Being a deckie who sailed with two triple-expansion steam engines and having spent a little time in the E/R my memory when maneuvering was the speed was controlled by the valve to the engine from the boilers. Full speed was about 60 rpm and half 30 rpm slow about 15 and dead slow about ticking over. There was a small steam engine that changed cams. I may be wrong but it was 67 years ago
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