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Let us now consider this force DG as acting in the diagonal of the parallelogram DHGI. It will be equivalent to two forces, DH and DI, acting along the sides. One of these forces, DH, is in the direction of the keel, and the other, DI, at right angles to the length of the vessel, so as to urge it sideways. The form of the vessel is evidently such as to offer a great resistance to the latter force, and very little to the former. It consequently proceeds with considerable velocity in the direction DH of its keel, and makes way very slowly in the sideward direction DI. The latter effect is called lee-way.

From this explanation it will be easily understood, how a wind which is nearly opposed to the course of a vessel may, nevertheless, be made to impel it by the effect of sails. The angle BDV, formed by the sail and the direction of the keel, may be very oblique, as may also be the angle CDB formed by the direction of the wind and that of the sail. Therefore the angle CDV, made up of these two, and which is that formed by the direction of the wind and that of the keel, may be very oblique. In fig.15. the wind is nearly contrary to the direction of the keel, and yet there is an impelling force expressed by the line DH, the line CD expressing, as before, the whole force of the wind.