The suitability of a certain shape of vessel, moreover, can to a certain extent be seen by observing the commotion which it makes in the water. Everyone has noticed the way in which a ship throws up a wave at its bows, and that bow-wave, as it is termed, represents so much energy being wasted. The power of the engines is absorbed to a certain extent in making that wave. It is impossible to make anything which when forced through the water will not make some wave, but certain forms cause less of it than others, and the designer of a ship seeks to find that form which will make the smallest bow-wave.
In like manner the eddies which a ship leaves in
its wake are the result of wasted energy, and the ship must be so shaped that they too will be reduced to a minimum.
Shipbuilders find that there are three things which retard a ship's movement: skin friction, or friction between the water and the sides of the ship; wave making at the bow and eddy making at the stern. The first depends largely upon the smoothness of the ship's surface, the second and third depend upon its shape. If a model behaves badly in the tank the fault may be either too much wave making or too much eddy making, and which of these it is the dynamometer does not of course tell. In many cases the experienced eye of the tank officials furnishes the clue to the trouble, but in some cases a cinematograph is used to make a complete series of photographs of the model and the water around it as it rushes from end to end. These can then be studied in conjunction with the chart and the cause of the fault discovered.
The real aim, it is obvious, of all these tank experiments is to find out the lowest horse-power necessary to drive the ship, or the best form of ship to get the highest speed out of a given horse-power.
The cost of keeping up these large tanks and making the models and conducting the experiments is very great, for not only are the premises very large (I know one in which the water alone cost nearly a hundred pounds) but a highly skilled staff is necessary. The saving effected in the cost of ships and the superior efficiency of the ships makes it well worth while however.
There is still one other point about this matter which will possibly be puzzling the observant reader. What are the models made of and how are they made? They are made of paraffin wax, and a very important department of the experimental tank is that where the models are formed.
First of all a rough mould is fashioned by hand in modelling clay and into this is poured melted wax, the result being a very rough model of the ship. This is then placed in the model-making machine.
Those of my readers who are familiar with an engineer's shop will know what a planing machine is like, and from that they can form an idea of the general structure of this remarkable tool. There is, first of all, a travelling table which, as the machine works, travels to and fro. Spanning this table is a beam which carries on its under side two revolving cutters, so that as the table passes beneath them the cutters can operate upon anything placed upon the table.