Propeller FAQs

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  1. How do I determine what is the best Michigan or Federal propeller for my boat?
  2. Can one propeller be best for both maximum boat speed and maximum pulling power, say when pulling a skier or fish nets?
  3. Although my engine(s) are rated to turn at a maximum RPM of X, for example 4,400 RPM, I never run our boat at wide open throttle (WOT). Wouldn’t I be faster or more efficient at cruise speed Y, for example 30 knots, with a larger propeller than suggested? This would lower my cruise RPM Z, for example from a current 3,200 RPM down to 3,000 RPM.
  4. You suggested a 13” diameter x 14” pitch propeller for my boat. Would a smaller diameter propeller with more pitch provide the same performance?
  5. What is the minimum allowed clearance between the propeller blade tips and hull bottom? How much space should exist between the propeller and strut or keel?
  6. I have a right-hand propeller but need a left-hand. Can it be changed?
  7. How can I tell if I need a left or right-hand propeller?
  8. My boat is not as fast as I expect it should be though I’ve tried several types of propellers. Why?
  9. For a twin screw vessel, should the propeller rotation be inboard or outboard?
  10. I run on lakes at elevations over 1 mile high. Should I adjust propeller size accordingly?
  11. To change from a 3-blade propeller to a 4 blade propeller, what diameter or pitch modifications must be made to result in the same engine performance?
  12. On a sailboat under sail, should the propeller be rotating or locked when the engine is not operating?
  13. A propeller with a diameter larger than will fit under my boat was suggested. Can a smaller diameter still be used without hurting my performance significantly? What other adjustments to the propeller should be made to compensate?
  14. My propeller has severe pitting and I was told it was caused by electrolysis. What could be the cause and how do I prevent it in the future?
  15. How much can you change the propeller pitch for an existing propeller?
  16. Should I have my propeller dynamically balanced?
  17. What is a “cupped” propeller and what benefits does it offer?
  18. Can I use heat to install or remove my propellers?

How do I determine what is the best Michigan or Federal propeller for my boat?

Your dealer, service center, and our distributors should be able to help in obtaining a free propeller analysis and review. They will have a form to complete with relevant information needed to properly evaluate your vessel. The key to obtaining a good propeller recommendation is providing accurate and current information on the boat, engine(s), existing propellers, and current performance. Don’t guess on answers! Also, it’s important to have actual maximum engine RPM and top boat speed values in order to determine what may be a more optimum propeller style and size.

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Can one propeller be best for both maximum boat speed and maximum pulling power, say when pulling a skier or fish nets?

In general, no. If maximum pulling power is desired, the propeller size, particularly the pitch value, is often reduced to allow higher engine RPM (where more power is produced) at lower boat speeds. For maximum top boat speed, the optimum propeller pitch value will typically be higher.

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Although my engine(s) are rated to turn at a maximum RPM of X, for example 4,400 RPM, I never run our boat at wide open throttle (WOT). Wouldn’t I be faster or more efficient at cruise speed Y, for example 30 knots, with a larger propeller than suggested? This would lower my cruise RPM Z, for example from a current 3,200 RPM down to 3,000 RPM.

Not really. Increasing the propeller size may reduce the cruise RPM for the same cruise speed but will also increase the load on the engine and therefore increase the fuel usage needed. In effect, you may reduce the cruise RPM but you burn the same amount of fuel and you increase the stress on the engine. More important, if a propeller size is chosen that prevents the engine(s) from reaching their recommended maximum rated RPM at WOT, there is a potential risk of reduced engine life and other overload results such as high exhaust temperatures, excessive smoking, etc.

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You suggested a 13” diameter x 14” pitch propeller for my boat. Would a smaller diameter propeller with more pitch provide the same performance?

A propeller with smaller diameter and larger pitch might provide the same load on the boat’s engines but may not have the same efficiency as the suggested propeller. Propeller diameter is chosen to be optimal for the boat’s particular combination of horsepower, RPM and speed. Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.

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What is the minimum allowed clearance between the propeller blade tips and hull bottom? How much space should exist between the propeller and strut or keel?

The clearance between the propeller blade tips and the hull bottom should be at least 15% of the propeller diameter and ideally, 20% or more. So for example, a 20” diameter propeller would have 3” minimum clearance and better still 4” or more. A common guideline for recommended propeller to strut or keel clearance is 20% of propeller diameter measured between the propeller blade edge and strut leg or keel. This is often measured at a point on the propeller blade edge about 70% of the distance from the shaft centerline to the blade tip.

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I have a right-hand propeller but need a left-hand. Can it be changed?

Sorry, no. Propeller hand (rotation) cannot be changed. A right-hand propeller is a mirror image of a left-hand propeller. Even if it were possible to mount the propeller backwards on the propeller shaft (not possible on tapered shafts), the propeller would still have the blades angled for the originally designed rotation.

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How can I tell if I need a left or right-hand propeller?

Stand astern of the boat (behind the hull) looking forward. If the propeller shaft turns clockwise, the correct propeller will be right handed. Counter clockwise – left hand propeller.

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My boat is not as fast as I expect it should be though I’ve tried several types of propellers. Why?

The propeller, while a very important part of the system that moves your boat, isn’t the only factor contributing to achieving maximum boat speed. 
The main factors influencing top speed are hull design, weight, trim, and bottom cleanliness; engine horsepower and condition; and propeller style, size, and condition. 
For the majority of cases where actual top speed is significantly below the normal or expected speed, the causes tend to be increased boat weight, adverse trim, or engine problems. Hull fouling can, of course, be another cause of decreased performance. Other than damage or too many recondition cycles, propellers tend to be extremely consistent over time.

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For a twin screw vessel, should the propeller rotation be inboard or outboard?

Twin engine boats typically have a right hand propeller on the starboard engine and a left hand propeller on the port engine. So, when viewed from the stern forward, the propeller rotations would be outboard looking at the tops of the propellers.

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I run on lakes at elevations over 1 mile high. Should I adjust propeller size accordingly?

Yes. Gasoline engines lose power when operated above elevations over 3000 ft. A propeller with the pitch reduced will compensate somewhat by allowing the engine to reach the desired RPM operation point. The overall vessel speed will be reduced though because the engine is not able to develop as much horsepower due to the reduced air density at elevation. If you frequently operate on different waters with significantly varying elevations, it may be necessary to change the propeller for each condition. An inboard ski boat used on the coast then hauled to mountain inland lakes is a good example.

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To change from a 3-blade propeller to a 4 blade propeller, what diameter or pitch modifications must be made to result in the same engine performance?

The best solution is to have a free propeller analysis performed with the 3 and 4 blade propeller sizes accurately calculated.
 For many vessels, the same diameter propeller is used but the 4 blade propeller pitch is reduced by 1” compared to the 3 blade.

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On a sailboat under sail, should the propeller be rotating or locked when the engine is not operating?

Numerous studies suggest the propeller drag is reduced by allowing the propeller to free wheel. Note, it is important to check with your transmission manufacture to insure the transmission can be placed in neutral while the engine is off. Some transmissions may suffer damage due to a lack of lubrication without the engine running. The following is a link to an interesting article regarding fixed vs. free propeller operation.
http://www.catamaransite.com/propeller_drag_test.html

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A propeller with a diameter larger than will fit under my boat was suggested. Can a smaller diameter still be used without hurting my performance significantly? What other adjustments to the propeller should be made to compensate?

If the difference between the optimal and allowable diameter is minimal, say 1 or 2 inches for a medium size pleasure boat, a corresponding increase in propeller pitch will be effective at maintaining reasonable propeller performance. For some situations, it may be suggested to increase blade number, say from 3 to 4, to provide more blade area on the diameter constrained propeller. For some rare cases, particularly on re-power projects, a engine-gear combination is selected with the wrong gear reduction ratio. This can result in a significant propeller selection problem with no real solution other than changing to a different gear reduction ration.

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My propeller has severe pitting and I was told it was caused by electrolysis. What could be the cause and how do I prevent it in the future?

Pitting on the propeller blade surfaces is usually caused either by electrolysis or galvanic corrosion. Both causes are related in that they result because of two dissimilar metals being located next to each other.

Galvanic corrosion occurs when two dissimilar metals are submersed in an electrolyte, such as sea water and the less noble metal begins to dissolve. The prevention is to have adequate sacrificial anodes, such as zincs, installed on the boat.

Electrolysis is similar but typically caused by stray electrical current. This may be generated in the vessel or may be caused by stray current in a marina. For this type of problem, there is a variety of solutions, depending upon the root cause. Sacrificial anodes will help protect the propellers but the source of the stray current should be identified and corrected.

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How much can you change the propeller pitch for an existing propeller?

In general, Michigan Wheel suggests the original design pitch be changed no more than 2”, either increased or decreased. The actual amount will depend upon the original propeller’s size, design, and material plus the ability of the shop performing the pitch adjustment. If excessive adjustments are attempted, it is possible to develop adverse stresses in the propeller blades that could impact the durability of the propeller. In addition, the efficiency of the propeller may be reduced.

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Should I have my propeller dynamically balanced?

All propellers are statically balanced when manufactured or properly repaired. Dynamic balancing uses a special machine to rotate the propeller and measure the amount of imbalance. In general, dynamic balancing insures the amount of imbalance is kept to a minimum or within a specified tolerance. It does not guarantee a propeller will perform without vibration though since there are a number of factors not relating to propeller balance that may cause vibration. Also, providing the static balance equipment is of good quality and properly maintained, statically balanced propellers will usually be just fine.

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What is a “cupped” propeller and what benefits does it offer?

A cupped propeller means the blade trailing edges are formed into a curved shape. In appearance, it looks as if the trailing edge was bent upwards into a small radius. Trailing edge cup is incorporated into propeller designs to improve their efficiency when operated on higher speed boats and/or for propellers for turning at high RPM. Typical use might be for example express cruisers with gasoline engines operating at speeds over 30 knots. The propeller will be operating in a cavitating condition. The trailing edge cup allows the cavitating propeller blade to generate more thrust, thus improving propeller efficiency.

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Can I use heat to install or remove my propellers?

Heat should not ever be required to install a propeller, see our Propeller Installation Guide for additional information. Propeller removal is best performed using a suitable propeller puller. There are a variety of puller systems available and most service yards have them. Occasionally, some service yards will heat the propeller hub for certain large commercial vessels. This can be problematic though because too much heat may damage the Bronze propeller material. It has also been reported rubber or synthetic bearings have been damaged due to heat applied at the propeller-shaft connection. For these reasons, we therefore do not recommend applying heat to the propeller during removal.

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