Re: Vincent lifeboat engine and double acting pistons

Here's what I know about them. ............

The Vincent Marine Engine


J. Kim Siddorn

A classic example of being conceived in good time but born too late, the Vincent Marine Engine was originally made for an air/sea rescue role and fitted to airborne lifeboats, a complete aluminium shell which could be dropped to men in the water from an aircraft.

Vincents gave up motorcycle production during the war, concentrating on making specialist components at their Stevenage factory, one of which - for instance - was rocket fuses. Phil Vincent's agile mind was never far from a novel solution to a perceived problem and he found an Air Ministry brief for an engine in which frugality of fuel consumption was vital - and the other parameters were tough, too. It had to be capable of being started by hand, fully radio screened, waterproof (sic!), light in weight and smooth running and able to run for long periods with little maintenance. The construction had to be compact so that the hull of the lifeboat would fit cleanly against the fuselage of the aircraft that carried it and be able to withstand a 5G deceleration as it hit the water. The specification called for 15 b.h.p. at 3,000 r.p.m. from an engine that had to be able to run satisfactorily on any petrol, from 70 octane "pool" to highly leaded 120 octane aviation spirit.

Worked up from an original concept by Phil Irving in 1942, the 500cc Lifeboat Engine (as it became known) was produced with Pacific rescue missions in mind. For a fuel consumption of 50 gallons, the boat seemed to be capable of a total mileage of over 1,000 sea miles at an average speed of 5 knots compared to an Austin marine engine's best performance of 500 miles at 4 knots on the same quantity of petrol and the prototype passed its AID inspection out of the box.. However, over 800 hours of development, designing a reverse gear, electric starter etc and a vacillating Air Ministry kept Vincents busy until 1949 and that was too late, only fifty being made. Of this single batch, there are around a dozen left, this example having the s/n 44.

For the technically minded, the device weighs just 256 lbs (116 kilos) and has a watercooled inlet manifold and exhaust. It is a two-stroke cycle, twin crankshaft opposed six, the pistons moving inwards to form a common combustion space, thus, this six has three bores. The centre bore of the three provides compressed mixture for the outer two cylinders, the mixture not being compressed in the crankcase as in normal two strokes. The pumping cylinders have cross heads like a steam engine, allowing straight, round section connecting rods to be sealed from the crankcase with gland seals. The mixture is circulated both "above" and "below" the pumping pistons, transferring to one set of power pistons on the inward stroke and to the other set on the outward stroke, thus making best use of the capacity available. The inlet charge must be well and truly mixed by the time it gets squashed and ignited!

A great advantage of this layout is that the port timings of the pistons on one crankshaft can be varied in relation to those on the other crankshaft. One pair of pistons control the transfer ports and the other the inlet ports, so the port timing can be readily varied to give the best results. As the gas moves in only one direction, it is called a Uniflow engine. In a conventional two-stroke, the same piston has to control both transfer and exhaust. This system may be simple, but port timings will always be a poor compromise compared to that on a two crank system and very wasteful of fuel, especially at full load or idle, ask any Petter M type owner! The development of the Lifeboat Engine progressed to a stage where the fuel consumption became extremely good even by four-stroke standards.

Looking nothing like an engine, the lumpy rectangular thing might be anything at all. Only the two spark plugs sticking up out of the block and the brass-bodied Amal carb give it away.

Wearing its original coat of Admiralty grey paint, the author's unit in these pictures is in pretty good condition, somewhat chipped and worn as one might expect in something which has been around for over fifty years. The only things that are missing are the ancillaries cover that covers the starter, generator and magneto and the recoil starter that mounts at one end and contains a huge clock spring. No-one has these items out there, I suppose?

At the time of writing in August 2002, the engine has only been run briefly in order to check it out, prior to being fitted with tanks and a cooling system.


The marine engine was doubled up into a flat six twelve piston engine intended to drive a 10Kva generator after the war. It was unsuccessful due to the impossibility of maintaining square faces during the machining of the huge and complex crankcase casting. It went through very considerable development, but it was just too complicated for its own good. There was a description of this in issue No: 156 of Stationary Engine Magazine for February 1987.


Kim Siddorn


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