General Concepts of Ballasting

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  • Pub No.: 5638
/ How much ballast to use and where.

(Includes using concrete as ballast)

The principal function of ballast in a boat is to increase its stability by means of the increased weight. For the underlying principles and theory of stability a standard text on design should be referred to. The three greatest factors influencing the stability of a boat are: (refer to figure 1).
1.    The vertical position of the center of buoyancy (B).
2.    The vertical position of the center of gravity. (G).
3.    A point called the meta center (M).
    These various points are determined by design calculations and are too lengthy to explain here. The point G will always fall above the center of buoyancy in a motor boat and it is only the resistance of the waterline plane to capsizing that keeps the boat in an upright position. Ballasting a boat or keeping the weights low will lower the point G and increase the distance between the points GM; and increase in the bdam of the boat raises the point M and increase the distance GM, thus improving the stability. In this case the ratio of the distance between BM and GM approaches unity. The stability increases or decreases as the cube of the beam dimension. So it is readily apparent that an increase in the beam or an increase in the waterline plane has a beneficial influence on stability.

16 pages, 2 plate(s)

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