Leyland Marathon...The "Nearly" Truck of The 1970s?

gingerfold:
You do realise that you are about to open up a new can of worms that will be meat and drink to our learned friend Carryfast . Co-incidentally I have just started reading a book about the British T Class submarine of WW2 and I did go to bed last night pondering about what designers consider when deciding on the power of the engines. The T Class boats weighed about 1100 tons fully fuelled and armed and they had two diesels of 1,250 bhp each, so a power to weight ratio of 2.27 bhp / ton, if that has any relevance at all, and that gave 15 knots surface speed. Torque figures aren’t quoted, thank goodness, so presumably it must all be about horse power with a marine engine? Then I suppose the diameter of the propeller(s) must come into the equation.

I think the torque is proportional to the slip speed of the propeller, so that means the boat engine does not need a high torque peak at low speed, like a lorry engine. You would have to specify the engine to suit the propeller size, like you choose the diff ratio of a lorry/car. I know bugger all about boats, so these are just my idle deductions.

The boat you describe goes well for its power output, considering the drag of the water on the hull. 2.27bhp/ton is the equivalent of a Gardner 5LW in a 44 tonner. I reckon such a vehicle would have to be geared for a top speed of about 30mph, so boats aren’t far behind.