Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 22Get this manual
Accompanying this weather-inspired loss of power is second but more subtle lossAt rigging time in early springthe engine was equipped with propeller that allowed the engine to run within its recommended RPM range at full throttleWith the coming of the summer weather and the consequent drop in available horsepower, this propeller willin effect, become too largeConsequently, the engine operates at less than its recommended RPMDue to the horsepowerRPM characteristics of an engine, this wi ll result in further loss of horsepower at the propeller with another decrease in boat speedThis secondary loss can be regained by switching to smaller pitch propeller that allows the engine to run again at recommended RPM To obtain optimum engine performance under changing weather conditions, the engine MUST be propped to allow it to operate at or near the top end of the recommended maximum RPM range at wide-open-throttle with normal boat loadThis will allow the engine to develop full power while operating in an RPM range that discourages damaging detonation boat is planing"hook causes more lift on bottom near transom and allows bow to dropthus greatly increasing wetted surface and reducing boat speed Hook" frequently is caused by supporting boat too far ahead of transom while hauling on trailer or during storagebRocker: The reverse of hook and much less common"Rocker" exists if bottom is convex in foreand-aft direction when viewed from the side, and boat has strong tendency to porpoise Surlace Roughness: Moss, barnacles, etc on boat or corrosion of motor's gear housing increase skin friction and cause speed lossClean surfaces when necessaryGear Housing: If unit is left in the water, marine vegetation may accumulate over period of timeThis growth MUST be removed from unit before operation, as it may clog the water inlet holes in the gear housing and cause the engine to overheatd