RESISTANCE POWER AND SPEED.
A ship, in moving through the water, experiences resistance due to a combination of causes, which combination, according to modern accepted theory, is made up of three principal elements.
1st—“Frictional” or “skin friction” resistance, due to the particles of water rubbing against the ship’s hull;
2nd—“Eddy-making” resistance, due to local disturbances or eddies amongst the particles of water—almost wholly at stern of ship;
3rd—Surface disturbance of the water by the passage of the ship, resulting in the creation and maintenance of waves: known as “wave-making” resistance.
The conditions which govern each of these elements, and their relative importance, may be generally indicated.
Surface-friction resistance, especially for vessels moving at moderate or slow speeds, is much greater than the resistance due to other causes—that is if the hull is ordinarily well formed. Its amount depends upon the area of the immersed surface, upon its length, upon its degree of roughness, and upon the velocity with which the water glides over it—i.e., upon the speed of the vessel.
Eddy-making resistance only acquires importance in exceptional cases, e.g., in ships having unusually full sterns. In ordinary well-formed ships it is of small amount, and is caused mainly by blunt projections such as shaft tubes, propeller brackets, and stern-posts.
Wave-making resistance is much more variable than surface-friction resistance. Its amount depends on the form and proportions of vessels, and on the speed at which they move: being greatest, of course, in ships of full form and in those moving at high speeds.
The sum of these three main elements of resistance constitutes the total resistance experienced by a vessel if “towed” through the water, that is, the resistance considered apart from the action or influence of the propelling instrument. In the case of a steamship, however, propelled by a screw or paddle-wheels, the resistance is augmented, more or less considerably, according to the form, surface, and disposition of the propelling instrument.
By the employment of various formulæ deduced by scientific authorities from theory and experiment, an approximation can be made before-hand to the total resistance of a proposed vessel, and from this an estimate of the power required to drive her at a certain speed. Moreover, through the law of comparison propounded by Mr Froude, the resistance of a ship can at all times be deduced with fair accuracy from the resistance of her model, certain corrections well determined by experiment having to be made.
The power of marine engines is expressed either in “nominal” or “indicated” horse-power. Nominal horse-power is a term practically obsolete so far as being a measure of the efficiency of engines, and only exists as a conventional method of commercially measuring the sizes of engines. Indicated horse-power measures the work done by the steam in the cylinders during a unit of time, and 33,000 units of work per minute, or 550 units of work per second, constitute one horse-power. The effective mean pressure of the steam is ascertained from diagrams drawn by means of the instrument known as the “Steam Engine Indicator,” and hence the term “indicated” horse-power.
The development by a vessel’s engines of the power requisite to drive her at a certain speed is always very considerably more than the power required simply to overcome her total resistance at that speed. This excess of power developed over power usefully employed in overcoming resistance is known as “waste work.” It amounts in many cases to as much as from 50 to 60 per cent. of the gross indicated power, and it is absorbed mainly as follows:—In overcoming frictional and other resistances of the engines and shafting, working air pumps, &c., and in overcoming the frictional and edgeways resistance of the propeller. The residue of power usefully employed is known as the ‘effective’ horse-power. The respective causes of ‘waste’ and their relative amounts are problems constantly demanding solution. Progressive speed trials with actual vessels and experiments with small scale models are daily contributing to their solution, and to some extent to their reduction.