But with Froude's experiments all doubt on the matter vanished. It was no longer a question of 'condemned mathematics.' Froude had the happy knack of writing so that the proverbial schoolboy could understand him; and the schoolboy could see the value of resistance to motion through water weighed out as simply and accurately as a pound of currant bun. These experiments for the determination of the frictional resistance of water, published in 1874, were supplemented presently by experiments on models of actual ships, and also by towing a full-size ship, the 'Greyhound,' her resistance at various speeds being recorded by means of a dynamometer on board the 'Active,' the vessel towing her. The results of the experiments on model and ship were set out in a curve, when it was found, after the necessary corrections were made, that both curves were of precisely similar character. A basis of comparison between model and ship was thus established, the measure of this being set forth in what is known as Froude's law of comparison, which may thus be stated. The equivalent speed of a ship and the model it represents will vary as the square root of their lengths. Thus, in the case of a ship 100 feet long represented by a model 4 feet long, the equivalent speed of the ship would be five times that of the model, and at these equivalent speeds would present similar phenomena connected with resistance as the model does. This fact enormously increased the knowledge of investigators, and it was belief in it that gave the writer absolute confidence in carrying out the design of the 'Vanduara,' though he possessed experience in small boats only. Mr. Froude also split up the several elements of resistance to motion through the water into their component parts, assigning a value to each, and showing what was due to surface friction and eddy-making, and what to wave-making. Scott Russell had already argued for a given length of fore and after body for any given speed, and this was recognised by yacht-builders to some extent by their gradually lengthening out their vessels; but the disadvantages as well as the advantages of length could only be thoroughly realised on investigating Froude's experiments. An example is given of such an experiment in the diagram, which shows the resistance curve of a model of the 'Merkara,' built by Messrs. Denny Bros., at Dumbarton, where the several resistances are shown, each in its place. In this diagram the resistance due to surface friction is indicated by the dotted line, and the total resistance by the full line. Up to a speed of 250 feet per minute (for the model) the resistance is almost entirely due to skin friction, but after that the wave-making becomes more and more serious until at 370 feet per minute the wave-making takes more power than the surface friction.
While surface friction thus plays a very large part in the resistance of all vessels, and more especially in that of ocean-going steamers and ships, which from their large dimensions seldom attain serious wave-making speeds, yet undue importance may be placed upon friction, and, in the smaller yachts, especially, surface may be inordinately cut away. A notable example of this was the 'Thistle,' built in 1889 to compete for the America Cup; here the surface was so cut down that sufficient lateral plane was not left to hold her to windward, and although she sailed the water as fast as the American champion, the 'Volunteer,' she drifted bodily to leeward.
A short history of Mr. Froude's discoveries in resistance was advisable before touching on 'Jullanar,' as this wonderful vessel, whether the result of intuition or of early and immediate appreciation of Froude's investigations, was a remarkable example of the modern theories regarding naval architecture.
The same year that 'Jullanar' was built, I designed my first racing yacht, the 5-ton 'Clotilde,' but whilst I had the advantage, through my friend Mr. John Inglis, jun., of specially early access to Professor Froude's investigations, I cut her away in a somewhat timid fashion, though sufficient for her at that time to be compared to a 'cart-wheel,' with the accompanying prediction that she might 'run on land, but would never sail in salt water.'
Meanwhile, with splendid audacity, and with no timid reverence for precedent, Mr. Bentall built the 'Jullanar.'
An Essex plough and agricultural implement maker, Mr. E. H. Bentall had but little training in naval architecture, but from boyhood had been fond of yachting and of yacht modelling. He fancied he could do something in the way of improving the form of the existing racing yacht. After cutting several half-models, he got one that pleased him, and on a piece of his own property adjoining the Blackwater river in Essex, the famous yawl, afterwards to be known as 'Jullanar,' was laid down.
'JULLANAR'
126 tons. Built by E. H. Bentall, Esq., 1875.
Great length was taken in proportion to beam, as length means capacity for speed, and beam in those days was doubly taxed. Draft was untaxed, and was used boldly to obtain stability and weatherly qualities; but while such proportions would have been impossible with the ordinary form of forefoot and sternpost, as the boat would have been clogged up with wet surface, this was got over by cutting all deadwood clean away both forward and aft, in such daring fashion as was not attempted until 'Thistle' was built, years afterward and I should not have essayed such a form of profile in her had not 'Jullanar's' success given me a precedent. Add to these features the fact that every line in the vessel was easy and fair, and the only wonder is that the famous yawl was not even more phenomenally successful than she was.