2001 Mercury Mariner 50-60HP Factory Service Manual, Page 36Get this manual
GENERAL INFORMATION
1Hook: Exists when bottom is concave in fore-and-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 storage2Rocker: 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 porpoise3Surface Roughness: Moss, barnacles, etc on boat or corrosion of outboard's gear housing increase skin friction and cause speed lossClean surfaces when necessaryWATER ABSORPTION It is imperative that all through hull fasteners be coated with quality marine sealer at time of installationWater intrusion into the transom core andor inner hull will result in additional boat weight (reduced boat performance), hull decay and eventual structural failureCAVITATION Cavitation is caused by water vapor bubbles forming either from sharp edge or angle on the gear case, from an irregularity in the propeller blade itself or from improper engine installation (too high)These vapor bubbles flow back and collapse when striking the surface of the propeller blade resulting in the erosion of the propeller blade surfaceIf allowed to continue, eventual blade failure (breakage) will occur