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22 Tae MARINE. REVIEW SOME STERN FACTS. BY HORACE SEE, ASSOCIATE.* It ig remarkable that the natural laws are so often disregarded, particularly in the mechanic arts, and in no branch is it more strikingly illustrated than in the vessel driven by the screw propeller. We might say that heze fantastic shapes are the most popular, as shown in the propeller and: the stern of a vessel. The propeller, like the Indian's boat, has received a dig from every passer-by, and all sorts of shapes suggested for obtaining the highest efficiency. The expanding pitch with. the tip of blade bent aft radially so as to assume a form like a spoon was at one time a prime favorite, but today has given place to the true screw. The latter, after a thorough investigation, was accepted by the writer to be the one, when properly designed, best suited for propelling a vessel, whether the stern was full or lean, as it offered the least resistance in its path whilst obtaining maximum effect. The success with the bronze blade is in the same line, as it is to the thin blade offering less resistance than the thicker iron one, and not to the kind of metal, that this result is due. The frequency, therefore, with which this partiality for the spoon-shaped blade was expressed led the writer, when a contract had been obtained some twenty odd years ago for. two vessels similar in every respect, to suggest, in order to solve the problem, that one be fitted with a blade in the form of a true screw and the other with one spoon shaped. These wheels were made solid, the outline as well as thickness of the blades being the same, the only difference being that the one was bent to form a true _ screw, whilst the other was expanded or twisted to make the spoon shape. When the vessels were ready to launch with the wheels in place they were visited by quite a number of persons, from whom the general expression was in favor of the spoon-shaped blade,.as it was said by them to take hold of the water better than the true screw. This view was quite true in one way, but not in that desired for obtaining the best results in propelling the vessel, as the one with the true screw was driven at a speed one knot greater than that with the spoon-shaped wheel. This result could not be attributed to the form of the one vessel or the character of its machinery being superior to that of the other, or to the handling of the vessels, but to the wheels themselves, as a new wheel of the true-screw pattern when it replaced the spoon-shaped one brought the speed of this vessel up to that of the other. Again, when the one first fitted with the true screw broke this wheel and had the spoon-shaped one that had been taken off the other vessel placed on it, then her speed also was reduced as in the case of the vessel originally fitted with this wheel. A tunnel arched longitudinally with its lowest point at bottom of vessel forward of the screw, increasing and attaining its highest point over it and then descending to a point at or below the water line at the stern, is another form that has fascinated many. The United States Gov- ernment has had a number of vessels of 'this type built, the contractors for which have lost a considerable sum of money--some having gone into bankruptcy--by failing to secure the contract speed. The original and altered form of stern are shown in Fig. 1. These boats were designed to maintain a speed of 29 knots for two consecutive hours. The Perry having attained but 282 knots on the pro- gressive speed trials, the contract speed demanded was afterwards reduced to 26 knots, and the duration of run to one hour. The hook at stern was also removed and new propellers placed on two of the vessels. The Paul Jones gave the best general results--attaining after the change * . i Sie : ee read at the Milan wperins of the International Navigation KIiGST. UL. tre. II. 28.91 knots on the progressive speed trials and 27.4 on the one-hour's run. The location of propellers and the form of run, still being bad, worked against attaining a higher speed. The change of trim in this vessel is not giv- en, but that of the others averaged 514 ft. The selection of this form of stern is more remarkable in view of the experience in 1889 with the dynamite-gun cruiser Vesuvius, contracted to attain a speed of 20 knots per hour, when tried in 44% to 5 fathoms or comparatively shallow water. The trials, four in number, with this depth of water were unsuccessful. The writer, who was present only on the last one, suggested after that failure a course where the depth of water would not be less than 15 fathoms. This met with favor, and when the next or final trial was made the vessel readily attained a speed of 21.65 knots or 1.65 in excess of the contract requirements. She had extreme- ly fine lines, with a wedge-shaped and not an overhanging stern. On this-occasion she stood up without squatting aft and with but very little disturbance in the water, whereas, in the former trials the stern not only was de- pressed, but a wave was formed with the crest of it so high as to cover the deck at this point, as shown in Fig. 2, thereby demonstrating that the water where depth is limited will, when leaving the stern, ascend naturally without mechanical means. This now brings meto the tunnel stern with an ad- justable flap, as shown in Fig. 3. Why a 'flap at all, is difficult to understand, not only in view of the ex- perience with the above-mentioned torpedo-boat destroyers, but also from the experiments with a shallow-draught launch fitted with a tunnel and hinged flap; of which it is stated in the 'Transactions of the Institution of