In this arrangement it is obvious that only the "roll" and not the "pitch" of the vessel can be utilised as the medium through which to obtain propulsive force. But it is probable that fully eighty per cent. of the movements of a vessel during a long voyage--as indicated, say, by the direction and sweep of its mast-heads--consists of the roll. Each ton of goods moved through a vertical distance of one foot in relation to the hull of the vessel, has in it the potentiality of developing, when fourteen or fifteen movements occur per minute, about one horse-power. A cradle containing 200 tons, as may therefore be imagined, can be made to afford very material assistance in helping forward a sailing ship during a calm. In such tantalising weather the "ground-swell" of the ocean usually carries past a becalmed vessel more waste energy than is ever utilised by its sails in the briskest and most propitious breeze.
For sailing ships especially, the rocking form of wave-motor as an aid to propulsion will be recommended on account of the fact that when the weather is "on the beam" both of its sources of power can be kept in full use. The sailing vessel must tack at any rate with the object of giving its sail power a fair chance, and thus, when it has not a fair "wind that follows free," it must always seek to get the breeze on its beam, and therefore usually the swell must be taking it sideways. It would be only on rare occasions that a sailing vessel, if furnished with rocking gear for using the wave-power, would be set to go nearer to the teeth of the wind than she would under present conditions of using sail-power alone. The advantage of the wave-power, however, would be seen mainly during the calm and desultory weather which has virtually been the means of forcing sail-power to resign its supremacy to steam.
For checking the rocker in time of heavy weather special appliances are necessary, which, of course, must be easily operated from the deck. Wedge-shaped pieces with rails attached may be driven down by screws upon the sides of the vessel, thus having the effect of gradually narrowing the amplitude of the rocking motion until a condition of stability with reference to the hull has been attained.