Mon, 20 Oct 2003 18:02:00 +0000, DSK wrote:

Lloyd Sumpter wrote:


Tell me, is your boat's stability increased by filling a fuel tank
down low in the hull? Fuel is definitely lighter than water. How
about a big cooler full of ice & beer?


Compared to AIR, yes. Compared to lead, no. I'd show you the
mathematics if you feed me beer.

Math, huh? Do you do numbers any better than you do logic?

So... because lead ballast is better, as ballast, does that mean water
cannot function AT ALL as ballast?

No. Of course not.



Yes, it does.

(actually, it's physics, not math, but you owe me a beer anyway)

Draw a diagram if you like. Now, the keel imparts a righting moment on the
boat because it exerts a downward force offset (in the x-dir) from the
center of rotation by the distance from the CofR (ie how deep the keel is)
and the angle of heel. Problem is, a water-filled keel does not sink and
therefore exerts NO downward force in water! (actually it does sink a bit
because the fibreglass the keel is made from sinks. You'd be better off
with a solid fibreglass keel...) Remember the water-filled bottle?

People think that because water is "heavy" in air means that it's also
"heavy" (ie exerts a significant downward force) in water. This is simply
not the case. Imagine this: get a boat with no keel and a mast. Put a rope
on the mast and pull on it, giving the boat heel. Now, put a boom out the
upward side with a bucket filled with water. As long as the bucket is in
air, it exerts a righting moment against the rope. But when it's in the
water, the only righting moment from the bucket is from the bucket itself
- the water exerts no righting moment whatsoever.

Now, having a water-filled keel that exerts virtually no downward force is
still preferable to an air-filled keel, which exerts an UPWARD force.
Also, water in the keel will help the boat's "stability" in that it
increases the overall mass (F=ma) so movement is dampened.

But "ballast"? No.

Lloyd Sumpter
"Far Cove" Catalina 36