Joe's point is correct, but there are some statements that are either stated poorly or are not correct. Hull form is part of the issue.
Weight distribution affects righting moment.
The above statement encompasses everything talked about. Weight distribution includes amount of weight, displacement, and hull form. Tender boats maintain their CB close to center relative to CG when they tip, stiff boats have a greater movement between the two as they tip. A keel is part of the hull form that affects this, it's part of the bottom shape. A catamaran's CB (Center of Buoyancy) moves way out to one side as soon as one hull lifts from the water, but it's center of gravity stays relatively centered on the boat. A narrow deep ball keeled boat doesn't move its center of buoyancy very far from the center of the hull when it heels, but its CG (Center of Gravity) swings way out with the keel as it tips.
These things are affected by the distribution of weight. The relative weight to volume formula of the various parts of the hull in and out of the water.
its shape displaces 175 liters of water when fully submerged, which works out to about 386 lbs. This is the amount of force the body of water is exerting on the board to keep it above the surface.
A surf board that weighs 10 lbs displaces 10 lbs of water to stay afloat. The buoyant force is 10 lbs. If its overall volume can displace 386 lbs, that's its load limit, the total weight, including the board, it can keep afloat. With all its volume below the surface, its buoyant force is 386 lbs.
The point, for righting force, is how that weight is distributed and how that distribution changes relative to buoyancy as the boat heels. Water or lead, it doesn't matter. Joe's initial point is valid because he's essentially right that if you removed the area of the centered water ballast tank from the inside of the hull, changing the hull form to what looks like a catamaran, the affect of righting moment isn't changed. But then, you are also changing the overall volume of the hull, reducing both volume and weight, as well as the hull form. They are all tied together. In the case of the posted cross section, once the boat tips far enough over, that any part of the ballast tank comes above the waterline, it will have an advantage over the same hull where that area has been opened up. The opened tunnel area will be above the water, and not add to the counter weight in conjunction with the shifting CB, while the one with the water tank will begin pulling against the heeling force on the opposite side of the Center of Gravity from the Center of Buoyancy.
I think our arguement here is in mostly how to communicate our ideas. I understand what Joe has been saying, I understand what others have been saying. Some are incomplete in the picture they draw and some have not started their points well enough to avoid confusing other, but most have not said anything that is wrong.
This is a long tangent to take from a post about choosing a trailersailer, but it is both a fascinating discussion and relevant to the initial question, since many trailersailers use water ballast. However, for a boost on a trailer, being able to empty a large water tank will allow for a lighter trailer, a smaller row vehicle and still provide the stiffness of some ballast.
I hope the OP has the patience to follow along and that everyone takes something away that they can use.
-Will (Dragonfly)
And that could explain why water ballasted boats like the hunters are tender at first then settle into a groove once healed, but I always assumed that was cause the ballast was up high rather than in a keel where it would be more effective.
