There is no universal “80 per cent throttle” rule. Use the engine maker’s rated-speed range, duty cycle and propeller guidance, then validate with the installed boat.
A healthy cruising point usually allows the engine to reach normal temperature, avoids continuous smoke and leaves useful rpm reserve. If the boat cannot reach the specified full-load rpm with a clean hull and normal loading, do not simply cruise harder. Diagnose propeller match and engine condition.
Long periods of very light load can promote low temperatures, deposits and cylinder glazing in some engines. Prolonged full output can exceed a recreational rating. The correct remedy is not periodic punishment at maximum throttle. It is a duty pattern within the manual and a propeller that matches the engine.
Electrical generation changes load. A large alternator can impose several kilowatts of mechanical demand after conversion losses, especially just after starting with a depleted battery bank. Monitor belt temperature, alternator temperature and engine behaviour in upgraded systems.
Record clean-bottom performance at several rpm points. A later loss of speed at the same rpm suggests hull, propeller or environmental change. A loss of attainable rpm suggests load, fuel, air, exhaust, control or mechanical change.