First separate engine rpm, engine power and boat speed.
- Normal rpm but low boat speed points towards propeller fouling, damage, current, hull fouling, clutch slip or inaccurate instruments.
- Engine cannot reach normal rpm with black smoke suggests too much load, insufficient air or restricted exhaust.
- Engine cannot reach rpm with little smoke can suggest inadequate fuel delivery or a control that is not reaching full travel.
- Rpm hunts or falls after several minutes often points to fuel restriction, tank venting or air entry.
Check the hull and propeller condition, gear selection and control travel. Compare no-load and loaded rpm only within manufacturer rules. Inspect filter restriction, water separator, fuel shut-offs, vent, air intake, charge-air hoses and exhaust elbow.
A dirty bottom can transform engine behaviour. On a twin-engine catamaran, compare rpm, fuel flow, temperatures and exhaust appearance at the same conditions. The healthy side becomes a powerful reference, but propellers and instruments must be comparable.
Do not compensate with more fuel or altered governor settings. Injection calibration, turbo tests, compression tests and exhaust back-pressure measurement belong with competent technicians using the specified method.