The Swaffham Bulbeck Gondola

The chassis is built from 4” x 2” timber which once held up the roof of a conservatory. The actual hull is of clinker built construction which can be lifted off for maintenance access. It runs on four invalid carriage wheels with pneumatic tyres – and yes! we had a spare wheel!

Steering is by tiller operating a correctly configured “Ackerman” linkage on the rear wheels. Stub axles, king pins and so on were fabricated in my workshop, the king pins running in bushes carried by four small castings originally sold as hand-rail supports.

The front wheels are driven by two permanent magnet D.C. motors originally designed for the ill-fated Sinclair C5 tricycle. The Tricycle was rubbish but the motors were superb. Made in Italy they are a masterpiece of production engineering having only about eight or ten separate parts. They are very powerful and take 30Amps @ 12vD.C. when delivering full power. (This means that at full load the two motors were delivering just short of one Brake Horse Power). The output shaft of each motor carries a drive bush which bears directly on the rim of the tyre in a reciprocal version of the old fashioned type of bicycle dynamo. The gives a huge gear reduction but is, above all else, silent in operation. A clanking gear driven drive would have destroyed the illusion of a gliding Gondola and the chosen solution is regarded as a major design success.

An electronic box of tricks of our own design controls the motors. A 50kHz oscillator provides a square wave output and the mark:space ratio can be varied. This signal drives a pair of MOSFET power transistors which feed the motors. Thus, the motors receive a full 12v on each half cycle providing maximum torque but the variable signal from the MOSFETs interrupts the current waveform according to the mark:space ratio setting. This therefore governs the actual power delivered to the motor and, in practice, against a constant load, this provides a speed control.

A large relay provides for forward/reverse switching. A remote control box on the tiller arm is fitted with the forward/reverse selector and speed control. A dead-mans-handle is also fitted so that power is removed totally in case the Gondolier falls overboard. There are no brakes as the outfit comes to an immediate halt when power is removed due to the inertia of the motor armatures when “seen” through a large gear reduction.

Where possible the whole thing was engineered from the contents of various engineers' scrap-boxes to keep down cost. The chassis has appeared in the Swaffham Bulbeck shows three times – twice underneath a Gondola and once under a parody of a Victorian steam launch in a production of H.M.S. Pinafore.

Much fun was derived from its development and construction!

Philip Upton, Burwell, June 2004

 

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