The following text may have been generated by Optical Character Recognition, with varying degrees of accuracy. Reader beware!

Tae Marine REVIEW a tlt A : ry a l j : 17. 20 " Spieeds Knots. . - Gyrinus model, towed backwards, to il- Resistances - Justrate the clinging of water to the rounded form of stern, which the bow then represented; and although this gave no trouble at ordinary speed, the effect at extreme speeds is surprising. ~The real stern lifts and the bow is de- pressed until the vessel makes a large angle with the line of motion (Fig. 6), "as was found to be the case when Mr. Froude towed the Ramus model. _It is often supposed that air passes under skimmers, but this is only likely "to take place when the water surface is broken. It is well known that a jet of water impinging on a surface, even -- -at an acute angle, does not all pass under in the direction of the jet. A small part at a point near to the sur- "face has its motion reversed. This fe- "versal renders the passage of any air between the jet and the surface im- "possible. If, however, the surface of the moving water is broken or churned into foam, then that mixture of air and water will pass along the surface. What will be the. effect of this seems uncer- tain, .but the late Lord. Kelvin was thoroughly of opinion , that the, fric- tion of this mixture would, be .greater than that of solid water... The form of boat used by M. Fauber is adapted. to eject any air from under it, and, per- haps, from this some advantage was ob- tained which tended to balance the loss which might be expected to arise from the use of so many short skimming 'surfaces, VM ALAA GMesi2t0ns RO ee ; ere OA. Pa ey A BC Is The author also tried models intend- ed to carry air under them in a hollow. This construction is favorable to high speed; for, the. angle of the surface rubbing on the water can be consider- ably reduced by the adoption of this plan, so reducing the resistance due to gravity on the incline. At the same time the friction due to air on the sup- porting water is much less. than that due to water on a solid; and if the air could be carried with the model, the power required to forcé it under would be small and would only amount to that necessary to make up --waste-..-It may be safely taken that when air goes under a vessel without forcing, it will fail to give any support. It is evident that the air under a vessel to serve this purpose must have a pressure above that of the atmosphere. In a skimmer in which the water rubs directly on the bottom surface, the cen- ter of pressure is always forward of the center of area; but in a vessel in which the supportyits due to air in a hollow, it must coincid®& with the cen- ter of gravity of the area of the hol- low. It is much more advantageous to have such a definite center of support than the ever-shifting position of the center of the lifting force due to con- tact with the water surface. The prac- tical difficulties, however, to be over- come in producing a type embodying this principle are many. Care must be taken, for instance, to. avoid disturb- ing the. water surface by. the surfaces which must surround the hollow. con- taining the air. If this be not done, waves will form and increase the rub- bing surface, besides taking up energy in their formation. With a model the author designed to overcome these dif- ficulties the resistance had not proved very small, but it was observed that the wake of the model indicated very little disturbance of the water surface. This model was very wide and _ nearly circular in plan, and it skimmed at quite a low velocity. It may be! inter- esting to state that this form of model was made in 1873. A similar model was towed from a launch at about 8 knots; this speed was more than sufficient to make it rise to the surface. When air was forced under it, the resistance was greatly reduced, but it was never very low. At the best it fell to a value of about one-quarter the weight of the model. At a lower speed--about 5 knots--a greater resistance was record- ed. ' It has often been proposed to force air under a vessel of ordinary form, and Ericson is said to have tried this, but found it was not a'success. In 4 skimmer, however, if the air can be carried with the model for the most part, there must be an advantage in this system, and perhaps it might also be used with advantage in very shallow ves- sels, not intended to skim, but having surface of bottom, and the to be pumped against much head of water. a very large not requiring air