'They do things different in Norfolk' Update

In January 1938, English Mechanics magazine published an article featuring ‘some fine examples of IC engines and boats seen in the Norwich area’. One of these was a ‘graceful little craft’ "Lyn" made during 1937 by John Duffield of Brancaster in North Norfolk. The method of hull construction was the remarkable feature as the upper part of the hull is ‘scooped out’ of a block of pine and the floors nailed on afterwards. To quote " Possibly in no other way could low weight, great strength and a great beauty of line be so efficaciously combined" The single step hull was reminiscent of Captain Bowden’s ‘Jildi Junior’ and with its home built 15cc two-stroke motor it was indeed a very attractive boat. The engine was described in detail in a later article in English Mechanics. "Lyn" had a short life of just one season to be replaced with "Lyn 2" of more conventional shape for 1939 but still with the same engine. After the war the engine was transferred to a three-point hull before a new hull and a new engine in turn replaced that. The earlier hulls were presumably broken up but the engine and all the parts made during its development were retained, and after passing through a series of owners featured in pit box in Dec 06. The post-war hull and engine currently resides with John deMott.

Having all the parts to hand, it seemed a worthwhile exercise to find out more about the engine and try to build it in each of its guises and states of development.

Two-stroke technology was at a very early stage in the mid 30s when this engine was conceived with piston porting and deflector pistons the norm along with capacious crankcases. Edgar Westbury propounded the theory that excessive crankcase volume did not affect performance?????

John Duffield started by using a steel rear rotary disc valve with large and long duration port opening, and maximised scavenging of the cast aluminium crankcase by padding the crankshaft. He then arranged for two transfer passages in the cast iron cylinder, one at each side, with windows in the piston skirt and a pair of exhaust ports at the front and rear of the engine. This gave true loop scavenging and a forerunner of the very precise schnuerle porting.

The positioning of the exhausts was far from satisfactory and so the first modification was a pair of cast aluminium manifolds and 5/8" aluminium pipes that took the exhausts overboard. The false fin with a joint each side also proved to be problematic and a one piece, air-cooled aluminium cylinder head was made to replace the water-cooled version. The engine was very effective, but in order to improve it still further a new and large venturi was made. The needle adjustment system was refined even further with 75 ‘clicks’ per rev, geared down 3:1. Imagine 225 ‘clicks’ to one turn of the needle. That is ‘fine adjustment’. A strangler was still retained for starting the engine.

Apart from new engine mounts being made for successive boats, this is how the motor remained until 1948 when John Duffield gave the engine another makeover and abandoned the spark ignition system in favour of a glow plug. A new magnesium cylinder head was made, with a better combustion chamber shape to take advantage of the smaller diameter of the glow plug. The Rotary valve and disc was rebuilt with a new cast backplate and a thick tufnol disc running on a steel spigot. The inlet tract was very large at a shade under ½" and the opening period massive. A new peripheral jet venturi followed the latest trend on high performance speed motors. With the deep backplate and thick disc, recessed for the big end, crankcase compression was maximised. In this state of tune the engine was competitive, but could not match the outright speed of the George Lines’ Sparky.

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