Shortcut to GM

The art of loading a ship so as to get the best stability possible is no longer something that depends on the feel and the rule-of-thumb calculations of the officer in charge of stowage. But all the guess has been taken out of it by a simple gadget which has been installed on a number of U.S. ships.

A ship's stability is measured by her GM--the height of her metacenter* (M) above her center of gravity (G); this distance determines a ship's ability to right itself after heeling over before waves, wind, or torpedoes.

The new device for calculating foolproof GM is called a Ralston Stability & Trim Indicator (Kenyon Instrument Co.). It is essentially an aluminum tray, engraved with a longitudinal cross section of the ship drawn to scale, which balances on a pair of knife edges. Weights, gauged to actual weight of cargo to be carried, are then placed over each hold on the diagram. The tray is then balanced by means of a sliding block, and the balance point is translated by graph directly into GM. Another pair of knife edges at right angles to the first can then be used to gauge the fore-&-aft trim of the ship in precisely the same fashion.

By trial & error the best stowage of a ship can be quickly figured out when she is loading in port. If she is torpedoed at sea the same gadget may help in determining the corrective measures needed to keep the ship from capsizing.

* The metacenter is the junction point of a ship's middle line and vertical lines through the centers of buoyancy at various angles of heel.

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