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Old 27-01-2012, 03:35   #16
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Re: free wheel or lock the prop in gear

Plenty of studies have been done that prove free wheeling gives less drag, but like many the constant noise drives me to lock it in reverse and yes you may need to slow down to get it back in neutral after leaving it in reverse for a long time.
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Old 27-01-2012, 04:29   #17
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Re: free wheel or lock the prop in gear

There is a lot of misunderstanding about the drag caused by fixed versus freewheeling shaft and prop.
The relevant parameters are the blade /disk area ratio and the residual torque resistance.
Disk area= area of the full circle around the blade tips
Blade area = area projected by the blade on a plane perpendicular to the shaft:
Blade /disk area ratio of a hélicopter rotor is about 5%
Blade /disk area ratio of a 2 thin blades fixed prop is about 20%
Blade /disk area ratio of a 3 wide blades fixed prop is about 80%

Residual torque resistance is caused by friction of the shaft bearing, packing gland , gearbox, friction of water on the spinning blades, torque driving a shaft alternator
...
When the engine of a helicopter fails in flight and no longer delivers torque, the pilot adjust the rotor to negative collective pitch , the rotor autorotates against the residual torque resistance as the helicopter descends and keeps generating enough lift to glide the helicopter into a mild forced landing.
Same with 20% Blade /disk area ratio 2 thin blades fixed prop : Allow it to spin and the Residual torque resistance causes a low incidence laminar flow of water on the blades and a lift, read drag, comparable to that which saved the helicopter and comparable to that which would be caused by a full disc.
So: With a low blade/disk area ratio prop, you have every reason to lock the shaft when sailing : Les drag, vibration and wear !


On the contrary, with an 80% blade/disk area ratio 3 wide blades props , the drag when the shaft is locked is 80 % of the drag which would be caused by a full disk.
If you leave the shaft spinning and if the residual torque resistance is small enough, the prop will spin fast enough, with low incidence and low lift (read drag)of the laminar flow of water over the blades, and the drag wil be much reduced. At low speed, say 4 knots on my 40' 9 tons sailing boat, I notice a .3 knots gain of speed when I let the shaft spin.
At hull speed expect no measurable gain as the locked prop's drag is negilgible compared to the hull's drag
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Old 27-01-2012, 07:20   #18
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Re: free wheel or lock the prop in gear

Quote:
Originally Posted by GALAWA View Post
So: With a low blade/disk area ratio prop, you have every reason to lock the shaft when sailing : Les drag, vibration and wear !


Like always I have an OPEN INVITATION to anyone wanting to fly to Maine, pay for my time, and view the actual real world of props being dragged through the water. You must however be willing to publicly be seen in a new video showing the look of disbelief on your face.

I still have a 16" two blade and a 16" three blade prop in my shop, the three blade in the video.

Actually, if we do this in the off season I have a Campbell Sailor 3 blade, Michigan MP 3 blade, Michigan Sailor 2 blade and a Flex-O-Fold two blade. All the props are 16" and between 9 & 12 pitch.

You're more than welcome to come see it for yourself. In every one of my "real world" tests the spinning resistance (bearing resistance) of the test jig was set to match that of my own boat as well as shaft angle.

The drag of the three blade prop was MORE THAN DOUBLE when locked compared to free spinning....

All that said, I lock my fixed prop unless I need that extra boost. Our vessel was free spun for a 5 year nearly 25,000 nm cruise by the PO and the gear box has never been rebuilt and now has over 3300 engine hours plus over 30,000 nm sailing/engine on her...... I still don't like the noise and pot buoy catching ability a free wheeling prop has, despite the cut in speed I get.
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Old 27-01-2012, 07:46   #19
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Re: free wheel or lock the prop in gear

Dear Maine Sail,
If I get you right, you confirm that a locked 3 blades prop drags twice as much as the same spinning 3 blades prop.
So why should I fear publicly being seen in a new video showing a look of disbelief on my face.

More seriously :
- What is the Blade/disk area ratio of the tested prop ?
I would guess 50% rather than 80%
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Old 27-01-2012, 08:02   #20
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Re: free wheel or lock the prop in gear

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Mechanically, its better to free wheel it,

In gear, it is moving the internals of the gear box with out load,
leading to chatter and slap inside the gear box amongst the gears and the bearings, Because they dont have a load on them.

And will in time destroy your gear box,
You need to think that one through again.....
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Old 27-01-2012, 14:46   #21
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Originally Posted by IslandHopper

You need to think that one through again.....
free wheeling theprop isok. if the transmission lube pump is on the shaft side of the trannie....if it is not then you can easily ruin your transmission

susan. ussv dharma...south pacific. still single handing at 76
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Old 27-01-2012, 16:45   #22
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Re: free wheel or lock the prop in gear

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You need to think that one through again.....
Nah! Just needs another beer.
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Old 28-01-2012, 19:34   #23
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Re: free wheel or lock the prop in gear

Ran into the Yanmar people today at a trade show, and had a great discussion on marinized Yanmars, including the prop in first, reverse, or neutral debate. All three stated categorically the transmission is to be left in neutral, and the prop to freewheel.
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Old 28-01-2012, 19:41   #24
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Re: free wheel or lock the prop in gear

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Originally Posted by shorebird View Post
Ran into the Yanmar people today at a trade show, and had a great discussion on marinized Yanmars, including the prop in first, reverse, or neutral debate. All three stated categorically the transmission is to be left in neutral, and the prop to freewheel.
What would they know, They have got it wrong.

The experts in here will tell them they have it wrong as well.




Are you on cbrxx.
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Old 28-01-2012, 19:56   #25
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Re: free wheel or lock the prop in gear

Quote:
Originally Posted by shorebird View Post
Ran into the Yanmar people today at a trade show, and had a great discussion on marinized Yanmars, including the prop in first, reverse, or neutral debate. All three stated categorically the transmission is to be left in neutral, and the prop to freewheel.
And what was the purpose of this this outcome? What were they suggesting could be achieved by selecting neutral and and allowing the pro to freewheel?

Reduced drag?
Limit possible transmission problems?
Reduce ( or create?) noise?
Something else?
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Old 28-01-2012, 23:05   #26
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Re: free wheel or lock the prop in gear

Simple

If you read the link I posted in Link #6 of this thread you will find that Yanmar transmissions (Kanzaki) will be damaged if left in reverse. The Yanmar service advisory # MSA08-003 -states that damage to the transmission will occur if left in gear. If you want to stop the transmission from spinning use a shaft brake. Or possibly pay for the consequences - which Yanmar wiill not pay for even if under warranty.
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Old 29-01-2012, 12:10   #27
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Re: free wheel or lock the prop in gear

Quote:
Originally Posted by mitiempo View Post
Simple

If you read the link I posted in Link #6 of this thread you will find that Yanmar transmissions (Kanzaki) will be damaged if left in reverse. The Yanmar service advisory # MSA08-003 -states that damage to the transmission will occur if left in gear. If you want to stop the transmission from spinning use a shaft brake. Or possibly pay for the consequences - which Yanmar wiill not pay for even if under warranty.
If that be the case for ONLY the Kanzaki's then it's good that most sailboats use either a folding or feathering prop.

So, one's choices are; buy a no-resistance prop or buy a new Hurth tranny.
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Old 29-01-2012, 12:22   #28
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Re: Free Wheel or Lock the Prop in Gear

SHAFT LOK INC.
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Old 29-01-2012, 12:42   #29
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Re: Free Wheel or Lock the Prop in Gear

With a borg warner, we put a shaft lock on to prevent damage to the transmission, due the different bearings that were not getting lubricated when free wheeling. I have a twin disc in the vessel I own now, and need to consult with the people who rebuild them to determine free wheel or shaft lock. Has anyone done any research on the drag of 4 blade props?
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Old 29-01-2012, 12:46   #30
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Re: Free Wheel or Lock the Prop in Gear

yanmar, as in my ericson , says freewheel. so does velvet drive, as in my formosa......doesnt matter in formosa as to speed--aint no stinkin difference, and if there is, i dont care. i am not concerned with speed. if i were concerned with speed, i would cruise a performance cruiser instead of a formosa.
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