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1903.] > - better. That was no doubt: perfectly true, and may lead to the impression that vessels built on this plan are not efficient for tow- ing. Such, however, is not the case. As a matter of fact they are very efficient for towing, provided the propellers are made sufficiently large. With a view to securing the maximum effi- ciency for speed when not towing, the propellers in all these boats MARINE REVIEW AND MARINE RECORD. 29 top. of the tunnel the water has a clear passage open to it to freely pass away in an opposite direction to that in which the vessel is traveling. 'The gain by the raising and lowering of this flap is illustrated by diagrams in Figs. 3 and 4. When the launch is light, drawing 11 in. with the same power, the speed is were. made of small size. Had the Sheikh and Sultan been in- tended for towing, we should, no doubt, >have adopted propellers of a diameter at least 50 per cent. greater than we did. I mention this fact, in passing, to avoid the impression that vessels on this plan do not tow well. ""T. now. come to a departure which we have recently made in the design of these tunnel vessels, and which has added considerably to their efficiency. It will be re- membered that the after part of the upper portion of: the tunnel, which. is.inclined downwards to- . wards the stern, is a source of re- sistance. As. already expiained, it--is compulsory when _ starting that the after extremity should be below the water in order to pre- vent air gaining access to the in- terior of the tunnel. Now, expe- rience has shown that once the boat is under way the after ex- tremity of this tunnel, when full, may be above the surrounding water, because the rush of water out of the tunnel in a sternward direction is such as to prevent the air passing in; in fact, the after part of this tunnel may be 6-or 8-in. higher than the water level without risk from this cause, at the same time improving the result, due to reduced resistance 'of the inclined surface. It has been shown that the Little John. increased from 9.2 miles an hour with the flap down to 10 miles an hour with the flap up; and when the draught is 28 in., loaded with 20 tons, the speed is increased from 6.9 miles an hour with the flap down to 8.25 miles an hour with the flap up, the power at both speeds being the same. As might naturally be expected, the increase of efficiency due to the lifting of the flap is greater when the boat is loaded, the lower speed in both cases being what they would have been if there had been no ad- justable flap, clearly showing the advantage of the flap. "We recently conducted experi- ments with a view to test the tow- ing efficiency of this method of pro- pulsion. We built for the Trent Navigation Co. a twin-screw tug called the Little John. It was 80 ft. in length by 14 ft. 6 in. beam; minimum draught, with steam up, 22 in.; displacement about 40 tons. Each screw was driven by a single inverted engine, 10 in. in diameter by 10 in. stroke; high pressure loco- motive boiler. The dimensions of the tug were determined by the size of the locks and the depth of the river. Up to the time of our building this boat the Trent Naviga- tion Co. had experienced much diffi- culty in dealing with their traffic. Side-wheelers were found too wide to pass through the locks owing to Robin Hood. after part of the tunnel should be below the water line when tne vessel is starting the paddle boxes. A twin-screw boat with propellers in the usual po- light, but if it were fixed in that position it. would be unnecessarily low down when 'the vessel is loaded, thereby involving greater resistance than if the after part of the tunnel terminated at a level sufh- cient for the loaded boat. | "In our more recent vessels of this kind, we have made the upper part of the tun- nel, from the propeller to the stern, hinged in such a way that it can be raised and lowered to any desired height, either by mechanical means or automatically. Fig. t shows the position of the hinged top, or | flap, when the boat is light, drawing II 1n., and Fig. 2, when the boat is loaded, draw- ing 28 in, It will be seen at a glance that when loaded; if the after extremity of the: tunnel were in the same position as it is when light, it would involve a greatly in- creased resistance, while by raising the Buchanan's Patent, 1856