Weight, not mass
I believe it is the same thing my mate.
Without going into an argument let me put this straight. The sketch delmarrey so kindly supplied shows exactly what I said. Notice that the COB moved, not COG. That is fixed, regardless of how the boat was loaded, providing nothing moves. Something else that changes in heel is the meta centre (GM).
Quoted by Jeff "the CG will move vertically and horizonatally relative to the center of bouyancy or to the water even if it does not move relative the boat itself. Typically hard chine boats 'roll up' meaning that the center of gravity rises relative to the surface of the water as the boat heels, while round bottom and vee bottom boats tend to 'roll down' as they heel."
In all respect Jeff, you have it the otherway round. COB always move relative to the COG. Example: if a boat is badly loaded, the boat will trim itself when the COB lined up with COG.
OTOH, a fairly common way some designers trim a hull (me included) is when we have done weight/mass calculations and the basic hydrostatics, and we found that the longitudinal COG and COB are out of alignment, we trim the hull. I usually move the keel with ballast as this is the most effective way to trim until the CB and CG is aligned.
While we are on this issue of COG and without highjacking this interesting thread the following.
There is also a misconception that with twin keelers, when heeled, the weather
side keel will pull down and increases the Righting moment. Nothing is further from the truth.....When the boat was trimmed statically by the designer, the CG of every thing on the boat, hull and deck included, was proccessed by the designer - on a vertical and horisontal plane. The keel are included.
Thus, as the twin keeler (and any other boat) heels, she pivots on the COG and only the COB changes due to the change of shape of the hull in the water as it heels. (Refer to Delmarrey's sketch.)
Yes, a round bilge hull is more tender
than a chined hull due to the shape of hull.