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22 that generally sufficient strength to. submerge 150 ft. is satisfactory. Prob- ably the minimum risk from sink- ing is obtained by a combination of high-pressure blow-out tanks, sub-di- vision into a number of water-tight compartments and a safety drop keel. This combination is not a new one, but one which already exists. Ves- sels so constructed can be saved from sinking if one, or even perhaps two, compartments are thrown open to the sea. Recent large submarines of the Holland type have heavy keels, and it would be interesting to know if all of this outside ballast is necessary to insure their stability. lf this ballast is not all necessary for stability, what fraction of it could be utilized for a safety drop keel, if desired? The amount of reserve buoy- ancy with which the vessel sub- merges is also an element of safety, but this is generally quite small, less. than one-half of one per cent of the total submerged displacement. Double Hull and Single Hull. The designation of submarines as "double hull" and "single hull' to differentiate between those that 'have a comparatively large emerged dis- placement for surface running in per cent of the 'total displace- ment when submerged, and _ those which have a smaller per cent of submerged displacement, does not seem to be an accurate way of des- ignating the two different types of vessels. I do not think that any sub- marine vessels haave a complete "double 'hull,' one water-tight and capable of resisting high pressure, and the other non-water-tight. The first published midship section of the French so-called 'submersible' Nar- val certainly showed this "double hull," but I have been told on good authority that as finally constructed the light exterior tanks of the Nar- val, which are empty when the ves- sel navigates on the surface, did not extend completely around the vessel. The Narval was designated as a "sub- mersible" owing to the fact that the emerged displacement for surface nav- igation, in per cent of the total dis- placement, was much larger than in previous under-water torpedo boats which had been designated as "sub- marines." In the Narvalthis emerged displacement for surface running was about 40 per cent of the total dis- placement when submerged. The con- struction of the Narval marked a great advance in the seaworthiness of under-water torpedo boats, and paved the way for the development of the THE MarINE REVIEW under-water torpedo boat into an able sea-going vessel. The program for the design of the Narval was due to Monsieur Bertin, the eminent French naval constructor, who was chef con- structor of the French navy when the "Narval was built. To Naval Construc- tor Laubeuf, of the French navy, be- longs the credit of developing the de- sign of this vessel. Underwater Torpedo Boats. Today it is difficult to divide under- water torpedo boats inta the two classes of vessels, "submarines" and "submersibles,' by differentiating one class from the other by thé amount of emerged displacement for surface running in per cent of the total dis- placement, and the consequent great- er or lesser development of the non- water-tight superstructure. i? re. mains for. the best per. cent of -emerged displacement for different services to be determined on the principle of the survival of the fittest. Certainly for the sake of durabili- ty the non-water-tight superstructure should not be developed to such a point that it can be "punctured by a boat hook," as Mr. Hay tells us. For- tunately the use of such extremely thin superstructure plating is gener- ally avoided. Sufficient emerged dis- placement in per cent of the total displacement not only permits higher surface speed to be obtained and fa- vorably influences hehavior in rough water, but also constitutes an ele- ment of safety, owing to increased buoyancy, when the vessel is pro- pelled at thigh speed on the surface, and in case of collision. These good qualities are obtained at the expense of some increase in resistance when submerged. [he control of § sub- merged running is not necessarily un- favorably affected. The Krupp Type. The present Krupp type of boat, which Mr. Hay mentions, has a very broad, low superstructure which tap- ers off to nothing at a considerable distance below the water line where it meets the circular water-tight hull. The conditions of stability of the Krupp type, either on the surface, or when submerged, are not typical of the best that can be obtained in a vessel with an emerged displacement for surface running of only 16.7 per cent of the total displacement sub- merged. A wide, flat superstructure, the width of which considerably ex- ceeds the diameter of the hull, woulld undoubtedly be largely accountable for the "broachine to" when turn- January, 1910 ing submerged, to which Mr. Hay refers. If we take the published dimen- sions of one of the Laurenti boats, La Faca, as cofrect, we find that this vessel, which is designated as a "submersible," is 140 ft. long, dis- places 185 tons when running onthe surface and 235tons when submerged, The emerged displacement for surface running is then 50 tons, or 21.3 per cent of the total displacement.- If we also take the published dimensions of the submarine Octopus, of our navy, 106.5 ft. long, 238 tons surface displacement and 275 tons total dis- placement, when submerged, we find that the emerged displacement for surface running. is 3/ tons, or 135 per cent of the total displacement. I wish that Mr. Hay would correct me if ;eam in ertor,:for.1.-do not-sea why he speaks of the boats of the Laurenti type as having an emergea displacement for. surface running of 60 per cent. -The emerged displace- ment of these Italian boats of about. 20 per cent is practically the same as tha@ of the recent large French boats, and somewhat in excess of that of the British boats which I understand is about 15 per cent. It would seem to me that we may ex- pect to find less and less difference in this percentage of emerged dis- placement in the boats of the future. A comparatively large ratio of length to beam of approximately 10 favorably affects surface propulsion, without affecting unfavorably sub- merged running, and is being gener- ally adopted in all types whatever may be the per cent of emerged dis- placement. Inefficiency of Hydroplanes. The 'question the number of rudders employed in different cases seems to be given' undue importance, Hydroplanes, located amidships, seem to be notably inefficient and to re- quire very large power to operate Hem, .. «wnderstand that a <few years ago amidship hydroplanes were experimented with on one of the Holland boats in spite of the fact that the company which builds these boats is a strong advocate of stern rudders only. For this company to have considered hydroplanes, even ex- perimentally, would seem to indi- cate a spirit of tolerance on the part of some submarine builders which may help to bring about greater uni- formity of ideas. No submarine has yet run submerged in equilibrium, or an even keel, whatever the disposi- tion of rudders. Experiments have shown that submergence can be ob- of