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.
STRUCTURAL STRENGTH.
Considering a ship as floating in a state of rest in still water, the volume of displacement represents a weight of water equal to the weight of the ship. This equality, however, does not exist evenly throughout the length of the vessel, or for individual portions: thus, amidships the weight of water displaced by a given length—in other words, the buoyancy—is usually considerably in excess of the weight of that portion of the vessel and her contents. Similarly at the extremities the ‘weight’ of a certain length exceeds the ‘buoyancy.’ Between the part or parts of the vessel in which there is excess of buoyancy over weight, and the part or parts in which the weight exceeds the buoyancy, there must obviously be sections of the ship at which the two are equal, and these are termed “water borne” sections. A ship circumstanced as described is in a condition similar to that of a beam supported at the middle and loaded at each end. Such a beam tends to become curved, the ends dropping relatively to the middle, and the ends of the ship tend to drop similarly, the change of form being called “hogging.” On the other hand, if the excess of buoyancy occurred at the extremities and that of weight amidship, the ship would resemble a beam supported at the ends and loaded at the middle. In such a condition the middle would tend to drop relatively to the ends: a change of form called “sagging.”
These general principles are much more readily and safely applicable to ships while floating in ‘still water’ than to ships when at sea—the strains experienced then being necessarily the results of far more complex and severe influences. The existence of waves and their rapid motions relatively to that of the vessel, and the pitching, heaving, and other movements thus caused, increase the inequality of distribution of weight and buoyancy and affect more materially the strains brought upon vessels. Consideration of the problem, therefore, involves a study of waves, both as to their formation and action, and necessarily leads to a mode of treatment which cannot have accurate regard for particular cases. Variable influences of immense importance are also constituted by the state of loading in vessels for merchant service. For a uniform basis of comparison in these calculations such vessels are usually assumed as loaded with homogeneous cargo—i.e., cargoes of equal density throughout.
This fundamental element of relative ‘weight’ and ‘buoyancy’ having been indicated, the chief strains to which a ship is subjected may now be stated. This may be done with sufficient regard to general accuracy, under four heads:—[7]
(1) Strains tending to produce longitudinal bending—“hogging” or “sagging”—in the structure considered as a whole.
(2) Strains tending to alter the transverse form of a ship, i.e., to change the form of athwartship sections.
(3) Strains incidental to propulsion by steam or sails.
(4) Strains affecting particular parts of a ship, or “local strains”—tending to produce local damage or change of form independently of changes in the structure considered as a whole.