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A high proportion of the steam engines used by UK steamboat modellers originated from just two manufacturers, Cheddar Models and Stour Valley Steam (SVS). All SVS engines, and most of those sold by Cheddar have been of the relatively uncomplicated oscillating variety. In this type of engine, the connection to the crankshaft causes each cylinder to swivel on its axis as the piston moves up and down inside it. This swivelling or oscillating movement between the two accurately machined surfaces of the cylinder and the faceplate against which it is held tightly by a spring, opens and closes holes drilled in the faceplate, and these holes allow steam to enter alternately into the top and bottom of the cylinder, or above and below the piston, and it is this alternately applied steam pressure that causes the piston to move up and down. These oscillating steam engines might sound rather rudimentary from a technical point of view, but at least as far as those from Cheddar and SVS are concerned, they certainly shouldn't be dismissed as toys or 'not proper steam engines'. Although admittedly quite simple devices, once correctly set up, they are more than powerful enough for our requirements, as well as being very reliable and long-lasting. Maximum operating pressure for these engines is 45lbs per square inch or 3 Bar, but the normal running pressure for a reasonable balance between performance and duration would be around 25lbs or 2 Bar. Oscillating engines like these, where the steam is used to move the piston in both directions are called double-acting, and all the engines produced by Cheddar and SVS are of this type. They respond well to a throttle control, and all that is needed for this is a single servo connected to the steam valve, which provides fully proportional speed control. This works both forward and reverse in the case of their twin cylinder engines, which are also self-starting, something that isn't normally possible with single cylinder engines. All SVS steam engines are supplied assembled and ready to run, but with Cheddar's products, although they were available factory assembled, the buyer also had the option in most cases of buying a set of finished components for self assembly. This was a quite a simple exercise for which little more than a couple of screwdrivers and a spanner (wrench) or two are needed. There was of course an appreciable cost saving if you assembled the engine yourself, and if all the instructions were followed, the finished steam plant should be virtually indistinguishable from the ready to run factory product in every respect. Modellers are blessed with varying degrees of skill though, and some self-assembled Cheddar engines will probably be rather better than some others, which is something that may be worth bearing in mind if you are offered a second hand self-assembled example.

Nearly all of the full size steam engines used in ships, railway engines and the like over the years, have been the rather more complex slide valve variety, though massive oscillating steam engines were used in some early paddle steamers. Their huge cylinders combined with relatively low boiler pressures made them somewhat inefficient, though as many thousands of modellers can confirm, this type of steam engine does work very well indeed when reduced to model size. In addition to their range of oscillators, Cheddar also produced two slide valve steam engines with the names Gemini and Proteus. These were a lot more complicated, much more powerful, and inevitably also considerably more expensive. Because they are so much more complex than oscillators, all were supplied factory assembled and ready to run. Both Gemini and the slightly larger Proteus are powerful enough to cope with the largest models, and either will easily power hulls up to six feet or two metres in length. Slide valve engines work at higher steam pressures. They will run smoothly on pressures of up to 90lbs or 6 Bar, though as with oscillators, to achieve a reasonable balance between performance and duration, lower pressures of 40 to 60lbs (3 to 4 bar) would be the usual range. But which engines will be suitable for which models? Well it's hard to generalise, as much depends on the weight of the model and the level of performance required, but smaller oscillators like the Heron and Vale from SVS and the Pintail and Plover from Cheddar, will quite happily power hulls up to 40 inches or one metre in length, or even a little more in the case of light fine-lined models like steam launches. The SVS Snipe and the Puffin and Pelican from Cheddar will provide adequate power for larger steam launch type hulls of around 50 inches or one and a half metres long, though for heavier displacement hulls like tugs, these suggested hull lengths should be reduced by about one third. Cheddar and Stour Valley Steam have not been the only UK manufacturers of model marine steam engines, but they are the only ones mentioned here, for the simple reason that they are the only ones that I have any real practical experience of. I have no connection with either manufacturer, other than being an enthusiastic user of some of their products.

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