Boat Propeller Advice and Tips

Why Buy a New Boat Propeller?

Propeller Performance

Pitch

• The angle of the blades is a good indicator of pitch.  The steeper the angle, the greater the pitch.
• Pitch inversely affects RPM level
• 2” of pitch is generally equal to 300-400 RPMs (2” increase in pitch =decrease of 300-400 RPMs).

Diameter

• Determine by measuring from the end of any blade to the center of the hub and multiplying by two.
• Diameter must be small enough to clear all engine and boat parts.
• Diameter is generally correlated to the size of the boat (small diameter prop with small boat, large diameter prop with large boat). 
• There are certain boat characteristics that will demand a different sized propeller than what their size may say.

Number of Propeller Blades

Three Blades

• Most popular for recreational boats.
• Offers better top end speed.
• Slower to plane

Four Blades

• Recommended for use with higher performance boats with high horse power outboards or I/Os.
• Faster hole shot and low end torque

Benefits of a 3 Blade Prop	Benefits of a 4 Blade Prop
Slightly higher top-end speed More pitch and style options Often lower priced than 4 blade props Slower to plane	Faster hole shot Better performance and fuel economy at mid-range RPMs Better holding power at rough seas Better low speed handling

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Materials

You have two options when deciding on the material for your boat propeller: aluminum or stainless.  Aluminum is less expensive and lighter, but stainless is stronger, has thinner blades and will last much longer.    

      

Aluminum	Stainless Steel
Most common and least expensive Generally painted with a protective coating Repairable, but will suffer from metal fatigue and loss of strength.	Improved performance due to thinner blades Five times as durable as aluminum Repairable at a high cost, but to like new condition Doesn't flex under pressure meaning it will stay optimally configured for a long period of time.
Benefits
Least expensive Satisfactory performance in most situations Weighs half as much as stainless steel Cheap to repair	Thinner blades won't flex under pressure More durable in abrasive environments so less likely to be damaged Although more expensive to repair, will be like new after
Challenges
Under high RPMs aluminum propellers flex, decreasing their pitch Thicker, thus creating more drag Wear badly in sandy situations Prone to damage from minor impacts	More expensive than aluminum equivalent Weigh twice as much as aluminum More effort to get spinning Fewer options available

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Parts

Hub

• Inner core of the boat propeller
• Central structure of hub is designed to fit a specific central drive shaft system.
• Slides over propeller shaft
• Hub is machined with splines that fit matching splines on the proper shaft
• Two different hub styles are available

Splines

• The teeth on the drive shaft and hub which mesh together.
• If the splines on the drive shaft don’t match the hub, the wrong boat prop is being used.
• The number of groves on a spline is its number of teeth.

 Removable Hub

• Hub and propeller are two separate pieces
• Multiple props will fit one hub
• Cost effective solution if props are changed for different conditions
• Hub kit includes all necessary hardware for installation.

Propeller Blades

• Attached to the hub and turn through the water, pushing the boat forward.
• Leading edge is edge that cuts through water, while the trailing edge is the back edge of the blade.

Cupping

• A slight curve at the trailing edge of the propeller blade to improve the prop’s ability to grip water.
• Allows for better hole shot and less slippage and ventilation.
• Reduced cavitation under heavy loads.
• Improved performance at high speeds
• Will decrease overall RPMs around 100-300 RPMs.

Rotation

• Right hand rotation is the standard for almost all applications
• Left hand rotation is used mainly for two engine applications to counteract torque and improve steering.

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Sizing

Satisfied with Current Propeller Performance?

• Boat motor make, model, year, and horsepower
• Current propeller’s diameter, pitch, material, and number of blades.
• Diameter and pitch are typically displayed together at various locations on the propeller and are listed with diameter first. (14x10 = diameter x pitch).

Not Satisfied with Current Propeller Performance?

If you're not satisfied with your current propeller the best way to determine what type of propeller is needed is the wide open throttle test.  Once it is known whether your boat has too much pitch or too little, you will be able to figure out which propeller will best fit your vessel.

The Wide Open Throttle Test

• All engines have a suggested RPM range for the engine to perform efficiently and safely at their wide open throttle (WOT).
• To find suggested RPM range consult our recommended RPM chart, look in your boats manual or ask the dealer or manufacturer.
• The right pitch will allow for the engine to run at or close to the suggested WOT range.
• In a safe area, run your boat at full throttle. 
• If the RPMs are higher than recommended, purchase a propeller with a higher pitch
• If the RPM is lower than recommended, purchase a propeller with a lower pitch

Common Prop Performance Problems

Ventilation

• Air from surface and gas from the engine get drawn into the propeller blades
• Propeller spins more rapidly without gripping the water
• Caused by very tight turns, a motor mounted too high, and an over trimmed engine.

Cavitation 

• An extreme reduction of pressure on the back side of the propeller blade causes water to boil.
• This causes damage such as metal erosion, pitting of the props, and damage to the leading edge, all reducing effective propeller thrust
• Incorrect propeller style for application, incorrect pitch, dings in the leading edge and poor design are the main causes.

Boat Propeller Care and Maintenance

For the sake of both performance and safety it's important to take proper care of your propeller and repair it regularly.  Not doing so will result in decreased pitched, possible damage to your engine and increased chance of cavitation. 

Boat Propeller Care

• Check propeller regularly for nicks, bent blades or any other damage.  
• Any distortion from normal can cause vibrations that can be harmful to the engine.
• Regularly disassemble your prop and make sure there is no fishing line, aquatic plant life, or anything else on the prop shaft. 
• After disassembling, apply a thin coat of waterproof lube to the splined area of the shaft before reassembling.
• Its best to have an extra propeller in case of emergency.

Propeller Repair

• Boat propellers can be repaired by repair shops.
• SavvyBoater.com does not repair boat propellers
• Stainless steel propellers cost more to repair but are like new after
• Aluminum propellers are less expensive to repair, but will not be as strong as when purchased.

Solas to OEM Cross Reference

BRP, Evinrude, OMC Stern Drive

Honda

Mercury, Mercruiser

OMC, VolvoPenta SX Drive

Suzuki

Yamaha

Solas to Michigan Wheel Cross Reference

BRP, Johnson, Evinrude, OMC Stern Drive

Honda, Volvo

Mercury, Mercruiser

Suzuki, Tohatsu, Nissan

Yamaha

Conclusion

• Finding the right pitch is the most important factor in achieving perfect propeller performance
• Use the wide open throttle test to determine which pitch is best for your boat
• Make sure that you don't run a damaged propeller for too long because it can damage your engine.

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