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.

The truth is all of these frustrating problems could stem from a quick fix, your propeller size! By adding the correct prop to your vessel you can essentially eliminate all 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 R.P.M. The target in selecting a propeller size is to figure out which style and size will maximize your vessels performance, while allowing your engine to operate within its recommended r.p.m. range. The correct prop will not allow your engine to over-rev, yet allow it to reach the minimum r.p.m. Where the maximum horsepower is produced, with ideal engine loading.

Over-revving test

• With existing propeller figure out max rpm.
• 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 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.
• 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.

If performance is not up to par

• If you are not satisfied with your boats 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 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.
• 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 alignment 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

Ventilation occurs when surface air or exhaust gasses are drawn into the propeller blades. When this happens, boat speed is lost and engine r.p.m. climbs rapidly. This can result from excessively tight cornering, a motor that is mounted very high on the transom, or by over-trimming the engine.

Cavitation

Cavitation (which is 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 to close to the propeller and others. (Research provided by BoatUS organized by Corsica River Marine Surveys)

Travis L. Palmer SAMS® AMS®, ABYC,

References
“Selecting the Right Prop.” BoatUS News. N.p., Apr. 2012. Web. 09 Sept. 2013.