In article ,
"R.W. Behan" wrote:

No. First: your example in the DC-3 included *feathering*; changing the
AOA to zero.

Ask yourself: how could it make a difference to drag to have such a prop
locked or free to spin?


Alan, look. Feathering an airplane propeller does indeed change the angle
of attack to zero, but that is how you STOP the autorotation. In a boat,
you stop the autorotation by shifting to reverse. The purpose in either case
is the same: to reduce drag.

Your second statement is a non-sequitur. There's no way in the world a
feathered prop CAN spin, right?

Good. You're starting to see.

Now. Change the angle of attack minutely. The propellor now
"auto-rotates", right?

Now. Is the amount of drag it is going to generate the same as the
amount of drag it would generate at a much larger AOA?

Answer that question correctly and then perhaps you'll see that you
can't simply equate the drag of a free-spinning propellor to a solid
disc of the same size as the props diameter...


Cheers. It's a lively discussion.

Dick



--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

On Mon, 28 Jul 2003 17:03:39 -0400, John Cairns wrote:

No, disagree. I purchased a Gori folding prop this season after some
experimenting last season with the prop in neutral vs. reverse. The manual
that came with the boat recommends locking in reverse to prevent
transmission wear, shifting from reverse to neutral while under sail
resulted in a 1/2 increase in boat speed. Did this on several occasions,
just to verify. I figured a folding prop should be worth at least 1/2 knot
also.

Hmmm...
If the AOA is optimised for auto-rotation at a particular boat speed,
then a turning prop would indeed create less drag than a locked prop
up to that speed. At other boat speeds though, drag increases as
auto-rotation increases.
A locked and feathered prop creates the minimum amount of drag.

--
GW De Lacey

When I have my fixed blade on, I lock it in the vertical position with
reserve gear. I don't see any reason to wear out the cutless bearing,
when sailing. When I have the folding prop I lock that in the horizontal
position with reverse gear. I see no reason to add wear to the blade
pivot. I use reverse to lock rotation
against normal rotation of free wheeling action of shaft

Works for me

OT

"nova" wrote...
why not lock in forward? what's the difference?

Only reason I've ever heard is that higher drag in the transmission will reduce
likelihood of the prop and engine turning (kinda like parking your
manual-transmission car in reverse).

John R Weiss wrote:

"nova" wrote...
why not lock in forward? what's the difference?

Only reason I've ever heard is that higher drag in the transmission will reduce
likelihood of the prop and engine turning (kinda like parking your
manual-transmission car in reverse).

In olden times reverse was the "lowest" gear available so the motor had
to turn much faster at any given speed and thus offered more resistance.
I dunno enough about sailboat trannys to say if the same holds true.

Regards
Howard