Why am I not reaching my maximum-rated rpm? Why is my engine over-revving? Why am I not getting the top-end speed I need for water skiing? Why is my low-end power low? These could be some of the questions you are asking your local marine mechanic or SAMS surveyor. All of these frustrating problems could stem from one issue: your propeller size. By adding the correct prop to your vessel, you can essentially eliminate all of the problems listed above. Although most of these problems are associated with outboards, they can pertain to inboards as well. The proper propeller size for your vessel 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 rpm. The target in selecting a propeller size is to figure out which style and size will maximize your vessel’s performance, while allowing your engine to operate within its recommended rpm range. The correct prop will not allow your engine to over-rev, yet it will allow it to reach the minimum rpm, where the maximum horsepower is produced with ideal engine loading.

Over-revving test

With the existing propeller, figure out max rpm. If the engine over-revs beyond the maximum recommended rpm, you may need to increase the pitch of the propeller. Increasing the pitch increment by two inches will result in approximately a 200- to 400-rpm drop. Also, switching from an uncupped to a cupped propeller will reduce your rpm. The cupped propeller of the same pitch and diameter will typically reduce your rpm by approximately 200. If you cannot reach maximum rpm, 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 four inches. Once your WOT rpm falls within the recommended range of the engine manufacturer, you have a propeller that is suited to your boat with respect to rpm.

If performance is not up to par

If you are not satisfied with your boat’s skiing performance or trolling speed, it may be a good idea in these circumstances to have multiple propellers to accommodate the different boating activities. Ski boats need more top-end speed; therefore, they 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 imperative, however, that the WOT rpm falls within the range specified by your engine manufacturer. If your engine is not able to reach this rpm range, it’s operating under an extremely loaded condition and premature failure is highly likely. Wrong props can wreck engines.

Fixes for outboard prop problems

Usually the best way to deal with a propeller issue with an outboard is to get a new prop if it’s damaged or if you’re not sure you have the correct prop, try on different ones, with the recommendation of a qualified dealer, until you’ve got it right.

Fixes for inboard prop problems

With inboards, typically, you’ll need to have the propeller(s) pulled and sent to a good prop shop so that they can work their magic on your existing prop to repair it or determine that you need another. Shops check balance, check align- ment of blades, add or remove “cup,” and many other adjustments as needed.

Words to remember

Prop size: The size of a propeller is defined with two sets of numbers, diameter and pitch, with pitch always following the diameter. Diameter: The diameter is two times the distance from the center of the hub to the tip of the blade. It can also be looked at as the distance across the circle that the propeller would make when rotating. Pitch: Pitch, the second number listed in the propeller description, is defined as the theoretical forward movement of a propeller during one revolution. Since there is almost always a small amount of “slip” between the propeller and the water, the actual distance traveled is slightly less. Cupping: Many of today’s propellers incorporate a cup at the trailing edge of the propeller blade. This curved lip on the propeller allows it to get a better “bite” on the water, resulting in reduced ventilation and slipping, and allows for quicker acceleration or “hole shot” in many cases. A cupped propeller also works well in applications where the motor can be trimmed so that the propeller is near the surface of the water. The cup will also typically result in a higher top-end speed.

Problems that could occur

Ventilation occurs when surface air or exhaust gasses are drawn into the propeller blades. When this happens, boat speed is lost and engine rpm climbs rapidly. This can result from excessively tight cornering, by a motor that is mounted very high on the transom, or by over-trimming the engine. Cavitation (often confused with ventilation) is a phenomenon of water vaporizing or “boiling” due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller’s blade surface due to the collapse of microscopic bubbles on the blade. There may be many causes of cavitation, such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, water flow obstruction caused by parts of the boat’s hull or running gear too close to the propeller and others. Research provided by BoatU.S. "Selecting the Right Prop" (boatus.com/boattech), edited by Travis L. Palmer, marine surveyor associate with the SAMS and owner of Corsica River Marine Surveys.