Propeller Pitch Perfect: How to Unlock Your Outboard's True Potential

Choosing the right aluminium propeller is like choosing the right gear for a bicycle. If you’re in a gear that’s too high, it’s hard to start moving; if it’s too low, you’ll be pedalling like crazy but going nowhere. Here's a basic guide to finding the right fit for your boat.

1. Understand the Two Main Numbers

Every propeller has two numbers stamped on it (e.g., 14 x 19).

• • Diameter (The first number): This is the distance across the circle the blades make.
    • Bigger diameter acts like a larger paddle, providing more "grip" for heavy boats.
    • Smaller diameter is usually for lighter, faster boats.
  • Pitch (The second number): This is the theoretical distance the boat moves forward in one revolution.
    • Lower pitch (e.g., 15) is like "low gear." It gives you better acceleration (hole shot) and power for towing but lower top speed.
    • Higher pitch (e.g., 21) is like "high gear." It’s better for top-end speed but makes the engine work harder to get moving.

2. Choose Your Blade Count

In some cases propellers come in 3-blade or 4-blade versions:

• • 3-Blade: The standard choice. It offers the best balance of top speed and fuel efficiency.
  • 4-Blade: Better for heavy loads, "hole shot" (getting on plane faster), and staying on plane at lower speeds. It’s ideal if you frequently tow skiers or tubes.
  • Note: it may be necessary to reduce pitch when changing from 3-blade to 4-blade, depending on the WOT rpm figure you are achieving. Please feel free to contact us to discuss.  

3. The "Goldilocks" Rule: Wide Open Throttle (WOT)

Full power is achieved when your outboard is revving to the manufacturers specified Wide Open Throttle (max rpm) figure. Increasing / reducing propeller pitch will add or remove load on the outboard engine, which will in turn increase or reduce achievable revs. 

To know if you have the right prop, you need to check your engine's RPM when you are at full throttle (WOT). Check your Owners Manual: Look for the recommended WOT range (e.g. 5,000–6,000 RPM). Even if you don't operate your boat at full throttle in normal use, in order to choose the correct propeller, the revs at maximum speed are required.

• • With a typical load, run the boat at full speed and record the revs achieved. If you don't have a rev counter onboard, consider purchasing a small "tachometer hour meter", available at very low cost from eBay etc.
    
    • RPM too low? Your outboard is overloaded and you need a lower pitch prop. Dropping 1 inch of pitch usually increases RPM by about 250-300.
    • RPM too high? You need a higher pitch prop to "slow down" the engine and prevent damage.

Aluminium or Stainless?

Aluminium is the most common material because it is affordable and sacrificial. If you hit a rock or a sandbar, the aluminium prop is designed to bend or break to protect the much more expensive gears inside your engine. It is the perfect choice for casual cruising and for engines under 150 HP.

Stainless is stiffer than aluminium and so will flex less under high load. Polished stainless looks great and in some circumstances, can assist performance. Choosing the wrong stainless propeller can be an expensive mistake, so we recommend going through the above process to find the correct aluminium propeller before considering a stainless alternative.

Summary Checklist:

These are the basics of propeller choice. Propellers for medium and larger outboards sometimes include options with more rake, cupping, or other attributes. You're not alone - we're always available at the end of the phone if you wish to discuss your propeller needs.