Between Prop Pitches

By tuning in a little extra cup at the trailing edge of the propeller blades, you might be able to avoid having to move up a size in pitch.

By tuning in a little extra cup at the trailing edge of the propeller blades, you might be able to avoid having to move up a size in pitch.

There’s a seldom-spoken-of situation in the world of propellers, and it’s the phenomenon of being “between pitches.” That’s when one prop isn’t quite right for your boat, but neither is the next one up. I had a buddy whose new-to-him boat was plagued with this phenomenon.

His case was rather involved, but just to make the numbers simple, let’s say you have a 26-inch pitch propeller and it’s hitting the rev limiter at top speed, but just barely. You know that going to a 28-inch pitch prop might be too much prop for your boat, so what do you do?

It’s possible for a competent shop to “fine tune” your propeller by adding some cup to the trailing edges of the blades. However, it depends on how easily your boat is getting to redline.

For example, if you can hit the rev limiter with a few people on board and about a half-tank of fuel, it might be better to move up to the next propeller size. If your boat only hits the rev limiter when it’s just you in the boat and the fuel tank is nearly empty, then adding a little cup to the blades might work.

Bear in mind, of course, that there is no great sin in not being able to get to redline, and it might actually be beneficial in the ranges in which you most often use a boat. For instance, a higher-pitch propeller will result in a more efficient setup that works better in the midrange and improves fuel economy. Even though the rpm may be lower, the performance may be better.

And let’s be honest, midrange is where we spend most of our time boating, anyway.

 

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