Celebrating 100 years of tethere

So goes the dialogue from Star Trek, but that has been the singular quest ever since Herbert Teague put a tether line on his boat in 1908. On a bitterly cold November day at a secret location in the wilds of Norfolk, a tin box was handed over that provided a tangible link to a group of engineers that had set about this task in a quite unique way.

In simplistic terms, to increase the power of an engine, increase the size of each bang, or chuff in the case of steam, and the number of bangs/chuffs per minute, and assuming it all holds together, the engine produces more power.

In January 1936, Professor Chaddock and John Innocent addressed a meeting of MPBA members, speaking for and against the ‘practice of supercharging’ to achieve the above aim with internal combustion engines. From 1908 until well into the 30s, IC engines had to play second fiddle to flash steam in terms of outright speed. Supercharging was not a new idea having been used in full sized practice for years, but here it was being suggested as a realistic option for small racing engines. Chaddock built and described his own eccentric vane ‘blower’ in a series of articles in the ME, but up in North Norfolk the idea stirred the imagination of three enthusiasts. The result was one of the most instantly recognisable tethered hydroplane motors ever, and two that have probably not been seen before.

It started with a comment in ‘Marine Modelling’ that a Dr Hewlett of Kings Lynn was building a supercharged hydroplane engine, although nothing further seems to have been mentioned about this motor. Amongst the other Kings Lynn members were John Duffield, whose innovative 15cc two-stroke engine has already been described, George Chapman, who went on to build ‘Wishbone’ with no less than four silencers on a single cylinder, and Bert Stalham.

Left to right in heading photo: Bert Stalham 'Tha'. George Chapman 'Bobsie III'. John Duffield 'Lyn'

Bert was a consummate engineer and travelled to many regattas with his ‘Ber’ and ‘Tha’ boats. The boats had some interesting features, including what was possibly the deepest prop ever seen. The four stroke single engines he used were well engineered, but it is with this magnificent supercharged Vee twin that the name of Bert Stalham will always be associated.

Engine was first seen at the 1951 ME Exhibition.

John Duffield had, by contrast, set about the more technically difficult task of producing a supercharged, single cylinder, 15cc two-stroke. He used the same eccentric vane supercharger design as Chaddock and Stalham, but controlling the charge going into the cylinder required a large degree of ingenuity. Each end of the engine has a very thin Zimmerman style Tufnol disc valve to control boost to transfer ports at the front and rear of the cylinder. These ports are above the level of exhaust ports, that are on one side of the liner only. A twin choke venturi controls the fuel into the supercharger while a high pressure plenum chamber with a 'pop off' boost control valve runs along the side of the motor This is connected to the disc valve chambers by thick wall plastic tubing. The discs only open for a fraction to allow a precisely timed ‘squirt’ of charge under pressure to enter the cylinder just after the exhaust ports have closed. The cylinder was topped off with a small insert comprising the combustion chamber and glow plug with a simple exhaust and silencer on one side and a decompression screw on the other.

With no crankcase induction, the crank bearings and supercharger drive bearings would have no lubrication at all. To overcome this, a bleed was taken from the end of the plenum chamber that force-fed oil from a sealed tank to bearings front and rear. A screw needle valve could be used to adjust the flow of oil.


Eccentric vane rotor, boost relief valve & twin choke venturi	Rear disc, rotor, supercharger case, cover plate.	Rear disc housing and supercharger drive.	Rear disc valve with minimal cut out.

Numerous records of the Stalham vee twin in action exist, but nothing about the Duffield motor, which begs the question as to whether it ever fired in anger. There are two answers to the question, but it would seem that there was a fundamental problem with the principle. As there was no natural induction to the engine, and with the drag of the two discs and the five vanes in the supercharger, it is questionable if the motor could be spun fast enough to get sufficient charge into the cylinder to start it, given the minimal time the ports in the discs were open. One can see that the motor might run quite happily and effectively once it was started, but that starting might well be nigh impossible. Further research might provide an answer to this conundrum. Conversely the engine did run very effectively, but only when the supercharger and second disc were removed and a venturi and needle valve fitted. The cylinder now has a more conventional set of transfer ports above the original ones and a schnuerle type boost port opposite the exhaust. The oiling system is retained with pressure provided by a slotted rotary valve at the rear of the engine.


Compare opening period of disc for natural aspiration with 'blower' disc.	Timed pressure bleed for lubrication system.	'Blower' ports top, new transfers below.	Front disc housing with crank, venturi, needle valve and inlet port.

John Duffield had a very light hull built for the motor, but ill health prevented him from running it. However John DeMott proved the effectiveness of the hull and engine, recording speeds in the high 60s with it until metal fatigue got the better of the skeg.

John DeMott seen here in the early 70s with John Duffield's boat and two 30cc boats of his own.

These two fine pieces of experimental engineering are a testament to the Kings Lynn group, but the story does not quite end there. The Duffield boat and engine have been rebuilt and it is hoped that they will run again this season, but through a quirk of fate or strange coincidence, details of Dr Hewlett’s engine have emerged and evidence that John Duffield had another go at producing a supercharged two-stroke.

Engine as it is currently, showing the pressurised lubrication system for the crank bearings.

An inline twin two-stroke motor with a supercharger between the cylinders. This is believed to be Dr Hewlett's engine that was mentioned in the Marine Models article.

Another of the prints is identified as John Duffield’s last engine. The internal layout of the motor is uncertain but given the length, might be a split single with a common combustion chamber. The supercharger, again with a plenum chamber appears to ‘blow’ into the crankcase, which would overcome the problem associated with the original motor. Again there is no evidence that the motor was run, but it does raise the question. ‘Where are they now’?

Thanks are due to John DeMott for the loan of John Duffield’s boat and engine and Peter Hill for the timely provision of photos detailing boats and engines from the Kings Lynn group.

On June 8th 2008 at Victoria Park, John Duffield's boat and engine were given a run for the first time in 35 years. With no information available about props and settings it was very much down to guesswork. The first run at around 45mph showed that there was not too much wrong and a second run some 10mph faster gives every prospect of the boat running successfully again in the future.

Thanks to Doug Etridge for the photo Left.

Postscript

On the 26th October 2008, at the Hydroplane Conference at Bletchley Park, Lynn Blowers was presented with the Peter Lambert Memorial Trophy for the fastest speed recorded during the season by a vintage hydroplane.