THE PROPELLER DISAPPEARS UNDER SAIL. BUT WHAT HAPPENS WHEN YOU NEED REVERSE?

The folding propeller has a wonderfully simple sales pitch: When you’re not using it, get it out of the way. Under sail, the blades fold together and present very little resistance to the water. For a sailor trying to eliminate drag, that’s extremely attractive. But eventually every beautiful day under sail ends with something less romantic:

A marina.

A fuel dock.

A crosswind.

In reverse.

That’s where propeller design suddenly becomes very real. Early folding props developed a reputation for weak astern performance because the blades had to fight the water flow as they opened. Modern designs have improved considerably. Some use blade overlap so that shaft rotation, centrifugal force and water pressure help the blades open — particularly in reverse.

Three-blade folders can provide performance under power close to a fixed prop in many conditions, although some may still have less “bite” during low-speed maneuvering. Four blades can provide smoother, quieter operation, particularly as horsepower and displacement increase. But adding blades doesn’t magically solve everything. As horsepower rises, blade loading becomes increasingly important. Get the design wrong and cavitation, noise and vibration can become part of the package. You may also hear the evidence from the aft cabin every time you motor.

So when somebody asks me: “Is a folding propeller good?” My answer must begin: “On what boat?” That’s the difference between buying a propeller and specifying one.

Coming to Annapolis? Bring us your boat model, engine horsepower and current propeller information at Bruntons Propellers at booth YB3. We’ll give you something much more useful than “folders are good” or “folders are bad.” We’ll talk about whether one makes sense for your boat.

Next: Why adding more pitch can actually give you less performance.