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 TM 55-1905-223-SDC
b. Thus, if the ship in the example is in a seaway and rolls to, 70 degrees, for example, it will come
back to the upright. But if the extreme angle of a roll exceeds 71 degrees, the ship will capsize, because
upsetting moments develop beyond this angle.
the metacentric height.
a. This distance is a measure of how much B moves when the ship inclines through an angle of heel,
-Θ. In the triangle GZM, FIGURE 5-1, the righting arm GZ will always be proportional to GM. This holds true
as long as M remains on the ship's centerline; i.e., from 0 to about 7. Therefore, GM is a measure of initial
stability for any given displacement, W. Unless otherwise specified, further reference to metacentric height will
refer to initial GM.
b. The point M does not come into being until the ship heels, but any slight inclination produces a
movement of B and consequently a metacenter is established. Note that the GM produced by a slight heel in
an upright position is a definite value which remains relatively constant up to about 7of heel.
c. Since at small angles of heel large values of GM indicate large righting arms, ships with large GM
are stiff and resist roll. Small values of GM reflect slowly developed righting arms, and these ships are tender
and roll slowly. Ships with very small GM are apt to hang at the extreme angle of a roll before starting back.
d. A study of FIGURE 5-3 will indicate that when G is below M, GM is positive and righting arms
develop, whereas when G is above M, GM is negative and upsetting arms develop. Thus, GM is an indicator
of whether initial stability is positive or negative.
5-3
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