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Propeller Help Guide

  • Why Buy Boat Propellers From iboats?

    Ease of Use Prop Selection Tool

    Easily find a replacement boat propeller for your outboard, sterndrive or outdrive. carries marine propellers designed for Mercury, Yamaha, Evinrude, Johnson, Honda, Suzuki, Tohatsu, Mariner, Force, Nissan, and Chrysler outboard motors. We also carry options for Mercruiser, Volvo Penta, OMC Sterndrive Cobra, and Yamaha stern drives.

    The Most Boat Props Online has the largest selection of propellers anywhere. You'll find discount prices on all the top quality brands including Michigan Wheel, Solas, Turning Point, Stiletto Precision Propellers, Quicksilver Props, Volvo Penta QL and Original Factory DuoProps.

    These performance brands are available in 3 blade and 4 blade as either value priced aluminum or durable stainless steel props. With the most in-stock props available anywhere, you're sure to find the right size boat prop diameter and pitch, and in standard, right hand rotation or counter, left hand rotation.

    Do you know the part number of your old prop? Use the Propeller Part Number Search tool for quick exact-matches on any propeller brand, including: Michigan Match, Vortex, Ballistic, Apollo, Hustler, Rascal, Express, Amita, Lexor, Titan, Saturn, Black Diamond, Thunderbolt, Torrent, Silverado, Lightspeed, or Typhoon propellers.

    Propeller Special Sales

    Find blowout pricing on hundreds of discount options using the boat prop calculator. As the largest online retailer of boat props, we often have manufacturers give us clearance props, inventory overruns, and other great deals. Shop now to find sales with free shipping and blowout prices up to 70% off.

  • How to Choose a Boat Prop

    Choosing a Boat Propeller

    The optimum boat propeller for your particular application is one that will allow your boat's engine - with your normal load aboard - to develop full-throttle RPM within the engine manufacturer's recommended maximum RPM range.

    Recommended RPM

    Every engine manufacturer states the recommended RPM range as part of the engine specifications. Damage to the engine can occur if a propeller that allows the engine to over-rev (attain a full-throttle RPM higher than the specification) is installed. Damage can also occur if the installed boat prop causes the engine to lug - not reach the minimum specification. Overall best acceleration and top-end speed will be achieved when your full-throttle RPM is near the midpoint of the manufacturer's recommended maximum RPM range.

    Matching Boat Prop to Engine

    Outboard motors 30 HP and higher, and Sterndrives require the most attention to correct propeller selection. Smaller outboards come with an all-purpose size that meets most requirements. An onboard tachometer to monitor engine RPM is also commonly installed with the larger engine installations. Most boats are delivered from the dealership or factory with propellers that have been properly matched for the most common usage. Make note of the part number that is usually cast into the metal itself and you'll be able to order an exact replacement using iboats' Propeller Part Number Finder.

    Pitch & Diameter

    The two important propeller dimensions that you'll need to consider are pitch and diameter. In most cases the boat propeller and engine manufacturers have pre-selected the diameter range needed for your engine. Your selection can be focused almost exclusively on pitch. Your vessel will respond to different pitches the way your car responds to different gear positions in your car's transmission. Moving to a higher pitch will result in a decrease in your full-throttle RPM, just like the effect when your car's transmission shifts to a higher gear. The reverse is also true: move to a lower pitch and your full-throttle RPM will increase. Most propellers are sized by two-inch intervals, example 17", 19", 21", etc, and a 2" pitch change will result in an average 300 to 400 full-throttle RPM change.

    Spare Boat Props

    It's always a good idea to keep a spare prop aboard, but here's a way to get double-duty from that investment. If your family is into tow watersports, it's likely that your vessel's performance can benefit from a change of 2" less pitch whenever you head out for a day of skiing. The improved acceleration will really be appreciated by the water skier you're pulling. Just don't forget to change back to your usual size standard prop on those days that you're out cruising.

    Prop Material

    Most boat props constructed of either aluminum or stainless steel and utilize a softer hub material to protect your engine's drive train. Aluminum propellers are less costly, but more prone to damage. Stainless steel propellers are usually about 3 times more expensive than their aluminum counterparts, but far more durable. Some boaters also experience better overall performance with stainless steel because the material's tensile strength allows the manufacturer to cast the blades thinner.

    Propeller Basics - Diameter

    Diameter Example
    Diameter is the width of the circle that the propeller would make when rotating.


    Diameter is the width of the circle that the prop would make when rotating. The diameter can vary by up to 1/2" with no discernable change in performance.

    Caution: Before increasing the diameter of your prop, make sure there will be enough clearance between the blades and cavitation plate or anode on your engine.

    Diameter is the first number listed on boat propeller dimensions.

    Propeller Basics - Pitch

    Pitch Example
    Pitch is the forward movement of the propeller after one revolution assuming there is not slippage.


    Pitch is a measurement of how far the propeller moves forward during one full revolution.

    Most boats experience some "slippage" between the blades and the water. This means that the propeller does not advance the full distance of the design pitch during each revolution. The amount of slippage varies from boat to boat.

    Pitch is the second number listed in the propeller dimensions.

    Propeller Basics - Cupping

    Cupping Example
    The curved lip on the propeller helps get a better bite on the water.


    The design on many propellers includes a cup on the blade's trailing edge. This cupping or curved lip on the blade edge lets the prop grab the water more efficiently.

    Getting a better bite on the water reduces ventilation, slipping, and often allows for a better hole shot too.

    Cupping works very well where the motor can be trimmed, positioning the propeller near the water's surface.

    Propeller Basics - Rake

    Propeller Rake


    Rake describes the slant of prop blades in relation to the hub. Rake affects how water flows through the propeller and can impact the boat performance.

    Aft Rake helps to trim the bow of the boat upwards, which often results in less wetted surface area and therefore higher top end speed. Aft rake propellers also typically "bite" better on ventilating type applications.

    Forward, or negative rake, helps hold the bow of the boat down. This is seen most often on workboat type applications.

    Propeller Basics - Ventilation


    Ventilation occurs when surface air or exhaust gasses are pulled into the boat propeller blades.

    When this happens the engine RPMs increase rapidly and the boat loses speed.

    Ventilation can be caused by tight cornering, a motor mounted too high on the transom, or by over-trimming the engine.

    Propeller Basics - Cavitation

    Cavitation Burn
    Cavitation is a phenomena of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade.


    Cavitation (which is often confused with ventilation) is a phenomena of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the blade surface due to the collapse of microscopic bubbles in the metal.

    There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, etc...

    Be advised disturbances in the water flow forward of the propeller can result in blade damage which appears to be blade cavitation, but is actually due to non-favorable water flow.

  • Propeller Characteristics


    Blade Examples

    3 blades... or 4?

    We recommend 3-blade propellers for recreational boats with 3, 4, and 6 cylinder outboards and I/0 engines.

    3-blades provide good "hole shot" and top-speed performance.

    We recommend 4-blade propellers for bass boats and those with high performance hulls running high horsepower outboard engines.

    Compared to 3 blades, 4-blade boat props provide better "hole shot" performance with less steering torque and less vibration at high speeds.

    When switching from 3 to 4-blades, generally a 1" pitch decrease is required. So, a 21" pitch 3-blade is replaced with a 20" pitch 4-blade. Pitch is increased when switching from 4 to 3-blades.

    Spline Count

    Spline Count

    Spline Count

    Spline Count is an important factor in fitting a prop. They are the little teeth you see on your propeller shaft and inside the hub.

    The number of splines on the shaft and inside the hub must match for the prop to fit your boat.

    A Pin Drive application has no splines. The prop is secured to a smooth shaft with a pin through the shaft.


    Propeller Rotation

    Opposite-direction propellers on twin engines eliminate steering torque because the left-hand rotation balances out the right-hand rotation.

    Standard & Counter Rotation Props

    Props spin either clockwise (standard or right-hand rotation) or counter-clockwise (counter or left-hand rotation).

    Most single engines use a standard rotation. Most twin engines use one of each. To determine what rotation you have, look at your prop as if on a boat. Position a blade pointing straight up. If the right-hand side of the blade is closer to the hull than to you, it's a standard rotation.

    Rotation on Twin Engines

    A boat with twin engines and two boat props spinning in the same directionats will have a lot of steering torque. Two right-hand (or standard rotation) propellers pull the stern hard to the right and the bow to the left.

    Using both standard and counter rotation with twin engines will eliminate the steering torque. This gives you better straight-Line tracking and helm control at high speeds.


    Performance Variables

    How does pitch and slip affect performance?

    Slip is the difference between actual and theoretical travel of the propeller blades through water.

    High slippage means that the boat is moving less distance for each rotation. This typically happens with boat props that are poorly matched.

    A correct fit will move forward about 80 to 90 percent of the theoretical pitch.


    What is the difference between Thru-Hub Exhaust, Over-Hub Exhaust, and Non Thru-Hub Exhaust?

    Thru-Hub Exhaust

    Thru-hub exhaust propellers are used on most outboard & stern drive engines. You have a thru-hub exhaust if there are exhaust holes on the engine gearcase around the shaft.

    Thru-hub boat props consist of a round barrel (the hub) to which the blades are attached.

    The exhaust passes through the barrel and out the back, without making contact with the propeller blades.

    This provides a good clean water flow to the blades, usually resulting in good acceleration and hole shot.

    Non Thru-Hub Exhaust

    Non thru-hub exhaust systems are found mostly on low horse power outboard and stern drive motors, and where inboard ski props are used.

    You have a non thru-hub exhaust if there are NO exhaust holes on the engine gearcase around the shaft.

    Over-Hub Exhaust

    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 are often used for attaining maximum top speeds.

    On some vessels, the hole shot can often suffer due to the extreme exhaust flooding that occurs around the blades during acceleration.

    Over/Thru-Hub Exhaust

    Over/Thru-hub exhaust propellers are a combination of thru-hub and over-hub exhaust.

    This allows some exhaust to escape at lower RPMS, providing a controlled amount of exhaust flooding.

    These types will allow the propeller to be slightly easier to turn during initial acceleration, allowing for a better hole shot on some engine/boat combinations.

    Aluminum Vs. Stainless Steel


    Most pleasure boats are factory equipped with aluminum propellers. Aluminum is a great choice for small and medium sized boats. They are inexpensive and under normal conditions can last for many years. When boating in an area where striking the bottom is likely, aluminum blades can flex, helping prevent damage to your engine.

    Stainless Steel

    Stainless steel boat props are ideal for large or heavy boats and those running in saltwater. Although more expensive than aluminum, stainless steel is more durable and will last longer. When running at speeds over 50 mph, stainless props will perform better than aluminum.

    Propeller Characteristics Q&A

    Common Boat Prop Questions & Answers

    Why do outboards with the same horsepower often need different prop sizes?

    The size of the boat propeller needed is largely influenced by the lower unit gear ratio. The stock gearing on outboards causes the shaft to turn slower than the RPM at the powerhead. Gear ratios, such as 12:21, are a measurement of the difference between the number of teeth in the crankshaft gears and the prop shaft gear. So, although the horsepower of two outboards may be identical, lower gear ratios use larger propellers and vice versa.

    How can you tell if your engine is running within the manufacturer's recommended RPM range?

    To check if your boat motor is performing within the RPM range recommended by the engine manufacturer you will need a tachometer. Many designs and styles of tachometers are available to help measure performance.

    Will a different propeller fix torque issues like listing and hard steering?

    The propeller is not typically the problem with steering issues. The usual causes are hull irregularities, steering system hook-up, or outboard engine mounting.

    Check the boat hull for any distortion that could cause a problem.

    Make sure that the steering system has enough adequate sized pulleys, is properly swiveled, and has the right steering cable tension. Engines that use right-hand rotation propellers should have the steering wheel on the starboard (right) side of the boat. The driver's weight offsets the tendency of this side to lift from torque action.

    Outboard engines must be mounted in the exact center of the transom and setting level. When a boat is underway the engine tilt should make the prop horizontal, otherwise it can have a pull to the side.

    Why does my outboard vibrate excessively when the boat prop hardly looks used?

    This is fairly common. Propellers that aren't showing any signs of impact may still have been damaged. Bent or distorted blades will cause the motor to vibrate even if the damage is indiscernible.

    Will a different sized prop help me troll better?

    Standard sizes, with a standard pitch, typically troll too fast. Throttling down to very low speeds causes the prop to overload, thus allowing the engine to idle faster. Lowering the pitch is always best for trolling. Use the lowest pitch possible for the best trolling results.

    Will a different sized prop help me in water skiing?

    Original outboard propellers are typically selected knowing the exact boat that it will end up on. So they are pitched a little high to prevent it from exceeding the top RPMs on a light boat. A prop change is usually a big benefit for water sports. The original equipment option tends to give poor speed, acceleration and performance when used on heavier boats or when towing a skier. This poor performance can make it more difficult for skiers to get up. Lowering the pitch or changing to a 4 blade propeller can improve hole shot.

    My engine came without a propeller, how do I know if I need a right or left-hand prop?

    Stand behind your boat and look at the prop shaft. If it rotates clockwise then you need a right-hand or standard rotation. If it rotates counter-clockwise then you need a left-hand or counter rotation.

    Should I change the propeller size on my sterndrive engine?

    Boat manufacturers typically run extensive tests on I/O boats to ensure that the propeller size performs well. However, if the boat displacement changes, due to the addition of heavy aftermarket equipment, you should consider a different boat prop. The diameter would remain similar to the original, but with a lower pitch.

    Should I change the propeller size if I boat on high elevation lakes?

    Gas engines produce less power when they are run at elevations above 3000 ft. Lowering the boat prop pitch will help keep the RPMs within the desired range. However, due to the lower air density, the engine will not have as much horsepower and the boat will have a slower top speed. Boaters who frequent high and low elevation lakes often use two different sized props to accommodate for their boating elevation.

    What is electrolysis and how does it affect my propeller?

    Electrolysis is caused by two distinct metals being near each other. A stray electrical current, from the boat or another source, causes pitting on the boat prop blades. Sacrificial anodes can help protect the prop, but the best solution is to find the source of the electrical current.

    Galvanic corrosion is very similar to electrolysis. However, galvanic corrosion occurs in salt water. The less noble of the two adjacent metals begins to corrode. Reduce the corrosion and pitting on your propeller by using adequate sacrificial anodes on your boat.

  • Prop Performance

    Determining RPMs

    Manufacturer's Recommended RPM
    Check your owner's manual or contact your dealer to find the manufacturer's recommended RPM range.

    Wide Open Throttle (WOT) RPM
    For safety and efficient performance, make sure that your engine is running within the manufacturer's recommended RPM range. Matching the right prop for the load is the most significant factor of RPM adjustment.

    Effect of Prop Pitch on RPM
    Changing the boat propeller pitch will increase or decrease the RPM's. Adjust the pitch so the RPM's fall within the recommended range. A 2" pitch increase (21" to 23" for example) acts like shifting a car gears up. It typically results in a 300-400 RPM decrease.

    Test for Maximum RPM
    Using your existing boat propeller or a new propeller, make test runs to determine the maximum RPM and boat speed. Vary the trim angle for optimum performance.

    RPM Higher Than Recommended
    If the actual WOT RPM's are above the recommended RPM range, install the next larger pitch option to decrease your WOT RPM. Re-test the WOT RPM.

    RPM Lower Than Recommended
    If the actual WOT RPM range is below the recommended range, install the next smaller pitch option to increase your WOT RPM. Re-test the WOT RPM.

    When you combine all these factors, you have the information you need to select the correct propeller for maximum performance, safety, and fuel efficiency.

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