Now moving one rung further up the evolutionary tree the obvious next step is tom stick two slide valve cylinders together and get a vertical twin slide valve engine.
Stuart make a popular but somewhat expensive model in their twin 10V and you will find a number of similar engines around from other manufacturers. In reality the twin slide valve engine was used in very early marine applications but was very soon superseded by the advent of the compound engine. The basic vertical twin supplies steam to both cylinders evenly and both exhaust to a common manifold. In a compound engine the exhaust from the first stage is then used to drive the second stage piston. This has the effect of getting considerably more of the energy from the steam and makes the engine more efficient. This was then progressed to a triple expansion compound engine which provided marine propulsion for many years during the first half of the 19th century.
A compound engine is very easy to spot as the cylinders increase in size as the steam is expanded from one cylinder to the next. The tops of a triple expansion are quite distinctive in their size progression,
The model is manufactured by a German company by the name of Hansen who make a very nice range of brass engines. The standard twin is self starting but not reversible and with this type of engine it is not simply a case of putting steam in the other way as the slide valve has now allowed us to admit steam at just the right time of the cycle and shut it off just as we want as well. To reverse a slide valve engine you have to use a complicated reversing mechanism which does not lend itself very well to smooth model operation. For a start it requires two servo’s, one for the reversing lever and one for the speed control so it is not the smoothest of operations and the reversing mechanisms seem prone to sticking.
The solution, as with this example, is to reverse the propeller externally so this engine is destined for an open launch model coupled to a controllable pitch propeller. This means that the engine can be turning at the same speed, although I am probably going to control that as well so as not to waste steam, and the reversing process will be controlled by reversing the propeller pitch. It took me a while but I eventually came up with an example of a Controllable pitch prop which will suit this job perfectly. Hopefully the engine will tick away nice and smoothly and the boat will go backwards and forwards without the engine changing speed or direction. Should prove interesting.
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“Dirty British coaster with a salt-caked smoke stack, Butting through the Channel in the mad March days" |