1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 16Get this manual
For boaters to realize optimum engine performance under changing weather conditions, it is essential that the engine have the proper propeller to allow it to operate at or near the top end of the recommended maximum RPM range at wide-open-throttle with normal boat loadNot only does this allow the engine to develop full power, but equally important is the fact that the engine also will be operating in an RPM range that discourages damaging detonationThis, of course, enhances overall reliability and durability of the engine (3 If excessive, can make the boat veer left and right (bow steer)2Boat Bottom: For maximum speed, boat bottom should be nearly flat plane where it contacts the water and particularly straight and smooth in fore-and-aft directionaHook: Exists when bottom is concave in foreand-aft direction when viewed from the sideWhen boat is planing, "hook" causes more lift on bottom near transom and allows bow to drop, thus 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 fore-and-aft direction when viewed from the side, and boat has strong tendency to porpoisecSurface Roughness: Moss, barnacles, etc on boat or corrosion of outboard's gear housing increase skin friction and cause speed lossClean surfaces when necessary3Gear Housing: If unit is left in the water, marine vegetation may accumulate over period of time in certain types of waterThis 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 overheat