Originally Posted by PCP
No. He certainly can explain better but he was talking about two boats with the same displacement, an heavy one and a light one and that means that the light one to have the same displacement has to be considerably bigger.
Regarding two boats of the same length and beam, one heavy and other lighter, assuming that both are well designed it does not work the same way.
The heavier one will have a lot more ballast (assuming that both have similar keels and the same ballast ratio) and if you are correct in saying that the lighter boat can take more weight without sinking it is also true that the same weight will have a bigger influence in raising the boat CG making it more unstable at considerable angles of heel diminishing in much the AVS.
In short, the load will represent a much bigger proportion regarding the ballast of the lighter boat than to the ballast of the heavier one. If the load is excessive, even if properly stored it will only increase stability at smaller angles of heel making the boat dangerous if by any reason it heels considerably.
That's why each NA gives for each boat a max load one that will allow it to have a non dangerous AVS.
Originally Posted by SloopJonB
The only factor which affects how a boat will sink from adding weight (stores, gear etc.) is the Pounds Per Inch Immersion. It is dependent on the waterplane area, not the displacement of the boat. A heavy steel boat and a lightweight cored boat, each with 1500 Lbs per inch immersion will both sink at the same rate from additional weight being added.
Boy, I did one hell of a job screwing that up! Ha Ha!