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"HYDROPLANES" OR "SKIM- MERS."* BY SIR JOHN I. THORNYCROFT. ESSELS which greatly re- duce their displacement when traveling at high speeds are RS Jel) generally called "hydro- planes', but this name is not altogether satisfactory, as the surfaces on which they glide are not always planes. To call such vessels "gliders" or "skim- mers" has been suggested as more-ap- propriate, but the former title suggests smooth motion, and this is not always a characteristic of their behavior. In this paper, therefore, the latter word will be used to describe boats which at high speeds are heavier than the water they displace. "Skimmers" are very ancient, and are still used by some of the inhabitants of islands in the Pacific, in what must be their simplest form. A single slab. of wood, rounded at the edges, is employed, and supports a man where huge waves form a sufficient slope to enable him, asststed by gravity, to attain the necessary start. After the start the man will even assume a stand- ing attitude on the board which skims along before the advancing wave. The problem' of making a boat glide steadily on the surface of the water is not an easy one. Steadiness can probably be gained by using a number of planes, but this is likely to increase the frictional resistance. When a num- ber of planes are used to support a given load, each must be of less length 4g Pigs de than when only one is used. The fric- tion per unit of surface being greater for short surfaces, it is improbable that a smaller total surface will be sufficient, and the necessary power required for 'a given speed must be greater. Mr. Froude's opinion was that the' best re- sults could 'be obtained' from' a single plane, but this must 'maintaiw"a partic- ular angle to thé water stirface. About thé year 1872 he made a model, in which three surfaces were attached to a frame, and towed in such a position that the wake of -either of the tnree did not in- terfere with the water on which any one *A paper read at a meeting of the Motor oe Club, London, on Thursday, March 4, ThE MARINE REVIEW of them had afterwards to run. Froude proved by theory that the angle the plane should have to the line of mo- tion should be such that the resistance due to surface friction should be equal to the resistance due to gravity, of the horizontal component necessary to balance the weight of the vessel on the incline of the supporting surface. This inclination he found to be about 1 i 17, so that the total resistance amounts to about %.5 of the weight or dis- placement of the vessel. This friction, however, depends on the value to be attached to the surface friction, and this also varies with the quality of the surface and the length of the sur- face rubbing. This inclination is there- fore not a definite angle. Figs 42 Mr. Froude found, when experiment- ing with the Ramus model that the front plane lifted entirely above the water surface as the speed was_ in- creased, and the center of gravity ap- parently overhung all natural support, as: shown. in Fig. 1. It is evident that this effect can only be produced by the pressure on part of the surface being less than that of the atmosphere. The author had made some experiments to pillustrate, this, with, a model, the bottom .of which -was,., for the most part, a simple plane,, but at the after end the sutface could be turned to a slight angle, and the result was that in some cases the model even jumped clear of the water surface. When the bottom sur- face was left flat throughout its length, the model glided smoothly over the water; but when the tail part was bent down, the model promptly dived. <A photograph taken of this operation, a little too late to show the model, clear- ly showed the hole in the water where Mr. Fic. Zi it disappeared. These experiments seem- ed to show that the endwise vertical section of the bottom of a skimmer should be a straight line; although a hollow curve would seem to promise a more even distribution of pressure on the bottom. "In a boat intended for skimming or sliding on the surface of the water with the least possible disturbance, there are a number of elements to consider. Un- fortunately, these do not all lead to the same proportions of design. In_ the first place, the lifting force depends on the amount of surface and on_ the speed; while the friction for a certain amount of surface will decrease with greater length. The speed at which skimming will commence, however, in- creases with the length: This limit Eig 3: