Do you put in gear or not when sailing?

In reverse

Folding propellers won't fold if in neutral.
Bear in mind that Yanmar has released a service bulletin that advises AGAINST putting it in gear for specific gearboxes.
 
nickfabbri said:
Just read that and am still confused. If I have a 2 blade folding prop can I be in reverse?
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Nick,
I'm confused too, but I've always put my gearbox in reverse (as most people I know with a folding prop) and will continue doing so. The problem is that your warranty may be void with the gearboxes mentioned by Yanmar.
The forces on the gearbox would be very small with a folded prop compared to e.g. a 3 bladed "normal" propeller, so I think it is safe to put it in reverse, but only with a folding propeller.
 
Stu,
I have not read every part of the document but P24 seems to confirm that preventing the prop from rotating increases the drag. One other aspect not dealt with in this report is whether locking the prop will damage the gearbox. When I contacted Volvo Penta about the VP 2003 diesel engine and MSB2 gearbox combination they said that the prop should be allowed to rotate. Of course, this is not to say that it applies to all engines/gearboxes.


Roy
 
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Juggler7823 said:
Stu,
I have not read every part of the document but P24 seems to confirm that preventing the prop from rotating increases the drag. One other aspect not dealt with in this report is whether locking the prop will damage the gearbox. When I contacted Volvo Penta about the VP 2003 diesel engine and MSB2 gearbox combination they said that the prop should be allowed to rotate. Of course, this is not to say that it applies to all engines/gearboxes.


Roy
Click to expand...
Roy
it was me that phoned up VP and talked my way past the receptionist to a chap called Lyn, he told me that on the VP2030 MS2 they used to tell peeps to put in reverse BUT that the box would lock in reverse, they advised to flick the starter to unlock it. Allegedly peeps were confused by this so at that point in time, 2003 ish they changed the advice to leave in neutral.
The box itself is just a bog std box, gears etc in a bath of oil so no probs with a pump to worry about.
So if we go back to the paper, leave spinning and it causes less drag?
Stu
 
It took a while to get to the point but they seem pretty sure

"The experimental results confirm that a locked propeller produces greater drag than
does a freewheeling screw (up to 100% more drag was observed, this being at higher
speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic
resistance is significantly affected by the amount of frictional torque on the shaft, low
torque being accompanied by low drag."

I don't know how this would equate to speed difference but I think I'll keep my 2 bladed prop locked to cut down on noise and wear and tear.
 
Spyro said:
It took a while to get to the point but they seem pretty sure

"The experimental results confirm that a locked propeller produces greater drag than
does a freewheeling screw (up to 100% more drag was observed, this being at higher
speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic
resistance is significantly affected by the amount of frictional torque on the shaft, low
torque being accompanied by low drag."

I don't know how this would equate to speed difference but I think I'll keep my 2 bladed prop locked to cut down on noise and wear and tear.
Click to expand...
I suppose that a good idea would be to put a spot of paint or line on the top of the shaft, indicating when the prop is vertical?
Stu
 
No it requires energy to turn the prop so letting the prop will use more energy that allowing it to freewheel. Consequently, locking the prop will give less drag.
 
This is a well trod path. Somewhere in my stock of old books I should be able to find articles by William Atkin in the 1920's and Maurice Griffiths in the 1930's which say exactly the same thing - but not in such fancy language!

The two pertinent issues are

1 Can you measure the difference in speed under sail with locked or unlocked prop.

2 Does it do any harm to lock the prop by putting into reverse to stop the dreadful noise (particularly on my saildrive).

From all I have read, the jury is out on both these counts.

All I know is that I can't tell the difference in speed on my boat and the engine/saildrive has done over 2500 hours in 10 years and has probably been locked every time the engine is not in use..
 
Tranona said:
1 Can you measure the difference in speed under sail with locked or unlocked prop.

..
Click to expand...

You would think with their test tank they could tow a boat with a constant force and measure the difference in speed. Or you could get a tow from another boat on a flat calm day, boat in front keeps engine at constant throttle setting boat behind has freeweeling prop then a locked prop and measure the difference.
 
Spyro said:
You would think with their test tank they could tow a boat with a constant force and measure the difference in speed. Or you could get a tow from another boat on a flat calm day, boat in front keeps engine at constant throttle setting boat behind has freeweeling prop then a locked prop and measure the difference.
Click to expand...

Or in a practical way, just take it out of gear and see what the speed difference is - have done this with no consistent difference!
 
Tranona said:
Or in a practical way, just take it out of gear and see what the speed difference is - have done this with no consistent difference!
Click to expand...

If you mean when sailing I would think there are too many variables. To notice a difference that you could rely on, you would need a decent speed for that you would need a constant wind (speed and angle) and that would mean waves so that's why I suggested a tow on a flat calm day.
 
I frequently experiment with a locked or rotating prop. Sometimes I think I can detect 0.1 or 0.2 knot improvement with the prop spinning, but it could just be wishful thinking. Then after about 10 minutes I get fed up with the noise and stick it in gear. I then feel annoyed with myself that I might be deliberately inducing more drag than I need, but finally convince myself that one of the most important pleasures of sailing is the complete absence of mechanical noise.
 
I put mine in gear when sailing (if I remember to) because I think the prop will be less likely to catch a bit of rope or net. Even if that costs me a fraction of a knot less speed, I'd rather that than the hassle of a fouled prop.
 
With the generator engaged on the free-wheeling prop (as in Skipper Stu's other post) the drag would be somewhere in between the locked and the free-wheeling 'extreme', depending on the state of your batteries?
That would probably be acceptable given that it's putting amps back into the batteries while you're going a little slower (or not)?
Except from the potential mechanical noises...?
 
skipper_stu said:
I suppose that a good idea would be to put a spot of paint or line on the top of the shaft, indicating when the prop is vertical?
Stu
Click to expand...

We always did this on racing boats, some even had a window to heck the prop.

With the gearbox questions and reverse, check with manufacturer of gearbox. Ours seized up after sailing for a few hours with gearbox in reverse. I'd just changed from a folding prop to a fixed as the volvo had thrown a blade off. After that I checked the manual and it was very clear that you sail with the shaft spinning if you have a fixed bladed prop.
 
Seven Spades said:
No it requires energy to turn the prop so letting the prop will use more energy that allowing it to freewheel. Consequently, locking the prop will give less drag.
Click to expand...

Not correct for boat propellors. This myth comes from helicopters and aeroplanes, and is called autorotation. There is a lot of gumph on it here...

http://en.wikipedia.org/wiki/Autorotation_(helicopter)

If the engine fails in flight in a helicopter, autorotation is used to enable a safe landing. Upon engine failure the pitch of the main blades must be set (rapidly!) to a point that allows this. The blades are kept spinning by the upflow of air through them enacting on a section of the blades where the pitch is such that that an upflow drives them. Then another section of the rotating blade has its pitch such that it pushes air down when rotating, hence keeping the descent rate to a level that is survivable on landing.

All helicopters must be able to perform a safe autorotation landing on engine failure to be licensed to fly. The blade and pitch system is carefully designed to enable this. This ability to do this leads to a lower efficiency of the blade system in powered flight, but is a price worth paying. So helicopter blades are designed to give lift when powered, and still give some lift when unpowered and autorotating.

Now to propellors on boats. These are designed to give thrust when powered. They are also designed to give some thrust, although less and less efficiently, when powered in reverse. There is no design consideration at all to give thrust when autorotating - in fact the opposite, you don't want it. So the thrust that marine propellors will give when autorotating is tiny, and much less than the drag of a locked propellor.

If I can remember, I'll get a video of this effect and post it here. My main engine is outboard so I can demonstrate this by releasing the hold-down catch and holding the engine down by hand. At 5 knots I can hold the engine down by hand with a freewheeling prop. Knock it in gear to lock the prop, and the kick up force nearly flips me off the boat. Once you've seen this, you'll never lock a prop again to reduce drag!
 
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