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CHAPTER XXI.

The Wave-Line System of Construction and Its Advantages.

Ir requires a man of fully as much wisdom and knowledge of his profession to turn the wave principle to account, and build a ship, promising every good quality it can give, as to build a vessel of traditional form. All it does is to enable an accomplished naval architect to combine with certainty the properties of high speed, small resistance, economical transport, and sea-going qualities, under circumstances where formerly it was guesswork merely.

Practical constructors should alter their system with great caution. It is quite possible to understand the wave principle and yet to design a bad ship. Knowledge of the wave principle does not supersede the knowledge of other principles of naval construction; it merely adds to their number.

There is hardly a vessel in the naval or mercantile marine in the world at the present day, possessing high speed with a moderate consumption of fuel, which does not possess a number of the characteristics of the "wave" principle.

The following are the main points of practical construction determined by the "wave" principle.

1. The entrance of a Ship designed on the Wave Principle may have a hollow water-line.

With a hollow water-line, one can obtain many qualities irreconcilable with a convex bow-line. In the materials of wood, the structure is much more easy and stronger with a hollow than a convex bow water-line; and in any structure the hollow line has the virtue of diminishing the room for carrying weights in those parts of a ship where it is injurious to sea-going qualities to carry much weight. In releasing us from the dominion of the convex bluff bow, the wave-principle has left us free from much that tended to bad ships and slow ones. To carry weights near the middle, and relieve the ends, is to give a ship some of the best qualities we are able to bestow.

2. The run of a ship may have a convex water-line.

The same maxims which used to prescribe a full bluff bow,

prescribed with equal force the long, fine run of the water-lines of the stern. "Cod's Head and Mackerel Tail," was the motto of the "old school," which happily now retains few disciples. The importance of this maxim was, however, not founded on fancy merely, but on a practical wish to improve the steering of the bluff-bowed ships. It is now most certain, that bluff, full bows have a great tendency to make vessels, forced through the water with great power, steer very wild, and obey their rudder inefficiently. It was to counteract this fault of the bluff bow that the extremely fine run was contrived.

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The fault of the extremely fine run was not merely that it sacrificed a great deal of the excellent stowage of the ship in a place where it was much wanted, by making the whole of the after-body meagre and thin, but that it failed to cure the fault of steering wildly under heavy press of sail or steam. The wave principle provides for a fine run, and admits of it; but it does so in the right manner and at the right place. It approves of fineness of water-line aft, and gives as much deadwood before the rudder as the old school could desire. But it shows that the place where the fineness should lie is below the surface, deep down, and not higher up. It shows that fineness below should be well aft, and not where capacity is wanted.

Fulness for capacity, it gives in the after-body, well up towards the surface of the water, and gives a large, capacious, upper after-body, exactly in the place where room in a ship is valuable—valuable in money, valuable in convenient stowage, and valuable in reference to those movements of a vessel which test the excellence of her performance in a heavy sea.

3. The entrance of a ship designed on the wave principle may be as long as the run, and even longer.—This is also the contrary of the maxims of naval architecture of the old school. That the run must be long and fine, and the bow comparatively short and full, was nearly the universal system adopted. The question left open to opinion and discussion, was merely whether the length of the after-body should be longer than that of the fore-body in some one proportion rather than another. I have seen ships

designed with nearly every variety of practice in this respect, some in a proportion of 2 to 1, others 3 to 2, others 4 to 3, and others 5 to 4. Nearly every builder had his own special proportion for this purpose.

The wave principle releases us from this maxim also: The

bow may be made as long as the stern of a ship. This proportion has the advantage of enabling the designer to obtain balance of weights in the varying circumstances of lading and draft of water more easily than with an excess of length at one end.

There is a class of vessels which require this exact balance; and especially where it is reckoned important that a vessel should navigate either end foremost, such an arrangement is very useful. The wave principle leaves us free to adopt this arrangement. I have found it useful to design a number of vessels in this manner, with the hollow wave-lines at both ends of equal length and width. And further, the entrance of a ship may be made longer than the run. This is especially valuable where the length is very limited, so that it is difficult to obtain by any means a fine hollow bow. By giving greater length to the bow than to the run, à long fine water-line of entrance may be obtained, with its widest part nearer the stern, and its length therefore exceeding half the length of the ship. To gain capacity, the lines of the after-body may be made extremely full above, and may be fine only below. In circumstances of very limited length, this treatment may be turned to great account; especially in building the smaller class of sailing vessels, yachts and steamers, where good speed is wanted under restricted dimensions.

4. The main breadth of a ship may, therefore, be placed nearer the stern than the bow of a ship. This is almost an evident consequence of what precedes. The chief water-line of entrance being made longer than the run, naturally throws the main midship-section further back than the middle. This, however, may be only partially true; because it by no means follows that the greatest breadth shall be necessarily found in the same position in all the water-lines; it may be further forward in the upper water-lines, and further aft in the lower, or the contrary. I have often seen it expedient to place the greatest breadth well abaft the middle in the upper water-lines of a ship, and well forward of the middle in the lower water-lines of the same ship. This expedient will be found especially useful in forming designs for speed upon dimensions that are much restricted. This distribution of main breadths, fore and aft the middle, is indicated clearly by the wave principle; and I am not aware that it is to be found in any other system of scientific construction.

5. The foregoing maxims release us from the trammels of previous systems; and so enable us to give to a vessel forms

which may suit the specific objects we may wish to attain. This alone is a great boon to the shipbuilder, who wishes power to adapt the shape of his vessel to the various uses of nautical and mechanical art.

The maxims which follow do much more than release us from impediments. They enable us to accomplish definite objects of practical value in an exact method. One of the most important practical qualities of a ship is that which enables her to carry her profitable load at least cost. Cost in a ship, propelled by steam. or other power, has, as one of its main elements, the resistance which the water opposes to the passage of the ship. This resistance is little for slow velocities, and very great for high velocities; increasing generally as fast as the square of the velocity, and in many shapes of vessel much faster.

It has always been of great value to know how to give a vessel, of given length and breadth, such lines as should enable her to divide the water in the easiest way, so as to experience least resistance from the water moved by the ship out of her path. But now, and especially of late years, the demand for high velocities in vessels propelled by steam, has given this problem of the form of. least resistance paramount importance in naval architecture, especially with reference to ships of war.

The problem is frequently presented in this shape: A vessel is to be built of a given breadth, of a given length of bow, and a given length of run; and the question is, what kind of lines will give this vessel the power to divide the water so as to suffer least resistance, and waste least power; or, it may be, to attain the highest possible velocity with a given propelling power. The following maxims of the wave system show how this is to be done :

6. When it is required to construct the water-lines of the bow of a ship, of which the breadth and length of bow are given, so as to give the vessel the form of least resistance to passage through the water, or to obtain the highest velocity with a given power:

Take the greatest breadth of the vessel on the main section of construction, or midship breadth, and halve this breadth. At right angles to this, draw the centre line of length of the bow. On each half-breadth, describe a half-circle, dividing its circumference into (say) eight equal parts. Divide also the length into an equal number of equal parts. The divisions of the circle, reckoned successively from the extreme breadth, indicate the breadths of the water-line at the successive corresponding points of

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