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.
Figure I—Diagram showing in transversal section some principles of canoe stability and construction.
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 asymmetrical[3] 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. The greater all these three magnitudes are, the greater the stability of the canoes. A shallow canoe, without much freeboard, will be easily forced into the water; moreover, if sailed in rough weather, waves will break over it, and fill it with water.
(i) The volume of the dug-out log naturally depends upon the length, and thickness of the log. Fairly stable canoes are made of simply scooped-out logs. There are limits, however, to the capacity of these, which are very soon reached. But by building out the side, by adding one or several planks to them, as shown in [Figure I (4)] the volume and the depth can be greatly increased without much increase in weight. So that such a canoe has a good deal of freeboard to prevent water from breaking in. The longitudinal boards in Kiriwinian canoes are closed in at each end by transversal prow-boards, which are also carved with more or less perfection (see Plates [XXIV c], [XLVII]).
(ii) The greater the distance B—C between dug-out and outrigger float, the greater the stability of the canoe. Since the momentum of rotation is the product of B—C ([Fig. I]), and the weight of the log C, it is clear, therefore, that the greater the distance, the greater will be the momentum. Too great a distance, however, would interfere with the wieldiness of the canoe. Any force acting on the log would easily tip the canoe, and as the natives, in order to manage the craft, have to walk upon the outrigger, the distance B—C must not be too great. In the Trobriands the distance B—C is about one-quarter, or less, of the total length of the canoe. In the big, sea-going canoes, it is always covered with a platform. In certain other districts, the distance is much bigger, and the canoes have another type of rigging.
Figure II—Diagrammatic sections of the three types of Trobriand Canoe.
(1) Kewo’u (2) Kalipoulo (3) Masawa