- Should I select a 3-blade propeller or a 4-blade propeller for my OBSD engine?
- What's the advantage of a left-hand propeller for my OBSD engine?
- Should I get an aluminum or stainless steel propeller for my OBSD engine?
- Why do outboard motors of the same power sometimes take different propeller sizes?
- How can I be sure my motor is operating within the recommended RPM range?
- Will a different propeller correct bad torque action (listing and hard steering)?
- What is the correct transom height for my outboard engine?
- I have a twin engine outboard. Can I get a propeller of opposite rotation and run one motor in reverse?
- What is the best tilt-setting or shaft angle?
- Is it advisable to have outboard propellers repaired?
- What is the purpose of the rubber cushion hub in an outboard propeller?
- The rubber hub in my propeller seems to be slipping. Is this possible?
- My outboard seems to vibrate excessively, yet the propeller hardly shows signs of use. Why?
- Are there advantages to plastic propellers?
- Is an adjustable pitch OBSD propeller possible or satisfactory?
- Can I troll better with a different sized wheel?
- Can a different propeller help me in water skiing?
- Why would I change my OBSD propeller?
- Will a propeller change help my I/O installation?
- Why won't my sterndrive perform as well as another sterndrive of comparable length, beam, weight, and I/O installation?
- Why do I shear so many pins without apparently hitting anything?
- Does it make any difference where the outdrive is vertically positioned on my sterndrive?
- My houseboat plows when the sterndrives are tilted to the furthest lock bar position. In an attempt to correct the bow-down attitude, the propellers cavitate. Why?
- What is the difference between thru-hub exhaust, over-hub exhaust, and non thru-hub exhaust?
- How do I determine what is the best Michigan or Federal propeller for my boat?
- Can one propeller be best for both maximum boat speed and maximum pulling power, say when pulling a skier or fish nets?
- 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.
- You suggested a 13” diameter x 14” pitch propeller for my boat. Would a smaller diameter propeller with more pitch provide the same performance?
- 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?
- I have a right-hand propeller but need a left-hand. Can it be changed?
- How can I tell if I need a left or right-hand propeller?
- My boat is not as fast as I expect it should be though I’ve tried several types of propellers. Why?
- For a twin screw vessel, should the propeller rotation be inboard or outboard?
- I run on lakes at elevations over 1 mile high. Should I adjust propeller size accordingly?
- 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?
- On a sailboat under sail, should the propeller be rotating or locked when the engine is not operating?
- 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?
- 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?
- How much can you change the propeller pitch for an existing propeller?
- Should I have my propeller dynamically balanced?
- What is a “cupped” propeller and what benefits does it offer?
- Can I use heat to install or remove my propellers?
Should I select a 3-blade propeller or a 4-blade propeller for my OBSD engine?
We recommend 3-blade propellers for recreational boats with 3, 4, and 6 cylinder outboards and I/O engines. These propellers provide good “hole shot” and top-speed performance.
We recommend 4-blade propellers for bass boats and boats with high performance hulls running high horsepower outboard engines. Compared to 3 blades, they provide better “hole shot” performance with less steering torque and less vibration at high speeds.
What's the advantage of a left-hand propeller for my OBSD engine?
Two propellers spinning the same direction on twin engine boats will create steering torque. In other words, two right-hand propellers pull the stern hard to the right and the bow to the left.
Two opposite-direction propellers on twin engines eliminate this steering torque because the left-hand propeller balances out the right-hand propeller. This results in better straight-line tracking and helm control at high speed.
Should I get an aluminum or stainless steel propeller for my OBSD engine?
Most pleasure boats are factory equipped with aluminum propellers. Aluminum propellers are relatively inexpensive, easy to repair, and under normal conditions can last for many years.
Stainless Steel is more expensive, but much stronger and durable than aluminum.
Why do outboard motors of the same power sometimes take different propeller sizes?
This is due to differences in lower unit gear ratios. Stock outboards are geared so that the propeller shaft turns at a slower speed than the RPM at the powerhead. This is usually expressed as a ratio such as 12:21 or 14:28, referring to the number of teeth in the drive gears. In the first example, the crankshaft gear has 12 and the propeller shaft gear has 21. This means the propeller shaft turns only 57% as fast as the indicated RPM at the powerhead.
The lower the gear ratio, the larger the propeller that can be used and vice versa.
How can I be sure my motor is operating within the recommended RPM range?
This can only be checked with a tachometer. There are various kinds commercially available.
Will a different propeller correct bad torque action (listing and hard steering)?
Not usually. Most likely it is the result of any of several irregularities in the hull, the steering hook-up or the engine mounting. Steering wheel must be properly located relative to propeller rotation. If an engine has a right hand rotating propeller, steering wheel should be on the right or starboard side. This side normally would tend to lift as the result of torque action and the driver’s weight offsets it. Modern outboards have built-in features in the lower units to compensate for torque. Engine tilt should be such that the prop is horizontal when underway. If it is up or down, the propeller can have a definite pull to one side. See that the engine is at the exact center of the transom and is setting level. Steering linkage should have enough adequate sized pulleys, properly swiveled and with the right cable tension. Check boat bottom for warping, distortion, which could cause difficulty.
What is the correct transom height for my outboard engine?
On average boats, it is best to mount the engine so the cavitation plate is approximately 1” below the bottom of the keel, or 1” below the bottom of boats without keel.
For racing boats, better speeds can be attained by raising the engine to reduce lower unit drag and exhaust back-pressure. Best transom height can only be determined by experimenting. Get the engine as high as possible, or to the point just before the propeller cavitates excessively.
I have a twin engine outboard. Can I get a propeller of opposite rotation and run one motor in reverse?
No. This is a frequent question, and we would caution anyone against attempting to run any engine in reverse continuously at high speed. The power units are not made to withstand the reverse thrust and this type of operation can only result in lower unit failures. Some lower units have been available with reversed gearing so counter-rotating propellers can be used in a dual installation.
What is the best tilt-setting or shaft angle?
Proper tilt is extremely important, and is determined only through experimenting. In any boat, the tilt-setting can change as the load changes. Tilt adjustment determines the planing angle and if improperly set, the speed loss can be substantial or the boat may not plane at all. Tilting the engine in toward the transom pulls the bow down. Tilting it away from the transom pulls the bow up. Vary the angle to find the point where the boat assumes the best planing position.
Is it advisable to have outboard propellers repaired?
That depends on the material. Those made of sand-cast aluminum are repairable at about 1/3 to 1/2 the new propeller price. Die-Cast propellers generally are not repairable. The material is very brittle, breaks off easily in the straightening process and will not weld satisfactorily. (Original equipment propellers are usually made of die-cast). It is advisable to discard such propellers and replace with the more durable sand-cast aluminum.
Authorized propeller repair stations located across the country offer extensive service.
What is the purpose of the rubber cushion hub in an outboard propeller?
It is not intended to prevent blade damage as is sometimes assumed. This device protects lower unit parts by cushioning the shock of propeller impact. Its prime purpose, however, is to prevent excessive breakage of shear or drive pins that otherwise would occur due to the jolt or shock that is experienced in the process of shifting gears.
The rubber hub in my propeller seems to be slipping. Is this possible?
It is a definite possibility, but it does not occur too frequently. Take a look at the propeller. If the blades are visibly bent or distorted, you very likely are experiencing cavitation — and cavitation is often mistaken for a slipping bushing. Have it checked by a reliable prop service station. The bushing can be replaced if needed, it or the blades can be restored to proper accuracy to eliminate the cavitation.
My outboard seems to vibrate excessively, yet the propeller hardly shows signs of use. Why?
This is not unusual. Propeller blade damage very often is not discernible to the naked eye and blades can become bent or distorted without showing signs of impact or abrasion.
Are there advantages to plastic propellers?
Nothing to date has been developed that has all the qualities of propellers made of metals. A good propeller must be durable, repairable and above all, it must perform well. So far, the available plastic propellers fail to meet these important requirements.
Is an adjustable pitch OBSD propeller possible or satisfactory?
An adjustable pitch propeller permits more flexibility of operation than the standard props supplied with the outboard. However, it is only efficient at one setting since the blade is a true helical surface only at one particular pitch angle. Two or three well-designed, rigid propellers of different pitches (needed for different loads) will give better performance than an adjustable propeller.
Can I troll better with a different sized wheel?
Propellers with low pitch are always best for trolling. The lower the pitch, the better. Standard propellers with relatively high pitch troll too fast and in throttling down to extremely slow speed, they tend to overload permitting the engine to idle faster while moving boat slowly.
Can a different propeller help me in water skiing?
Yes, in most cases. Original equipment propellers are pitched a little on the high side. Not knowing the boat the engine will be used with, the manufacturer pitches the prop a little high so the engine does not exceed top RPM if placed on a light boat. However, on a heavier boat, or with water skiers, this propeller tends to overload the engine, resulting in poor speed, poor acceleration and sluggish performance, making it difficult to get a skier up. This is corrected with a lower pitched prop. We also recommend 4-blade propellers for water sports, as the added blade area provides better “hole shot”.
Why would I change my OBSD propeller?
The stock propeller with which most outboards are equipped is a compromise. Since it has fixed diameter and fixed pitch, it is really limited in its use and it does not provide satisfactory performance for all the combination of hulls and loads that will be encountered once it is installed. One important fact to note is that the propeller moves the boat through the water at a specific engine RPM, and horsepower (HP) is directly related to the developed RPM. The engine cover is marked with a certain HP rating but in most instances the full benefit of the possible HP is never realized. Along with the HP rating equal emphasis should be placed on the RPM at which the rated HP is developed. This, of course, is where the propeller comes into the picture. Outboard engines are designed to run at peak RPM for full efficiency. Excessive RPM with its increased friction and wear is obviously harmful. It is equally harmful to run the engine so overloaded that it cannot achieve its rated RPM since this results in excessive carbon buildup in the cylinder with subsequent problems of poor fuel economy, pre-ignition, frequent spark plug failure, scoring of the cylinder walls and even burned pistons.
Will a propeller change help my I/O installation?
Generally the boat manufacturer exhaustively tests the various models to insure that the boat performs properly. If, however, the owner increases the boat displacement by installing heavy optional equipment, a different propeller will be required. In almost all cases, the propeller to be used would be comparable in diameter but have less pitch.
Why won't my sterndrive perform as well as another sterndrive of comparable length, beam, weight, and I/O installation?
Differences in hull design, such as hull dead rise, position of center of gravity and transom angles can account for significantly different performance. Transom angle is important since it governs the maximum tilt setting of the sterndrive and consequently the shaft angle.
Why do I shear so many pins without apparently hitting anything?
The engine may be turning too fast in the shifting range. The shear pin hole or slot may be sloppy or oversized. The shear pin may be of improper strength for the amount of power involved. Or, the cushion hub may be frozen tight.
Does it make any difference where the outdrive is vertically positioned on my sterndrive?
Generally it is best to mount the outdrive on the transom so that the cavitation plate is located approximately 1” below the bottom of the keel. (Or 1” below the hull if there is no keel.) Sterndrives on houseboats are generally located deeper in the water and may have strut extensions. High performance ocean racers will position the cavitation plate even with or above the hull bottom for best performance.
My houseboat plows when the sterndrives are tilted to the furthest lock bar position. In an attempt to correct the bow-down attitude, the propellers cavitate. Why?
The bow-down attitude results from the longitudinal position of the center of gravity being forward of midships. Merely changing shaft angle will not correct the severe plowing that would be experienced. As the lower unit is tilted toward the furthest lockbar position, effectively changing the shaft angle, the depth the propeller is immersed is decreased and ventilation will occur causing a breakdown of thrust. This phenomenon is sometimes mistakenly termed cavitation.
What is the difference between thru-hub exhaust, over-hub exhaust, and non thru-hub exhaust?
Over/Thru-hub exhaust propellers are a combination of thru-hub and over-hub exhaust propellers. This allows some exhaust to escape at lower RPMs, providing a controlled amount of exhaust flooding. These types of propellers will allow the propeller to be slightly easier to turn during initial acceleration, allowing for a better hole shot on some engine/boat combinations.
Over-hub exhaust propellers have the blades attached directly to the smaller tube that fits over the propeller shaft, eliminating the larger exhaust tube. These types of propellers are often used for attaining maximumtop speeds. (On some boats, the hole shot can suffer due to extreme exhuast flooding that occurs around the propeller blades during acceleration.)
Non thru-hub exhaust propellers are used for inboards using shaft driven propellers, sterndrives using through hull exhaust, and on some outboards that don’t route the exhaust through the lower topedo.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.