A.--By comparing the actual speed of the vessel with the speed due to the pitch and number of revolutions of the screw, or, what is the same thing, the speed which the vessel would attain if the screw worked in a solid nut. The difference between the actual speed and this hypothetical speed, is the slip.
567. Q.--In well formed screw propellers what is the amount of slip found to be?
A.--If the screw be properly proportioned to the resistance that the vessel has to overcome, the slip will not be more than 10 per cent., but in some cases it amounts to 30 per cent., or even more than this. In other cases, however, the slip is nothing at all, and even less than nothing; or, in other words the vessel passes through the water with a greater velocity than if the screw were working in a solid nut.
568. Q.--Then it must be by the aid of the wind or some other extraneous force?
A.--No; by the action of the screw alone.
569. Q.--But how is such a result possible?
A.--It appears to be mainly owing to the centrifugal action of the screw, which interposes a film or wedge of water between the screw itself and the water on which the screw reacts. This negative slip, as it is called, chiefly occurs when the pitch of the screw is less than its diameter, and when, consequently, the velocity of rotation is greater than if a coarser pitch had been employed. There is, moreover, in all vessels passing through the water with any considerable velocity, a current of water following the vessel, in which current, in the case of a screw vessel, the screw will revolve; and in certain cases the phenomenon of negative slip may be imputable in part to the existence of this current.
570. Q.--Is the screw propeller as effectual an instrument of propulsion as the radial or feathering paddle?
A.--In all cases of deep immersion it appears to be quite as effectual as the radial paddle, indeed, more so; but it is scarcely as effectual as the feathering paddle, with any amount of immersion, and scarcely as effectual as the common paddle in the case of light immersions.