Air-cooled Engines in Light Speedboats

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By R. R. POOLE, B.Sc.

The light speedboat is such a recent development of marine engineering that it has not yet outgrown the conservatism of long established practice.

From earliest times it has been both necessary and customary to provide vessels with ballast and all parts of a boat have in consequence grown to generous dimensions and unstinted weight. Marine engines are therefore naturally much heavier than their counterparts on land, and weight-to-power ratios of 20 lbs. per b.h.p. are not uncommon, as compared with some 10 lbs. per b.h.p. for ordinary car engines. Racing car engines run from 3 to 5 lbs. per b.h.p. and aircraft naturally lower still, down even to 1 in the larger motors. Now great massiveness may be right and proper in a slow moving displacement hull, but it is evident that the resistance to motion of a fast boat depends in some way on its immersion and so on the total weight.

This has not been lost sight of in the case of larger and higher powered racing boats in which aero engines figure prominently, and in the Duke of York’s trophy class, where supercharged high speed motors are used, identical in design with modern racing car engines. It is in the very light boats that one finds the engine weight becoming disproportionately high, though undoubtedly some modern outboard units are very light when the whole assembly is taken into account. Even here, however, there is room for considerable improvement, possibly, in the direction of aircooling, since a watercooled two-stroke of some 350 c.c. cylinder volume develops not more than seven or eight b.h.p., while one well known overhead camshaft motorcycle engine can be made to develop 25 b.h.p. with the same capacity, and is rather lighter so far as the actual power unit is concerned.

There appears to be no fundamental objection to aircooled engines, since rusting can be absolutely prevented by means of electrolyte deposits of zinc or cadmium, and the absence of a circulating system is an advantage not to be despised, particularly for racing on some of our unclean inland waters. The silencing of the exhaust by means of the waste cooling water in the usual types of engine is often insisted upon in competition work, and in many cases aircooled engines are definitely debarred. However this is by no means universal and in any case need not prevail indefinitely, so there may be a considerable future for the aircooled motor in both outboard and inboard driven boats.

In the former, some such engine as the A.B.C. opposed twin would appear to possess advantages over the water cooled two stroke of about the same capacity, on the grounds of both power and lightness, though its additional complexity might prejudice many in favour of the valveless unit. For inboard drives one’s mind turns towards a hefty single cylinder of perhaps 500 c.c. such as the Norton. This could be housed in the forward section of a single step skimmer, with its cylinder barrel and head projecting through the decking into an unobstructed air current, and readily accessible for carburettor and valve adjustment. Alternatively a V twin mounted transversely would give a very accessible arrangement and certainly a very high power ratio, both on weight and capacity bases.

The overhead valve 500 c.c. Norton develops nearly 30 b.h.p. at a maximum speed of about 4,500 r.p.m., a suitable speed for a direct driven propeller of 5 or 6 inches diameter. Overhead valve twins of some 850 c.c. are said to be capable of about 40 b.h.p. at roughly the same speed.

It would appear therefore that if real advantage would accrue from a saving of weight in a boat really designed for the purpose of travelling fast, the use of air cooled engines would be one of the first steps towards this end, and their performance in their more normal field testifies well to their reliability and general performance.

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