Mercury Mariner 8/9.9HP 4-Stroke Factory Service Manual, Page 29Get this manual
Propeller Selection For in-depth information on marine propellers and boat performancewritten by marine engineerssee your Authorized Dealer for the illustrated "What You Should Know About Quicksilver Propellersand Boat Performance Information" (Part No90-86144)For best all around performance from your outboard boat combination, select propeller that allows the engine to operate in the upper half of the recommended full throttle RPM range with the boat normally loaded (refer to Specifications)This RPM range allows for better acceleration while maintaining maximum boat speedIf changing conditions cause the RPM to drop below the recommended range (such as warmer, more humid weather, operation at higher elevations, increased boat load or dirty boat bottomgear case) propeller change or cleaning may be required to maintain performance and ensure the outboard's durabilityCheck full-throttle RPM using an accurate tachometer with the engine trimmed out to balanced-steering condition (steering effort equal in both directions) without causing the propeller to "break loose"Refer to "Quicksilver Accessory Guide" for complete list of available propellers1Select propeller that will allow the engine to op-