A sailing craft requires a water-tight, immersible vessel of some considerable volume. This is supplied to our natives by a hollowed-out log. Such a log might carry fairly heavy loads, for wood is light, and the hollowed space adds to its buoyancy.
Yet it possesses no lateral stability, as can easily be seen. A look at the diagrammatic section of a canoe Fig. I (1), shows that a weight with its centre of gravity in the middle, that is, distributed symmetrically, will not upset the equilibrium, but any load placed so as to produce a momentum of rotation (that is, a turning force) at the sides (as indicated by arrows at A or B) will cause the canoe to turn round and capsize.
If, however, as shown in Fig. I (2), another smaller, solid log (C) be attached to the dug-out, a greater stability is achieved, though not a symmetrical one. If we press down the one side of the canoe (A) this will cause the canoe to turn round a longitudinal axis, so that its other side (B) is raised, Fig. I (3). The log (C) will be lifted out of the water, and its weight will produce a momentum (turning force) proportional to the displacement, and the rest of the canoe will come to equilibrium. This momentum is represented in the diagram by the arrow R. Thus a great stability relative to any stress exercised upon A, will be achieved. A stress on B causes the log to be immersed, to which its buoyancy opposes a slight resistance. But it can easily be seen that the stability on this side is much smaller than on the other. This asymmetrical3 stability plays a great part in the technique of sailing. Thus, as we shall see, the canoe is always so sailed that its outrigger float (C) remains in the wind side. The pressure of the sail then lifts the canoe, so that A is pressed into the water, and B and C are lifted, a position in which they are extremely stable, and can stand great force of wind. Whereas the slightest breeze would cause the canoe to turn turtle, if it fell on the other side, and thus pressed B—C into the water.
Another look at Fig. I (2) and (3) will help us to realise that the stability of the canoe will depend upon (i) the volume, and especially the depth of the dug-out; (ii) the distance B—C between the dug-out and the log; (iii) the size of the log C.