Transmission nuetral or locked while sailing

[transmitterdan]

Originally Posted by Island Time O25

Also I can't really understand how or why the idea that locked prop has less resistance than free-spinning one ever started. That's like thinking that automobile wheels will have less resistance when locked than when turning.

Because the effect is opposite in props designed for air compared to props designed for water. Some can't understand the difference between air and water it seems.

[a64pilot]

Originally Posted by Island Time O25

Also I can't really understand how or why the idea that locked prop has less resistance than free-spinning one ever started. That's like thinking that automobile wheels will have less resistance when locked than when turning.

OK, on an airplane, a spinning prop is turning the engine over at a couple of hundred RPM, faster than the starter can. This power that is being wasted to spin that dead engine over is coming of course from the kinetic energy of the airplane and is drag.
Stop that thin, pencil looking prop and all the drag you have is the flat plate drag of the thin prop.

Many of us are pilots and naturally think turning prop = high drag as we are taught and shown that in aircraft.

Just it doesn't translate to a boat prop, obviously.

[a64pilot]

Look at this picture, my Baby Cessna and a B-25, see how thin the props are compared to a boat prop?

[Blue Stocking]

Hamilton-Standard, who built the largest aircraft propellors the free world has ever seen, says they feather un-powered props to relieve any positive hydraulic pressure from either side of the pitch control valve assembly. Ships do the same thing.

[missourisailor]

Originally Posted by a64pilot

Look at this picture, my Baby Cessna and a B-25, see how thin the props are compared to a boat prop?

Your Cessna 150 is on the right...right?

Seriously, "Killer B" was built in my home town. Many, many years ago.

I had the honor of working on and going on a test hop with her sister, Fairfax Ghost.
The last B-25 to leave KCK before the airport was shut down in 85.
Thanks A64, for stiring the memory.

[a64pilot]

Well she is a 140' little wheel in the back

Tom Reilly has Killer B now, it and he live in Douglas Ga. I think Tom may even have been given Killer B by her owner or maybe just a great deal, I'm not sure. There is a story there, I have forgotten it though.
He and a group are restoring, actually building really an XP-82, twin Mustang. A very rare bird in that it's an X plane, not a regular P-82. We have provided some parts and heat treat etc for them in their restoration so Tom brought Killer B over to the plant for us to see. My son and I flew in her back home in Douglas, we both got some left seat time. I had to bring out my 140 for some pictures as she was built in 1946, close to the B-25's build date.
Link to the XP-82 project.
http://xp-82twinmustangproject.blogs...p/home_17.html



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[Stu Jackson]

Originally Posted by Island Time O25

Hey, even Universal and Hurth people can't make up their minds. These are the two sentences in their manual, one in the beginning and the other at the end.

CAUTION:
DO NOT LEAVE GEAR IN FORWARD WHEN SAILING. GEAR MUST BE IN NEUTRAL FOR FREE WHEELING OR SHIFTED INTO REVERSE TO LOCK PROPELLER WHILE SAILING.
.......

CAUTION: TRANSMISSION MUST BE LEFT IN NEUTRAL WHEN SAILING, DO NOT LEAVE IN FORWARD GEAR WHEN SAILING WITHOUT POWER AS IT WILL DAMAGE GEAR.

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[Island Time O25]

Originally Posted by a64pilot

OK, on an airplane, a spinning prop is turning the engine over at a couple of hundred RPM, faster than the starter can. This power that is being wasted to spin that dead engine over is coming of course from the kinetic energy of the airplane and is drag.
Stop that thin, pencil looking prop and all the drag you have is the flat plate drag of the thin prop.

Many of us are pilots and naturally think turning prop = high drag as we are taught and shown that in aircraft.

Just it doesn't translate to a boat prop, obviously.

Yes but the aircraft prop blades are pencil thin and 3-blade fixed boat prop blades are so much wider. Every mention of any study I've read so far claims locked produces more drag than free-wheeling. Now, there may be other considerations to prefer locked prop but that's whole other issue.

[EWOL Props]

There is a solution for this.....

Avoid fixed propellers.

A good feathering propeller gives you same performances under power, benefits in reverse thrust and maneuverability, no drag and no doubts on if you should leave it in forward, backwards or neutral.....it will anyway take the feathering position and will not create torque during sailing.

Originally Posted by Island Time O25

Hey, even Universal and Hurth people can't make up their minds. These are the two sentences in their manual, one in the beginning and the other at the end.

CAUTION:
DO NOT LEAVE GEAR IN FORWARD WHEN SAILING. GEAR MUST BE IN NEUTRAL FOR FREE WHEELING OR SHIFTED INTO REVERSE TO LOCK PROPELLER WHILE SAILING.
.......

CAUTION: TRANSMISSION MUST BE LEFT IN NEUTRAL WHEN SAILING, DO NOT LEAVE IN FORWARD GEAR WHEN SAILING WITHOUT POWER AS IT WILL DAMAGE GEAR.

[Wotname]

Originally Posted by missourisailor

Of course what you are saying about the heeling due to counter rotational forces is true, vector graphics prove that. But the larger force is the drag due to the cavitation that the stationary prop is producing.

Cavitation is what you keep omitting.

Hmm… not sure if its cavitation or turbulence or blue cheese but the generic term drag covers it nicely but I do concur it is probably the larger force.

[jkangas]

Originally Posted by jmschmidt

Locked or in reverse gear. A free-wheeling prop creates drag equal to a solid disk the diameter of the prop. Same in airplanes.

Nope. Free wheeling prop has less drag. My boat slows down noticeably when I lock the prop.


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[missourisailor]

Originally Posted by a64pilot

Well she is a 140' little wheel in the back

Tom Reilly has Killer B now, it and he live in Douglas Ga. I think Tom may even have been given Killer B by her owner or maybe just a great deal, I'm not sure. There is a story there, I have forgotten it though.
He and a group are restoring, actually building really an XP-82, twin Mustang. A very rare bird in that it's an X plane, not a regular P-82. We have provided some parts and heat treat etc for them in their restoration so Tom brought Killer B over to the plant for us to see. My son and I flew in her back home in Douglas, we both got some left seat time. I had to bring out my 140 for some pictures as she was built in 1946, close to the B-25's build date.

Way too cool!!!!!

[missourisailor]

Originally Posted by Wotname

Hmm… not sure if its cavitation or turbulence or blue cheese but the generic term drag covers it nicely but I do concur it is probably the larger force.

Mummmmmm.... Blue cheese.

[missourisailor]

Originally Posted by EWOL Props

There is a solution for this.....

Avoid fixed propellers.

A good feathering propeller gives you same performances under power, benefits in reverse thrust and maneuverability, no drag and no doubts on if you should leave it in forward, backwards or neutral.....it will anyway take the feathering position and will not create torque during sailing.

Okay, here is a question for you.
On "fat ole cows" that have full keels, prop walk is a significant issue in reverse with fixed props.

Does your prop decrease prop walk?

[a64pilot]

Originally Posted by missourisailor

Okay, here is a question for you.
On "fat ole cows" that have full keels, prop walk is a significant issue in reverse with fixed props.

Does your prop decrease prop walk?

You know I went with another type of feathering / variable pitch prop, it completely eliminated my prop walk, and I sorely miss it.
Be careful what you wish for. With that prop walk, I could turn sharply to Starboard, make docking alongside a breeze, with no prop walk it's tougher.

I think "walk" is associated with pitch, greater the pitch, the greater the walk.
For the airplane crowd, it's not "P" factor