Propeller Sizing in 3 Easy Steps

Airboating Magazine recently caught up with Patti from Whirl Wind Propellers and asked a simple question we often see from our readers: ‘What prop should I choose that will make the most thrust for my airboat’?

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This is what Patti had to say: “Well that is a great question we get asked all the time. We have to get into the science a little to best understand what goes into sizing a propeller, so here goes. To get the most thrust from any given propeller and engine combination, the engine must develop the most horsepower (HP) possible – right! The prop just simply converts this HP into pounds of thrust."

Step 1: Horsepower

Sometimes people get hung up on engine Torque vs HP, and are not sure which is most important. In truth, HP is simply a mathematical calculation of Torque and RPM – so HP is Torque with RPM. Without RPM, Torque is meaningless. The formula for HP is simply (Torque x RPM) / 5252. Horsepower can be explained as the amount of work done in a given amount of time. Since the engine’s Torque curve doesn’t change linearly as the RPM changes, higher RPM means there’s more work being done every minute, even if Torque is flat or decreasing. If you’ve ever looked at an engine dyno sheet, you’ll notice the HP curve and the Torque curve always cross at 5200 RPM – because HP is mathematically calculated from Torque. So, Step 1 – determine max available horsepower from the engine.

Step 2: RPM