Equilibrium polygon

An equilibrium polygon is the submarine trim-balance diagram bounding the set of longitudinal trimming moments the ballast and trim tanks can produce across the range of variable-load conditions. Each tank’s fill range maps to a moment contribution; the vector sum of all combinations defines a closed region, and a load condition is controllable in trim only if its required moment falls inside the polygon. It is the standard tool for sizing trim and compensating tanks so the boat can hold level trim at any feasible loading and depth. The seed gloss describing a rudder-versus-drift maneuvering plot is incorrect; that is a steering diagram, not the equilibrium polygon.