Propeller Choices

These days there are a variety of propellers around, so which is the right one to use? Here’s a description of the types of propellors with pros and cons of each.

Fixed propellers

Fixed propellors are relatively cheap and efficient at their designed parameters, i.e. at one particular point in the rpm range, but under or over pitched under all other conditions. Not so efficient in reverse and cause drag on sailing boats under sail.

Folding Props

Folding props are similar in performance but less efficient than fixed props. They are mechanically more complicated and more expensive. There can be a delay after engaging gear while the prop opens and “bites”. They cause the least drag when sailing but this will be diminished if there is a large deviation in shaft angle from horizontal. If changing from a fixed prop more clearance will be required behind the prop to accommodate the folding blades.

Feathering Props

With feathering props, the blades can rotate on a hub so when in forward gear the blades rotate into position for forward thrust. In reverse gear they rotate around so the blades leading edge is the same in reverse making them just as efficient in reverse. When sailing, the blades line up with the shaft, or better still the water flow, depending on the design and minimise drag.

Most feathering props can have the pitch adjusted by the owner to match their boat and engine, but again only for one point in the rpm range. This procedure can be complex on some models and require hauling out the vessel. Obviously this is a more expensive sort of prop and mechanically complicated. If changing from a fixed prop more clearance may be required in the propeller aperture. The hub is very large which can make the smaller sized feathering propellers less efficient.

Variable or adjustable pitch propellers

Several externally adjustable pitch propellers are available but currently only suit larger vessels. The pitch of these propellers can be manually or automatically matched to the speed and conditions to produce maximum efficiency, save fuel or provide more power when needed.

Brunton-Autoprop makes an automatically adjusting variable pitch prop for suitable for smaller vessels and no doubt more will appear on the market if they haven’t already.

Which prop is right one?

To a large extent this depends on what kind of sailing or motoring you intend to do. For the average occasional weekend sailor a fixed prop is the easiest, most reliable and the cheapest option.

For racing boats and cruising yachts that like to race, a folding prop would make the most sense, with low drag in a fairly simple, cheaper unit.

Modern lightweight boats that are easily driven and manoeuvrable would probably consider a folding prop as the most sensible option. If a lot of motoring is envisaged, then a more efficient prop like a feathering model could be considered.

Yachts that are difficult to manoeuvre in reverse due to prop walk might want to consider changing to feathering a prop to give greater control while manoeuvring with the added bonus of less drag while sailing.

For yachts going on an extended blue water cruise, an automatic variable pitch prop would make sense provided you were happy with swapping a simple fixed prop for something mechanically more complex. Drag under sail would be reduced. As the prop would self pitch and be more efficient under a wide variety of operating conditions, fuel savings over a period should go a long way to paying for the extra cost of the prop.

Here a list of resources and opinions collected by Bill Dietrich.

This is intended as an overview of a complex area with many variables. Anyone thinking of changing their prop should consult a qualified professional with experience in this area before making any decisions.

Please add comments with your experiences in this area.