How to Select a Propeller for Outboard

Planning on hitting the lake or ocean anytime soon? With summer quickly approaching, many boaters and seamen are likely getting their vessels ready to go out on the water to enjoy the sun, air, and sea. When bringing your boat out after the cooler months, chances are your boat may need a bit of tuning up before it’s ready for the waves. One piece that should always be up to date for optimal safety is your propeller. Responsible for keeping your boat’s hull clean, your outboard or engine tuned, and your boat moving seamlessly through the water, this piece plays a large part in your boat’s performance. Luckily, finding the right propeller for outboard is a fairly simple process that immediately optimizes your boats performance. Whether this is your first time setting sail or you’re looking to improve performance by replacing an old propeller, knowing how to select a prop for outboard is an important bit of boater info to have.

Why You Have to Select the Right Propeller

The right propeller makes all the difference in how your boat runs on the water. Certain factors can affect how well your chosen propeller works with your boat, such as whether you choose stainless steel or aluminum blades and how you use your boat (sailing, water skiing, tubing, wakeboarding, long distance or high-speed sailing). This is to say that one propeller won’t fit all, which is why you have the task of selecting a propeller that works for your boat and how you use it. Though an ill-suited propeller will technically still work, it won’t be as effective or efficient as a well-chosen propeller. Keep this in mind as you explore your options.

Things to Consider

So, what should you consider when selecting a propeller for your outboard? Most boaters ask themselves the following questions to help pinpoint what’s really important in the decision process:

  • What Problems Do I Need to Solve?
  • Should I “pitch up” or “pitch down”?
  • Is my boat’s engine over or under revving?

Let’s explore these questions a bit more in depth:

What Problems Do You Need to Solve?

Think about your boat’s performance. What problems come to mind? Is it sluggish when it gets started? Is it not hitting its highest speed in a timely manner? Could it’s overall performance be better? Is your current propeller acting up, blowing out, or ventilating excessively? Could it improve in fuel economy? Whatever problems you identify with your vessel will help you when the time comes to select your propeller for outboard.

Should You “pitch up” or “pitch down”?

The pitch of your propeller, meaning the distance travelled during one revolution of the propeller’s blades, is closely related to the RPMs of your outboard motor or engine. When you increase the pitch of your propeller, it will decrease the engine RPMs. If you decrease the pitch, it will increase engine RPMs. To help you understand how it works, there is a general rule for pitching up or down: if you increase a pitch by just 2 inches, it results in a reduction of 300-400 RPM for the engine. On the flip side, if you decrease pitch by 2 inches, it will result in an increase of 300-400 RPM.

With these numbers in mind, you can do one of two things:

  1. If your engine is under revving, consider a propeller with less pitch
  2. If your engine is over revving, consider a propeller with more pitch

Is Your Boat’s Engine/Outboard Under Revving or Over Revving?

For optimal performance, your boat’s outboard should run within the correct designated RPM range. This range is measured at WOT, or Wide Open Throttle. Your outboard owner’s manual should be able to specify the range for your particular outboard. Typically, the range runs between 5000 and 5500 RPM for an outboard. If your owner’s manual does not have the information you are looking for, you can also talk to a dealer, mechanic, or manufacturer to get the best idea.

If you find that your outboard is running below the lowest  end of the range or above the highest endf the range, this means that it is under revving and over revving respectively. When you allow your outboard to under or over rev while at WOT, it can result in serious damage to your outboard, which affects your propeller as well. To remedy this, all you have to do is select the correct pitch for your propeller as we outlined above. Remember, under revving requires less pitch, and over revving requires more pitch.

Selecting a Propeller for Outboard

Once you have narrowed down your concerns and taken all factors into account, you can begin looking at the selection criteria for your propeller. Selection criteria helps you sift through all the propeller for outboard choices that exist, helping you zero in on the right propeller for you boat and your needs. The following criteria act as a sort of rubric for what you want and how you want your boat to perform. Be sure to consider the following:

  • Propeller Data
    • Propeller data consists of knowing the present diameter of your propeller, the present pitch, clockwise or counterclockwise propeller rotation, how many blades the propeller has, and the material the propeller is made out of (typically aluminum or stainless steel). You should also search for the manufacturer’s part number as well as the shaft diameter and number of splines.
  • Engine Data
    • Engine data refers to the number of engines your vessel holds. You must also pay attention to rated horsepower, gear case size, RPM at WOT, as well as the manufacturer, model, and year of the engine or engines.
    • Boat Data
      • For boat data, all you need to pay attention to is length of the boat overall, as well as hull material. You may also want to think about the boat manufacturer, model, and year as well as hull shape and desired top speed.

  • Propeller Size
    • Propeller size can make a big difference because it affects the level of power. Propeller size is learned with two numbers: the diameter and pitch. The diameter is always stated first. The diameter is the measure of two times the distance from the center of the hub to the tip of any propeller blade. Smaller propeller diameters are better suited to smaller engines or powerful, high-performing boats. Pitch is the theoretical forward distance, measured in inches, that the propeller travels during a single revolution. Since the propeller moves through water, there is always a bit of “slip” (or inaccuracy) between the theoretical distance and the actual distance traveled.
  • Propeller Rake
    • Rake refers to the degree that the blades slant forward or backward in connection with the hub. Rake is an important factor and can affect how water flows through the propeller, thus affecting pitch.
  • Material
    • As stated, most propellers are made from either aluminum or stainless steel. Most outboards are sold with aluminum propellers because they are easily repaired and fairly inexpensive. Though aluminum is more common for outboards, stainless steel offers a performance advantage due to the thinner, more rigid blades and more modern design.

Selecting a propeller for outboard entails taking each of these factors into account. Though switching your old propeller for a new one is an easy thing to do, a lot of thought must go into the job itself. Be sure to follow this criteria carefully so you can make the best choice for your boat.