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What Is an Outboard Propeller?

An outboard propeller is exactly what it sounds like: a propeller that is used with an outboard engine.

When you buy an outboard prop for your boat, you are looking for a propeller that works best with your specific combination of engine and boat. There is no one-size-fits-all prescription for propellers.

The most important way a propeller should fit your boat is running within the proper wide open throttle (WOT) operating range. This is measured in horsepower per R.P.M. (revolutions per minute). Each engine's manufacturer will indicate the appropriate range.

The basic goal of the best propellers is to achieve the highest horsepower at the lowest R.P.M. This means you will avoid over-revving and damaging your engine while achieving maximum speed and power levels. Keeping horsepower too low will result in lugging.

Over-revving and the strain that an inefficient propeller puts on a boat's engine can even lead to premature engine failure.

There are, of course, other factors that complicate this simple equation. These include your specific performance goals for your boat and troubleshooting issues like ventilation and cavitation.

Factors to Consider

The primary factors you'll want to consider when looking for a propeller are:

• material
• diameter
• pitch
• rotation
• number of blades
• rake
• cupping

Material

Outboard props are almost always made of either aluminum or stainless steel.

Aluminum propellers are more common and often come pre-installed on boats. They are cheaper, but also more prone to dings and other damage.

Stainless steel propellers, while more expensive, last significantly longer. The blades are thinner and stiffer. This allows them to function better in the water and to stand up to repairs more easily.

Diameter

Diameter, or distance from propeller blade tip to blade tip, is one of the primary measurements that determine propeller function. It is so important that engine manufacturers specify the diameter of propeller to use.

You should follow the manufacturer's instructions here. If you are unsure what diameter propeller your engine's manufacturer specified, you can follow our guide to find out without consulting your instruction manual.

Pitch

When you see props described using two numbers, like 14x19, the first number is diameter and the second is pitch. Pitch describes the theoretical forward distance the propeller would travel in one revolution if there was no slip at all.

In reality, all propellers experience some amount of slip in the water, but a prop that is well-matched with your boat should have 10% slip or less.

Aside from diameter, which is prescribed by the engine's manufacturer, pitch is the most important thing to consider when choosing an outboard propeller.

Lower pitch allows for better hole-shot and acceleration as well as better low-speed pulling power, as the propeller will encounter less resistance. Because each revolution pushes the boat forward a shorter distance, though, the top-end speed will be lower. This may make a lower pitch prop ideal for ski or wake boating.

A prop with a higher pitch will be able to achieve a faster top-end speed, but it's hole-shot and low-speed pulling power will not be as great. Higher pitch also results in lower R.P.M. overall.

Rotation

Each propeller is set to rotate either clockwise or counterclockwise. Most outboards require right-hand props or props that rotate clockwise. Dual outboards, however, typically need one right-hand and one left-hand (counterclockwise rotating) prop.

It is best to check your manufacturer's instructions to make sure the propeller you get rotates in the correct direction.

Number of Blades

Most propellers have either three or four blades.

Three-blade propellers work well for general recreation boats. They help a boat to reach top-end speeds more easily. The lower resistance they have also increases fuel efficiency and lowers their effect on the outboard's R.P.M.

Four-blade props have more resistance, which gives them better traction and bite. They may also provide a smoother ride with less vibration. They do not reach top-end speeds as easily but have better hole-shot and acceleration and low-speed pulling power.

The increased resistance of a four-blade propeller also makes them less prone to ventilation. Four-blade props handle better on tight corners and in rough waters.

Rake

The rake of a propeller describes the angle at which the blades slant backward from the line of travel. Rake is typically between zero and twenty degrees.

A higher rake will lift the bow higher out of the water and increase the boat's top-end speed. Too high of a rake can put a strain on the engine.

Cupping

Cupping is a relatively common feature of outboard props. It refers to a curved lip on the trailing edge of the blade, which gives the prop more bite.

Functionally, cupping on the top of the propeller works like an increased rake. Cupping on the side of the prop works like an increased pitch.

An added benefit of cupping is that using a propeller with cupping allows for the outboard to be trimmed. This brings the prop closer to the surface and reduces R.P.M.

Which Outboard Propeller Is Right for Me?

Now that you know all the measurements and specifications that differentiate outboard propellers, you can decide which one suits your boating needs best.

It may be ideal to choose two different propellers with two different pitches so that you can achieve top-end speeds or great hole-shot as needed. Check out our wide selection of outboard propellers to find one (or two) that fit your boat and your needs best.