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Sail in Forward, Reverse, or Neutral?

6K views 15 replies 15 participants last post by  Maine Sail 
#1 ·
I have a Hurth ZF 10M transmission on the 24 hp Universal diesel in my Newport 33. The ZF 10M manual indicates the transmission should be in forward or reverse when sailing – not in neutral. I would think it would be advantageous to sail with the prop in neutral to let it free-wheel and reduce drag. Is it truly necessary to operate under sail in with the transmission in forward or reverse?
 
#2 ·
The prop shaft free wheeling turns the output shaft of the transmission but in most transmissions that does not provide lubrication or cooling which can do a lot of damage.
We put ours in reverse to prevent it from spinning (Volvo).
 
#3 ·
In neutral, the trans may not lubricate as it should. Since the input shaft is not turning (only the output shaft) the oil in the trans may not circulate to the bearings.

Also, the freewheeling prop is not really less drag than a prop which is stopped. I know it is counterintuitive, but the spinning, freewheeling prop is more drag.
 
#16 · (Edited)
Also, the freewheeling prop is not really less drag than a prop which is stopped. I know it is counterintuitive, but the spinning, freewheeling prop is more drag.
It is correct that a spinning propeller creates more drag than a stationary one. I too fly airplanes and it is very well demonstrated when your engine quites. You will glide significantly further in a fixed pitch propeller airplane if the prop is stopped.
This has now been shown to be un-true by three independent studies specific to sailboat props. Apparently we can't compare an airplane to a sailboat prop..

One study was done by MIT, one by the University of Strathclyde Ocean Engineering Department and a real world test by Yachting Monthly May 2009 where they actually towed a real boat through the water and measured the strain with a digital strain gauge.

In each of these studies, all of differing designs, the fixed props caused more drag.

I also did my own study where I towed a Michigan Wheel prop through the water and found that when locked it caused nearly three times more drag locked then when freewheeling.

Sailboat props are apparently not helicopters...;)

Here's the graph from Yachting Monthly:


And a quote by the YM test crew.

"Our test shows that letting your fixed-blade prop spin, if the gear box manufacturer will allow it, halves the drag."
 
#12 ·
That is counter to my intuition, can you explain why the spinning prop presents more drag?
The free spinning prop is being spun by the water passing the blades. Whatever friction is in the system/shaft/etc... is what the sails have to overcome. Give your shaft a spin in the boat and you'll see what you're working against. Though it will spin on it's own, it is by no means friction free.
 
#6 ·
The Hurth will not be damaged by sailing in Neutral. One of the few transmissions where that is the case.
 
#8 · (Edited by Moderator)
It is correct that a spinning propeller creates more drag than a stationary one. I too fly airplanes and it is very well demonstrated when your engine quites. You will glide significantly further in a fixed pitch propeller airplane if the prop is stopped.

Also, weather the bearings receive lubrication or not, there is no need to turn the shaft when not producing power. It will still wear the bearings needlessly.
 
#9 ·
I'm not sure boat props follow the same theory as those on airplanes. Being a compressible fluid, the analogy to that of airplane props (in air) falls apart and if you were to substitute the dynamic properties of water vs. air in the equations of the Styrathclyde example, the coefficient of drag changes inversely.

Regardless, this issue has been debated and argued for decades and so-called experts constantly disprove each other. Seems to me you should either get a folding/feathering prop if drag is of great concern or risk the transmission by not following the mfg recommendations. Your choice.
 
#11 ·
#13 ·
Our engine manual also says to sail in reverse.

I added a loop to the keychain and hang the key from the shift lever when the engine is off. This way I remember to put it in reverse when I turn the engine off and remember to put it in neutral when I take the key off to restart the engine. :rolleyes:
 
#14 · (Edited)
I have a Hurth ZF 10M transmission on the 24 hp Universal diesel in my Newport 33. The ZF 10M manual indicates the transmission should be in forward or reverse when sailing - not in neutral. I would think it would be advantageous to sail with the prop in neutral to let it free-wheel and reduce drag. Is it truly necessary to operate under sail in with the transmission in forward or reverse?
This is quite easy..:D



This is what it actually says in the manual itself from ZF:

CAUTION. Idling position of the
propeller: gear shift lever must be
in «O» position.


This means "idling" as in motor off not running and that its okay to leave it in neutral. I don't know whay they chose the word idling but they did..


Use the shift
position opposite to the direction
of travel for locking the propeller
shaft, otherwise the transmission
will be damaged.


This describes how to lock the prop and keep it from spinning. It must be locked in a direction opposite of travel. If you leave it in forward the clutch plates will slip and cause damage to the gear box.

Oh and yes with most fixed blade Michigan Wheel type props there will be less drag freewheeling..
 
#15 · (Edited)
I'll second Maine, the manual for my transmission says Neutral or Reverse, but not forward. And I believe I have a Hurth too.

As for speed, I've switched between neutral and reverse while sailing and tried to tell the difference. I couldn't, so I generally keep it in reverse (which stops the prop) to cut down on the noise (albeit slight) from the spinning prop.

Regards,
Brad
 
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