Fixed vs Feathering Prop ?

[dniello]

What are your opinions between a fixed or a folding/feathering prop. Are feathering props as efficient? how are they in reverse? Any thoughts would be great.

[Captain Bill]

I have Autostream feathering props on my cat. They definitely offer less drag while sailing than a fixed prop does. If the props are not feathered the speed drops about a knot. They are not as efficient as a fixed prop. To produce as little drag as possible while feathered the blades are perfectly flat. You'll notice that fixed prop blades are cupped for better effciency. I'm told that my boat would cruise at 8 knots with a cupped folding prop. It cruises at 7. It's not clear to me if I could adjust the pitch to do a little better or not. The engine turns its max rated RPM at full throttle with the current pitch setting. The autostream prop reverses pitch (reverse pitch is a separate adjustment from forward) when the transmission goes into reverse. This provides much more reverse thrust than either a fixed pitch or folding prop. Downside is that these things are pretty pricey and take a bit more maintenance.

My 2 cents, change welcome.

[David M]

Cupping a prop has the same effect as adding an inch or so of pitch. Cupping the trailing edge of a foil does not necessarily make it more efficient.

The plus side of fixed props is fewer mechanical parts to break. The minus side of fixed props is of course less drag.

[Captain Bill]

Sorry, I used the wrong terminology. Cupping on a propellor is a specific treatment to the trailing edge of the blade. The blades of a fixed pitch prop curve to a flatter angle of attack as one moves outward from the hub. The speed of the blade through the water increases as you get away from the hub, thus the inner part of the blade has a higher angle of attack than the outer part of the blade. This allows maximum efficiency along the entire length of the blade. The feathering prop has a perfectly flat blade with the inner part of the blade having the exact same angle of attack as the outer part of the blade. Therefore the blade shape is not optimized for either the inner portion of the blade or the outer portion and is inherently less efficient.