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Old 25-08-2009, 23:31   #1
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Underway Propeller Riddle

hi all

I still cannot figure out whether a fixed 3 blades propeller should be free to turn ( neutral gear) or immobilized (forward or reverse gear ) when underway in a sailboat ?
if I trade mine against a 2 blade feathering prop will I get the same thrust or will I be loosing power, the engine is a 2GM20 18hp Yanmar.
thanks for the info
Georges
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Old 25-08-2009, 23:59   #2
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Originally Posted by gbendaly View Post
hi all

I still cannot figure out whether a fixed 3 blades propeller should be free to turn ( neutral gear) or immobilized (forward or reverse gear ) when underway in a sailboat ?
if I trade mine against a 2 blade feathering prop will I get the same thrust or will I be loosing power, the engine is a 2GM20 18hp Yanmar.
thanks for the info
Georges
Whether to freewheel or not has been argued in several threads here. The answer is, do the experiment yourself. Others here have done the experiment and posted their results. Some were faster freewheeling, some locked, it depended on the boat, probably including prop type, friction in the transmission and other factors.


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Old 26-08-2009, 01:01   #3
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Before you experiment, you MUST check what is recommended for your gearbox.

Some require that the prop is freewheeling to avoid the constant presuure from the thrust.

Other gearboxes are not sufficiently lubricated in neutral unless the engine is running to allow the prop to turn continuously.
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Old 26-08-2009, 01:08   #4
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We stick ours in astern before turning the engine off.

why?

Cos thats what Moorings / Sunsail tell their charterers to do. So it must be better for the engine.
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Old 26-08-2009, 01:14   #5
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if I trade mine against a 2 blade feathering prop will I get the same thrust or will I be loosing power, the engine is a 2GM20 18hp Yanmar.
thanks for the info
Georges

There are a couple of reasons for a 3 blade prop. One is to make sure you can extract the power from the shaft with little clearance to the hull. A two blade prop would need to be too big.

There are 3 bladed folding/feathering props.

We upgraded from folding to feathering and it was the best thing we did to the boat so far.
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Old 26-08-2009, 01:41   #6
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could you please send me a link to these previous threads
thanks
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Old 26-08-2009, 02:01   #7
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Before you experiment, you MUST check what is recommended for your gearbox.

Some require that the prop is freewheeling to avoid the constant presuure from the thrust.

Other gearboxes are not sufficiently lubricated in neutral unless the engine is running to allow the prop to turn continuously.
mine is a KANZAKI KM2
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Old 26-08-2009, 02:45   #8
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I think the Kanzaki gearboxes allow freewheeling but if locked one must use reverse, not forward, gear. The only scientific study I know of regarding freewheeling vs. locked can be found here.
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Old 26-08-2009, 03:59   #9
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A few earlier discusions of interest:

In gear or out when sailing?
In gear or out when sailing?

Prop shaft spinning when sailing
Prop Shaft Spinning when Sailing

Shaft Brakes
Shaft Brakes

Propeller generator

Propeller generator

Borg Warner Velvet Drive spinning
Borg Warner Velvet Drive spinning
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Old 26-08-2009, 09:47   #10
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What about shifting from reverse when sailing ,as recommended by manufacturer(when sailing at speed)back into neutral so I can start the engine?(ZF Transmission)I get the feeling it doesn't like it.
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Old 26-08-2009, 10:15   #11
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I could not shift from reverse to neutral while sailing the old boat (the new one has a folding prop). I would put pressure on the handle and then press the starter, with the turning of the shaft from the starter it would jump right into neutral and the engine would fire up.
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Old 26-08-2009, 11:15   #12
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a two blade folding will likely not give you the same thrust. Put it in reverse and not free wheel.
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Old 26-08-2009, 13:33   #13
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I believe it was the Yachting Monthly that published an extensive material on this issue last November or October. You can get a copy in your library or ask them for a copy via e-mail. It is worth it as they made real life tests and they went thru a good choice of props.

Briefly:
- yes 3 blade fixed is better that the folding or feathering, except for some expensive and good designs of the folding that do deliver,
- yes it drags a lot,
- freewheeling is less drag than locking, but not all transmissions are designed to be left freewheeling,

Note - if your 3 blade fixed was not the perfect match then you might match up or exceede it with a well matched folding / feathering!

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