How The Steamboat Works

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2 responses to “How The Steamboat Works

  • Mike Kerfoot

    I’m interested to know more about the boiler – this is pre Trevithick’s work with ‘strong’ (i.e. High pressure) steam and so I’m guessing Rumsey’s boiler works on an atmospheric principle – the upward stroke of the piston being powered by the weight of the beam more than the pressure of steam. Is this correct?

    • jamesrumsey

      The ingenious Trevithick was beginning to pursue high-pressure steam around 1798, finally getting results 1805-1810, Oliver Evans at about the same time in 1804. Rumsey’s engine however was a hybrid, using the steam expansively to raise the piston, and then condensing it for the work stroke, like an atmospheric engine. He’d never seen a steam engine before, only looked at the Newcommen variety featured in his copy of Desagulier’s Mechanic Exercises. But like his rival John Fitch he must have heard about newer steam engines that did not need a weight to raise the piston and had an external condenser, and he seems to have set out to make something like that but simpler… using a brass turn cock instead of a poppet vale or drop valve, for example, and at least initially perhaps not even trying to come up with a valve mechanism to make it function automatically. Watt could have told him that using steam to raise the piston was wasteful of heat, but Watt wasn’t there to teach him, so Rumsey had to find out for himself that his first boiler- made from a cast iron wash pot- didn’t make enough steam. So, he had one of the local gunsmiths forge weld a pipe boiler for him. That both supplied enough steam and was stronger. The working beam was for an air pump, and feed pumnps- something that he must have heard about on Watt’s design. Although pulling down the vacuum can put a strain of a couple hundred pounds on the beam, it doesn’t actually lift the piston. Nice if it did.

      In practice, running the Experiment meant always balancing the boiler pressure against the condenser vacuum. A high boiler pressure would quickly raise the piston, but the denser steam was harder, making a slow work stroke. Lower boiler pressure would mean there was a good firm work stroke, when the condenser collapsed the steam, but the piston would only slowly rise for the next cycle. The happiest medium was reached when there was about 7 pounds/square inch steam pressure. That would cycle the engine pretty well, give us about 12 inches of mercury of vacuum ( and it was also about the same pressure that Boulton and Watt ran their boilers, coincidentally- though their engines had a steam cutoff, so the effective cylinder pressure and steam consumption was much lower) But it was not easy for us to get that for long. Either the feedwater pump would lag and the boiler would start to run hot, or it would run ahead and the water level would build up leave us waiting for all that H2O to go to steam. or the ever-sticky control valve would stick and need to be pulled apart , cleaned and greased again. Of course we knew all this, knew we could convert the engine to being a double-acting atmospheric or higher-pressure engine, knew we could change to a drop valve that didn’t stick, and knew that a twin-cylinder pump would be far better than Rumsey’s single one…but the task was to find out what Rumsey’s boat did, not improve on it.

      Rumsey’s later steam engines, by the way, were also hybrids. Either he never really understood heat, or he was just wary of running afoul of the vigilant Boulton and Watt patents. Though these later designs replace all the working beam- and lever-operated cylinders of the normal beam engines with nesting cylinders, saving a lot of space, in their drawn configuration they waste heat.

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