In design, when the trim and compensation arrangements are being addressed, a procedure is required for sizing the tanks and assessing the amount of water in them, i.e. the weight of variable ballast. The method we now describe, which is widely used for the purpose, is to be read in conjunction with Figure A3.1.

1. (a) A standard condition of the submarine is defined, i.e. one with full crew, stores and fuel on board, with the boat in water of standard density 1.0275. A statement is drawn up of all the weights and locations of items which might vary during the course of a patrol, including buoyancy changes, especially those associated with extremes, e.g. maximum changes of seawater density between say 1.00 and 1.03 (or whatever is appropriate for the submarine under design).

2. (b) A graph is prepared with a horizontal axis for longitudinal moments and a vertical axis for forces, the origin representing the standard condition of the boat. The weight and buoyancy changes in the statement from (a) are plotted as a series of discrete points on the graph to the convention that reductions in weight forward are placed in the first quadrant. The aim is to cater for demanding combinations of weight and buoyancy changes which are sensible but nevertheless still serve to establish the boundary of extreme conditions in which the boat might find itself. Thus towards the end of a patrol a submarine, having consumed nearly all of its stores and fuel, might have to enter an area in which sea-water density is high; alternatively a submarine shortly after leaving base to go on patrol might have to enter water which is almost fresh. […]