yanmar 2gm20 - freewheel prop or not?

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Ardenfour

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as per title - when sailing do I lock the prop in gear or let it spin? Would need to be in reverse, it still seems to turn in forward. I seem to lose a fraction of a knot when the prop is locked.
 
Yanmar published a bulletin saying it should not be locked in gear. They say to either let it spin or fit a shaft brake if you want it stopped.

Pete
 
Well that is interesting as I am sure that when previously asked the general consensus was to lock it into reverse and that was certainly the guidance given to me by the engineer who rebuilt my Yanmar gearbox which he believed had suffered from being left to turn when sailing.

Thoughts you all?

PRV - do you still have access to that guidance note from Yanmar?
 
AFAIK Yanmar use two types of gearbox.

The one used on our high hours 3JH2E series should be locked while sailing.

This does sometimes mean it is difficult to engage neutral after sailing for many hours with it locked.

The gearbox used on our boat has epicyclic gears for the reduction and mating internal and external cones to lock the forward and astern modes.

As an aside, since fitting a Kiwi Prop which feathers when sailing the problem is noticably lessened.

Also, carefull adjustment of tickover speed is essential for easy engagement of all gears and neutral-I suspect it is the high hours on the unit, general wear and tear.
 
Ardenfour said:
as per title - when sailing do I lock the prop in gear or let it spin? Would need to be in reverse, it still seems to turn in forward. I seem to lose a fraction of a knot when the prop is locked.
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A very interesting question which I have often wondered about and some good answers. Please could anybody tell me,
what would apply to Volvo MD2040 with direct drive reverse gearbox
 
At the risk of returning to an age old argument, the least drag from a fixed blade propeller is when it is spinning unless it is 2-bladed with a skeg to tuck it behind when locked is better. The KM2P gearbox has been reported to jam when reverse is used to lock it but mine certainly doesn't. However, the Yanmar advice is to let it spin.
 
awol said:
At the risk of returning to an age old argument, the least drag from a fixed blade propeller is when it is spinning unless it is 2-bladed with a skeg to tuck it behind when locked is better. The KM2P gearbox has been reported to jam when reverse is used to lock it but mine certainly doesn't. However, the Yanmar advice is to let it spin.
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That's interesting, and counter to what I've been told - My belief is that a fixed prop generates less drag than a spinning one. As a simple experiment, pull a child's "windmill" through the air with the blades spinning, then do the same with the blades locked still...there's far less resistance with the blades still. I've always sailed with the prop locked in reverse, and never had any issues.
 
I have a 2gm20. Always sail with it locked in reverse. I've never had any problems getting it out of gear. I'd rather do that than listen to it spinning all the time and wearing out the cutless bearing and stuffing box packing.
 
Aardee said:
That's interesting, and counter to what I've been told - My belief is that a fixed prop generates less drag than a spinning one. As a simple experiment, pull a child's "windmill" through the air with the blades spinning, then do the same with the blades locked still...there's far less resistance with the blades still. I've always sailed with the prop locked in reverse, and never had any issues.
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It has been proved over and over again that a freewheeling prop generates less drag than a fixed one. A student did a PhD thesis on it at Strathclyde Uni, Maine Sail has done it using a catamaran with a prop mounted on it and measuring the drag and PBO had somebody do it again about two years ago.

The KMP gearbox does lock in reverse, or at least it becomes impossible to move the lever, but it seems to do no harm to start the engine with it in gear and then go into forward. We have noticed a strange phenomenon of this box. If we apply reverse immediately to start our Brunton feathering it will arrest the shaft. If we forget and sail for a while with the prop rotating, applying reverse does not arrest the shaft but applying forward does. Applying forward immediately does not arrest it.
 
vyv_cox said:
It has been proved over and over again that a freewheeling prop generates less drag than a fixed one. A student did a PhD thesis on it at Strathclyde Uni, Maine Sail has done it using a catamaran with a prop mounted on it and measuring the drag and PBO had somebody do it again about two years ago.

The KMP gearbox does lock in reverse, or at least it becomes impossible to move the lever, but it seems to do no harm to start the engine with it in gear and then go into forward. We have noticed a strange phenomenon of this box. If we apply reverse immediately to start our Brunton feathering it will arrest the shaft. If we forget and sail for a while with the prop rotating, applying reverse does not arrest the shaft but applying forward does. Applying forward immediately does not arrest it.
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So much for my kids windmill experiment...every day's a school day. Thanks for the explanation Vyv.
 
vyv_cox said:
It has been proved over and over again that a freewheeling prop generates less drag than a fixed one. A student did a PhD thesis on it at Strathclyde Uni, .......
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It's been done to death using props that spin very freely, but not props with a drag on them like churning half a gearbox in oil.

It's immaterial anyway, as all decent sailing boats have folding or feathering props. IMHO.
 
It has always intrigued me that the great Sir Robin raced around the world with a prop from a non functioning engine, spinning merrily away. I wonder how much faster he would have been without it.
I muse on that fact every time that I start to think about splashing out beer money on a folding prop. ;-)
 
Having read his book a good number of times I think the word "Raced" is perhaps not what he actually did-especially compared to how they go about it today.

Sir Robin sailed his boat around the world on his own and, knowing he had the slowest boat of any of his potential rivals, he sort of kept his fingers crossed he would get home first-which he did.

Raced-no not really........................
 
Ravi said:
It has always intrigued me that the great Sir Robin raced around the world with a prop from a non functioning engine, spinning merrily away.
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Whereas when Uffa Fox raced across the Atlantic he had the propeller taken off and the boat towed out of harbour :)

Pete
 
Ravi said:
It has always intrigued me that the great Sir Robin raced around the world with a prop from a non functioning engine, spinning merrily away. I wonder how much faster he would have been without it.
I muse on that fact every time that I start to think about splashing out beer money on a folding prop. ;-)
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Cutless bearings must have worn better in them days?
 
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