Some approximations from Edward L Attwood

Rick Robinson's "Spring Style" program produces a "tons-per-inch immersion" figure for a ship design. He must be using some approximation such as that shown on page 200 of Edward L. Attwood's book, War-Ships: A Text-Book on the Construction, Portection, Stability, Turning, Etc., of War Vessels. I am looking at a copy of the 6th Edition from 1917.

For ships with a fine waterplane, the approximation is:

tons per inch = (Length x Beam)/600

For fuller ships, such as a battleship, the approximation is:

tons per inch = (Length x Beam)/530

There is a table that compares the approximation with the real figure for four ship types, and they all are within 3% (usually better) of the correct figure. For wargame-oriented ship design, this is perfectly acceptable.