(JAXAshby) wrote:

The blades rotate until they are inline with the shaft axis, how is this
not feathering?

take a look at the blades. they rotate and thus *reduce* drag, but they
certainly don't "feather".

Blades can be set to in-line with the keel (two blade props), but the
prop still has the blades at about 90 degrees to the shaft. For two
blade props on boats where there is something of the keel sticking
down into the water past where the prop is (like on many sailboats),
this does reduce drag. I think however that many sailboats don't have
two blade props and this doesn't work well with a 3 or more blade
prop.

In feathering and/or folding (since I know there is a difference but
don't remember what it is), the blades move to align themselves along
the shaft - kind of like furling an umbrella.

Some props can be folded or have the pitch changed from inside the
boat and some you have to either haul or go into the water. I think
that is variable pitch. A fixed prop where everything is one solid
piece is a non-feathering/folding and constant pitch prop.


grandma Rosalie

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
Shameless Commercial Division: http://www.spade-anchor-us.com

On Mon, 30 Aug 2004 09:39:46 -0400, Glenn Ashmore
wrote:

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.

They are close enough in function to use the term constructively. I
don't think Autoprop's self-description of their product as a
"feathering prop" constitutes trade fraud in this instance.


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.

Agreed. As noted, my friend accepts the wear as adequate pay-off for
the motoring performance enhancements he was seeking. I wouldn't put
an Autoprop on a J-Boat, for instance, or any racer-cruiser. It's a
good compromise if you understand the pros and cons, not a universal
panacea for prop drag.

If left idle for any length of time they require
considerably more maintenance than the others to keep the blades
rotating freely.

He hauls in a TraveLift once a year (luckily his club possesses one)
and inspects and adjusts then as part of his general yearly hull
maintenance/cleaning/repainting. He says it's pretty straightforward
so far, but he acknowledges that they are complex pieces of machinery
for props.


Auto-Props are best where powered performance takes
precedence over sailing performance.

Debatable, if you consider the alternative as being a fixed prop or a
folding prop. I think you have to consider hull type, displacement and
engine output along with intended use. My friend takes his large steel
ketch out alone a great deal, and while he is fine sailing it solo, he
appreciates the degree of control his Autoprop gives him in tight
situations and in solo docking. Certainly that aspect--the degree of
control of a 15 ton boat-- is quite noticeable and is obviously worth
it to him in his use of a heavy displacement cruiser.

That's why I tried to give both pros and cons, as the Autoprop isn't
particularly well-known, being British. Getting one personally would
be senseless for my current boat, but seems a good compromise for him
and has bought him a few more years out of his 35 HP Volvo, even if he
has to rethink transmission isolation and so on.

But it's not for everyone. No "marine solution" is, except maybe for
those wooden tapered plugs people hang off seacocks. G

R.