The goal in propeller selection is to determine what style and size will maximize your boat's performance, while allowing your engine to operate in the recommended r.p.m. range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum r.p.m. at maximum horsepower.
If the engine over-revs beyond the maximum recommended r.p.m., you may need to increase the pitch of the propeller. Increasing the pitch increment by 1" will result in approximately a 150-200 r.p.m. reduction at wide open throttle (WOT). If you cannot reach maximum r.p.m. at WOT, then pitch should be decreased. These recommendations apply to single engine installations only. For most twin engine installations it is necessary to increase pitch by 4". Adding diameter will typically decrease rpm, while reducing diameter will raise the rpm.
Propeller selection will depend on your usage. Ski boats need more top end speed, and should choose a prop with a higher pitch. Cruisers and houseboats need more performance at displacement speeds, and should use a prop with a lower pitch to achieve low-end power. It is essential, however, that the WOT r.p.m. falls within the range specified by your engine. If your engine is not able to reach this r.p.m. range, it's operating under an extremely loaded condition.
If the propeller supplied with your engine is not providing desired performance follow these rules to help choose a different prop:
1. Consult owners manual to find recommended wide open throttle RPM range for your engine.
2. Determine current WOT RPM and speed using existing propeller. Vary trim to achieve maximum speed.
3. If existing propeller doesn't achieve RPM within specified range at WOT select larger or smaller pitch to achieve this.
-Adding 1 inch of propeller pitch will reduce WOT RPM by 150 to 200 RPM.
-Subtracting 1 inch of propeller pitch will increase WOT RPM by 150 to 200 RPM
4. A 4 blade propeller will usually generate 50-100 RPM less than an equivalent pitch 3 blade propeller.