# Engine to shaft or thrust hp

Not sure if this is the right spot for this question but, does anyone know an equation to calculate engine hp to actual hp, taking into account shaft size and wheel pitch/size? My guess is those are the only factors other than constants. But any insight to this would be helpful. Thanks in advance. - Paddy

[QUOTE=Tugboat paddy;192063]Not sure if this is the right spot for this question but, does anyone know an equation to calculate engine hp to actual hp, taking into account shaft size and wheel pitch/size? My guess is those are the only factors other than constants. But any insight to this would be helpful. Thanks in advance. - Paddy[/QUOTE]

By “engine” horsepower I assume you mean brake horsepower (BHP) BUT what do you mean by “actual” horsepower? Do you mean shaft horsepower (SHP) or effective horsepower (EHP) or something else?

There isn’t a formula for what I think you are asking but you need to start with what is between the engine and the propeller. If you know the engine BHP and want to make an estimate of the SHP being delivered to the propeller you need to account for the efficiency of the items between same, like reduction gear (maybe 3%) but getting to an estimate of EHP is of value only for a given specific condition like say design sea speed and sea state.

The EHP would be based on things other than the propeller itself. Like frictional horsepower and wave making resistance which depends on the hull form (and condition) itself. Once a designer knows that (and applying a margin for hull fouling etc) they back into a required SHP and therefore engine size

What is it you are trying to accomplish? Are you running your propulsion engine at maximum continuous rating and not seeing the speed you thought you’d make?