"Andy Champ" wrote in message
...
Keith wrote:
I always get confused in regards to 'centre of gravity' and 'polar
moment of inertia' when it comes to yachting.

As you seen to know what you're talking about - what's the difference?
In yachting terms ?

Centre of gravity (which is actually misnamed) is the point where you
could say the average mass of the boat is. Polar moment of inertia is
how far from that point the mass is. That's hard to explain - so wander
along to your local club, and borrow a laser mast for a minute. (Both
bits)

Put them together, and pick them up. Find the balance point, so you can
hold them in one hand. You hand is very close to the centre of gravity.

Now twirl them around. It's hard to get them going, and hard to make
them stop. That's because the polar moment of inertia is large, the
long thin shape moves most of the mass a long way from the CoG.

Take them apart, then tie them together with a couple of bits of string
or something so you have one short fat bit. (I think you might actually
be able to put the top mast inside the lower upside down).

Pick it up again, and twirl. It'll be a lot easier to twirl now the
mass isn't so far from the CoG.

Things are much more complicated with hulls than masts, but the
principle is there.

Brilliant explanation Andy, many thanks!
TonyB