Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 26Get 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 increasi ng wetted surface and red ucing boat speed"Hook" frequently is caused by supporting boat too far ahead of transom while hauling on trailer or durin 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 RoughnessMoss, barnaclesetc 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 th rough hull fasteners be coated with quality mari ne sealer at time of installationWater intrusion into the transom core andor inner hull wil resu lt in additional boat weight (red uced boat performance)hull decay and eve ntual structural failureCAVITATION Cavitation is caused by water vapor bubbles forming either from sharp edge or angle on the gear casefrom an irregularity in the propeller blade itself or from improper engine instal lation (too high)These vapor bubbles flow back and coll apse when striking the surface of the propeJ blade resulting in the erosion of the propeller blade surfaceIf allowed to continler ue, eventual blade failure (breakage) will occur