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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.