General Propeller FAQs

What's the best propeller for my MerCruiser?

Your engine should be propped so that it operates at or near the top end of its recommended rpm range at wide open throttle with a normal boatload (fuel, gear, people, etc.) An engine that does not reach the rated RPM at wide-open-throttle is in an "overpropped" condition, resulting in "lugging the engine." This high-torque operation puts a tremendous load on the pistons, crankshaft, and bearings in your marine engine.

On the other hand, an engine that revs past the recommended RPM will have higher than normal wear and can also be damaged.

Propping is not an exact science and may require some trial and error. The maximum RPM your engine can achieve is dictated by propeller pitch. Generally speaking, one inch of propeller pitch equals 150-200RPM. To increase your engine RPM, you should decrease propeller pitch and vice versa. Your local authorized dealer can assist you in choosing the best propeller.

Initially, the prop on your boat is selected by the boat manufacturer and/or selling dealer. However, the way you load your boat (people, fuel, gear, etc.) may indicate that a different prop is best for your use. Please notify your dealer immediately if your engine cannot reach the proper rpms at wide-open throttle.

You may visit the Propellers section of this site for additional information.

What is the "trim angle" of the engine?

Trim angle of an outboard or stern drive is the angle between the boat bottom and the propeller shaft formed by moving the engine/stern drive closer to the boat transom, called trimming "in" or "down" or "under," or moving the outboard/stern drive further away from the boat transom called trimming "out" or "up." When a boat is cruising on plane and the trim is adjusted so that the propeller shaft is parallel to the surface of the water, that is said to be running at "neutral" or "zero" trim. On outboards without power trim, this angle is adjusted by changing the hole in which the adjustable tilt pin is inserted.

The trim angle of the outboard/stern drive has a distinct effect on the planing angle of the boat which, in turn, significantly alters top speed and handling. The engine/drive should be trimmed in for best start-up acceleration and shortest time to plane. The engine/drive would then be trimmed out for peak performance. If trimmed "in" (under) too far, the bow drops and the boat runs too wet. In this condition, top speed drops, fuel economy decreases, the boat may over-steer in one direction or the other ("bow steering"), and steering torque will increase (to the right with a right-hand rotation propeller).

If trimmed "out" too far, the propeller may lose its hold on the water, fast vee-bottom boats may start to "walk" from right to left to right, etc. ("chine walking"), steering torque will increase in the opposite direction to that when trimmed in, and getting on plane may be difficult or labored. Porpoising of the boat may also occur.

What are the differences between aluminum and stainless steel propellers?

Aluminum is by far the most popular material used today for recreational boat propellers. It is relatively low in cost, has good strength, good corrosion resistance, and is easily repaired. However, compared to stainless steel propellers, aluminum propellers are more easily nicked or bent. Stainless steel is the strongest, most durable of all materials used for propellers. In fact it is about five times stronger than aluminum. This is its greatest advantage. The strength of stainless steel propellers will help maintain engine performance because these propellers are more resistant to the small nicks and bends normally found on aluminum propellers, often after running for a short period of time. Blades cast of stainless steel can be made thinner for better efficiency. Stainless steel is much more resistant to corrosion. Stainless props can be repaired easily, although more expensively than aluminum. They can cause some corrosion on nearby aluminum surfaces in saltwater if good anodic protection is not used. An aluminum propeller can usually be replaced with the nearest size and blade equivalent stainless steel propeller.

Why do I need a tachometer and speedometer?

A tachometer ("tach") measures engine RPM while the speedometer measures boat speed in miles or kilometers per hour, or knots. A knot equals one nautical mile per hour, so it is incorrect to use the expression, "knots per hour." Here’s the relationship between these three units of measurement:

An engine is designed to run at wide-open-throttle (WOT), within certain RPM limits. Without a tachometer, the operator has little opportunity to know if the engine is at a dangerously high or low RPM level. Once the correct propeller is selected, the engine will run at WOT within the recommended maximum RPM range. Any deviation from this established WOT RPM other than that associated with climatic conditions, elevation, or gross load changes, is an indication of a performance problem.

A speedometer, when used with a tachometer, also will give hints of engine or boat problems, should an unusual speed drop occur. With experience, a boater will be able to detect problems at part throttle using the combination of the tach and speedometer. For a good, solid speed reading, it is important to install the speedometer pickup as low and close to the center of the boat as possible without creating a water disturbance ahead of the propeller or water intakes.

What is the difference between 3-blade and 4-blade propellers?

4-blade propellers:

  • Plane the boat faster than 3-blade propellers 
  • Keep the boat on plane at a lower speed 
  • Give improved mid-range speed at the same rpm as a 3-blade propeller 
  • Provide quicker acceleration than most 3-blade propellers 
  • Run smoother than 3-blade propellers 
  • Have better holding power in rough conditions 
  • Are less likely to ventilate in sharp turns 
  • Provide better low-speed handling 
  • Are usually not quite as fast on the top end as a comparable 3-blade propeller

You may visit the Propellers section of this site for additional information.

How do elevation and climate affect performance?

Elevation has a very noticeable effect on the wide-open-throttle (WOT) power of an engine. Since air (containing oxygen) gets thinner as elevation increases, the engine begins to starve for air, like using a supercharger in reverse. If the boat has been set up at a lower altitude and then moved to a much higher altitude, there will be a noticeable reduction in power, thus RPM.

Although some performance can be regained by dropping to a lower-pitch propeller, thus bringing the WOT RPM back into the recommended range, a basic problem still exists. The propeller is too large in diameter for the reduced power output. The experienced marine dealer or a Quicksilver propeller repair station can determine how much diameter to remove from a lower-pitch propeller for specific high-elevation locations.

In some cases, a gear ratio change to more reduction is possible and very beneficial. However, this fix is only safe while the power level is reduced. If the engine is again run at a lower altitude, the gear ratio change must be reversed to prevent excessive torque on drive train parts.

It is a fact that weather conditions exert a substantial effect on power output of internal combustion engines. Therefore, established horsepower ratings refer to the power that the engine will produce at its rated RPM under a specific combination of weather conditions now established by the International Standards Organization (ISO).

Summer conditions of high temperature, low barometric pressure, and high humidity all combine to reduce the engine power. This, in turn, is reflected in decreased boat speeds - as much as two or three miles per hour in some cases. Nothing will regain this speed for the boater, but the coming of cool, dry weather.

How should I maintain and service my propeller?

Essential to good propeller maintenance is periodic inspection to detect even small dings, which can lead to blade failure if not dressed or repaired. A damaged propeller, even one that only appears slightly damaged by running through silt and sand, can significantly reduce performance efficiency and fuel economy, and can more severely damage itself through cavitation erosion emanating from the blades’ irregular leading edges. In one test with a damaged propeller, top speed fell more than 13%. Acceleration was off over 37%. Optimum cruise miles slowed 21%. Worse yet, damage usually is not done to each blade uniformly and, therefore, the damage can set up imbalance vibrations that can cause fatigue damage to other parts of the engine or drive. If you boat in shallow or rocky waters, you will want to check your propeller more frequently for possible damage.

Up to a point, dealers can have a propeller restored to like-new condition; however, extreme damage can be more expensive to repair than the cost of a replacement.

Minor damage corrected early on can prevent much more serious and costly repair later on to both the propeller and other parts of the engine.

Propeller repair, as well as customizing, may be available from your dealer. * Such customizing can include adjustments to pitch, rake, cup, and blade thickness.

To aid in the future removal of the propeller, liberally coat the propeller shaft spline with Quicksilver Anti-Corrosion Grease or 2-4-C. To assure that the propeller remains secure on the shaft during the season, periodically check the self-locking prop nut for tightness.

*Stainless steel propellers only

What is the best prop for skiing?

Designed with sterndrive ski boats in mind, the HighFive delivers unsurpassed hole shot, good top-end speed, better bow lift and handling, reduced vibration, and great holding in rough water. You may visit the Propellers section of this site for additional information.

How can I improve hole-shot and top speed performance?

No single propeller can deliver both optimum hole-shot and top speed performance, with the exception of the Enertia. You will need to decide which of these performance aspects is more important to you and select a propeller that is best suited for that purpose. You may visit the Propellers section of this site for additional information on the Enertia and all other propellers.

What are some common signs that it may be time for a new propeller?

Your boat takes longer to get on plane than it used to. It uses more fuel. It doesn’t seem to run as fast as it did when it was new. If your boat feels sluggish, one of the first things you should check is your propeller. Is it showing signs of wear and tear? Does it have more than its share of nicks, dings and missing paint? These symptoms can accumulate and significantly impair the performance of a perfectly good engine.

My stainless steel propeller is turning white, or is rusting or discolored. What can I do about this?

Stainless steel propellers can discolor and form areas of light rust. Brushed finish stainless props are more susceptible than brightly finished stainless steel props. Rust stains can occur from polluted water, galvanic corrosion from the boat or marina, or from oxygen depletion. The propeller can be easily cleaned with rubbing compound or Scotchbrite. Do not use steel wool. After cleaning, use chrome polish to protect the finish. Stainless steel props can also turn white from calcium or lime in the water. In this case, clean the prop with household mineral bath and tile cleaner. Again, do not use steel wool. Follow up with chrome polish to protect the finish.

What is Pitch?

Pitch is the theoretical distance, in inches, that a propeller moves forward during one revolution. Think of a screw as you turn a screwdriver once around. The distance that the screw travels into the wood is the pitch. Propeller size is specified in inches of pitch.

How do I know what pitch is right for me?

Consult the engine Operation, Maintenance & Warranty manual to find the recommended wide-open-throttle (WOT) range for your engine. If the current propeller is at WOT RPM within the specified RPM range, select a replacement or upgrade propeller with the same pitch as the current propeller.

If the current propeller isn’t at WOT RPM within the recommended RPM range, select a replacement or upgrade propeller with a larger or smaller pitch using the following rules:

  • Adding 1 inch of propeller pitch will reduce WOT RPM by 150 to 200.
  • Subtracting 1 inch of propeller pitch will increase WOT by 150 to 200.
  • If you’re upgrading from a three blade to a four blade propeller, remember that a four blade propeller generally turns 50 to 100 RPM less than a three blade prop with the same pitch.

Stainless Steel vs. Aluminum?

It’s a question of strength and performance. Aluminum propellers usually cost quite a bit less, however, stainless steel is over five times more durable than aluminum. If you’re looking for quicker acceleration, superior top speed, or better overall performance, you should consider a stainless steel propeller.

How can I prevent the propeller from sticking onto the shaft?

We recommend a liberal coat of one of the following Mercury Precision or Quicksilver Marine Lubricants on the propeller shaft: Special Lubricant 101, 2-4-C Marine Lubricant, or Anti-Corrosion Grease. These lubricants are available from any authorized Mercury Marine dealer.

Note: Always use the correct mounting hardware and torque the propeller nut to the correct specifications. Verify the propeller tightness after 20 hours of operation. DO NOT operate the boat with a loose propeller.

Three or four blades?

Four blade propellers usually:

  • Plane the boat faster than 3-blade propellers
  • Keep the boat on plane at a lower speed
  • Give improved mid-range speed at the same RPM as a 3-blade propeller
  • Provide quicker acceleration than most 3-blade propellers
  • Run smoother than 3-blade propellers
  • Have better holding power in rough conditions
  • Are less likely to ventilate in sharp turns
  • Provide better low speed handling
  • Are not quite as fast on the top end as a comparable 3-blade propeller