There seems to be a bit of confusion here. There are basically 4
different types of props with movable blades. All can improve sailing
performance to one degree or another.

Folding props: Blades with a fixed pitch are pivoted along the axis of
the shaft so that they fold back in the fore and aft direction. Blades
may be geared together or independent. In forward they are held open by
the forward thrust. Most have some camber so they are close to the
efficiency of fixed blades in forward but in reverse they are held open
by centrifugal force which means that you have to apply more power to
get them to perform in reverse. Folding props are preferred when sail
performance take preference over powered performance.

Feathering props: Blades are pivoted (more or less) perpendicular to the
shaft. They remain extended when idle but align themselves with the
flow to present the smallest cross section. The blades are geared to
the shaft so that they are held open by the torque. Pitch can be
adjusted by modifying the stops. In reverse the torque flips the blade
over so that you get the same pitch (and performance) in forward and
reverse. However, to achieve the lowest drag the blades usually do not
have any camber making them slightly less efficient. Feathering props
are preferred where a balance must be struck between sail and powered
performance.

Variable Pitch Props: Blades are assembled similar to feathering props
but are geared to a control shaft concentric to the drive shaft. By
adjusting the position of the control shaft relative to the drive shaft
from inside the hull the pitch can be varied to meet current conditions.
Usually the blades are cambered to optimize forward performance.
Variable pitch props are preferred where maximum performance under power
in all conditions is desired and cost is not a limiting factor.

Auto-Prop: Blades are pivoted similar to feathering props but are
independent of each other and can rotate a full 360º. The offset
geometry of each blade is designed to find its own most efficient pitch
by balancing torque against water pressure. Auto-Props can give close
to optimum performance in most conditions in forward or reverse. They
are not truly feathering however. With no torque water pressure forces
the blades back slightly which results in considerably more drag than
normal feathering props. Also they have considerably more mass which
puts a lot of strain on the drive train when shifting from forward to
reverse and back. If left idle for any length of time they require
considerably more maintenance than the others to keep the blades
rotating freely. Auto-Props are best where powered performance takes
precedence over sailing performance.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
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