The Prop Shop





Selecting the Right Prop

The proper propeller size for your boat and engine combination is based on the wide open throttle (WOT) operating range for your particular engine. You can find this in your operator's manual, expressed in terms of a certain horsepower at a certain r.p.m.

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. where the maximum horsepower is produced.

Using your existing propeller, determine your maximum obtainable r.p.m.. If during this test, you begin to exceed the maximum rated r.p.m. of the engine, reduce the throttle setting. If the engine over-revs beyond the maximum recommended r.p.m., you need to increase the pitch of the propeller Increasing the pitch increment by 2" will result in approximately a 200-400 r.p.m. drop. Also, switching from an uncupped to a cupped propeller will reduce your r.p.m (see "Cupping" paragraph). The cupped propeller of the same pitch and diameter will typically reduce your r.p.m. by approximately 200. If you cannot reach maximum r.p.m., 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".

Once your WOT r.p.m. falls within the recommended range of the engine manufacturer, you have a propeller that is suited to your boat with respect to r.p.m. However, you may not be satisfied with your boat's skiing performance or trolling speed. It may be advisable in these circumstances to have multiple propellers, each to accommodate different boating activities. In all likelihood, more than one propeller will be suitable for your boat and motor combination, depending 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 acheive low-end power. It is imperative, however, that the WOT r.p.m. fall within the range specified by your engine manufacturer. If your engine is not able to reach this r.p.m. range, it's operating under an extremely loaded condition and premature failure is highly likely. Our Manager of Technical Services Bob Adriance would like to remind you that the wrong prop can wreck an engine. "I've talked to mechanics that think using the wrong prop is the single greatest cause of premature engine failure," says Bob.


  • About Props
  • Prop Sizing
  • Prop Quotation Form
  • Prop Repair
  • Skeg Repair
  • Cross Reference
  • SST Prop 
  • SST Pricing
  • SST Cross Reference
 Contact Us

Rayplex Limited,
341 Durham Crt.,
Oshawa, Ontario,
Canada L1J 1W8
PH: (905)579-1433
FX: (905) 579-1431

Update June-2002