The reader, therefore, cannot fail to see how important are these methods, laws, and researches of Mr. Froude.
The above-described process for testing models is being continually conducted in the case of all new battleships and cruisers for the British Navy, and also is pursued by the naval constructors of other nations. In connection with the extensive programme of battleship construction which has been carried out of late years, Sir William White, the late eminent Chief Director of Naval Construction, states that it is not too much to say that these methods of investigation and experiment have placed in the hands of the naval architect an instrument of immense power for guiding him safely and preventing costly mistakes. Sir William White has declared that it would have been impossible to proceed with the same certainty in battleship design, were it not for the aid afforded by these methods.
Mr. Froude was not content, however, with experiments made with models. He ascertained by actual trials the total force required to drive an actual ship through the water at various speeds, and obtained from other experiments valuable data which showed the proportion in which the total resistance offered to the ship was divided between the skin friction and the wave-making resistance.
Then he made experiments on a ship of 1157 tons, viz. H.M.S. Greyhound. This vessel was towed by another vessel of 3078 tons, viz. H.M.S. Active, by means of a tow-rope and a dynamometer, which enabled the exact “pull” on this hawser to be ascertained when the Greyhound was towed at certain speeds. The following are some of the results obtained:—
| Speed in knots of H.M.S. Greyhound. | Strain in tons on towing-rope. | |||
| 4 | knots | 0·6 | tons | |
| 6 | ” | 1·4 | ” | |
| 8 | ” | 2·5 | ” | |
| 10 | ” | 4·7 | ” | |
| 12 | ” | 9·0 | ” | |
It will be seen that the total resistance increases very rapidly with the speed, varying in a higher ratio than the square of the speed.
In addition, the indicated horse-power of the engine of the Greyhound was taken when being self-driven at the above speeds, and it was found that only 45 per cent. of the indicated horse-power of the engines was used in propelling the ship, the remaining 55 per cent. being wasted in engine and shaft friction and in useless churning of the water by the screw.
It is an important thing to know how this total resistance is divided between skin friction and wave-making resistance.
Mr. R. E. Froude has kindly furnished the author, through the intermediation of Sir William White, with some figures obtained from experiments at Haslar, showing the proportion of the whole ship-resistance which is due to skin friction for various classes of ships going at certain speeds.
| At full speed. | At 10 knots. | |||
| Battleships | 55 per cent. | 79 per cent. | ||
| Cruisers | 55 ” | 84 ” | ||
| Torpedo-boat destroyers | 43 ” | 80 ” |