Power describes the rate of doing work. Torque is turning force. Engine speed is measured in revolutions per minute. They are related:
Power (kW) = Torque (N·m) × RPM ÷ 9550
One kilowatt is approximately 1.34 mechanical horsepower. Manufacturers may publish metric horsepower, brake horsepower or shaft power, so compare figures on the same basis.
The rated maximum output is not automatically the power an engine should deliver continuously. Manufacturers publish duty ratings, operating envelopes and power curves. A recreational high-output rating, intermittent rating and continuous commercial rating represent different allowable loads and operating times.
For a displacement yacht, more installed power does not remove hull-speed physics. Once wave-making resistance rises steeply, a large increase in power may produce only a small increase in speed. Reserve power still matters for head seas, wind, charging load, manoeuvring and maintaining control, but a larger engine adds weight, cost and installation demands.
At sea trial, a correctly matched installation should normally reach the manufacturer’s specified full-load rpm range with the vessel in the stated test condition. Failure to reach it can indicate over-propping, fouling, restricted fuel or air, exhaust back pressure, a control problem or engine wear. Overspeed may indicate under-propping or a tachometer error.
Continue with engine sizing and operating rpm and loading.