JumbleDuck
Well-Known Member
In general a folded or feathered prop produces least drag, then a freely spinning one, then a locked one. So the idea is that my locking a folder you encourage it to fold. I think.
Put engine in reverse when sailing?
- Thread starter dolabriform
- Start date
In general a folded or feathered prop produces least drag, then a freely spinning one, then a locked one. So the idea is that my locking a folder you encourage it to fold. I think.
[70521]
Well-Known Member
Having a Chuckle here. Visions of skippers forcing the youngest crew member over the side to check if the prop is vertical.
I could mark the prop shaft but don't fancy ripping my bunk apart and faffing about sorting out the prop every time we used the iron sail.
A visit to the boat show is planned this year to discuss the finer points of silent sailing.
JumbleDuck
Well-Known Member
It all depends on the resistance torque on the shaft. If the propeller can rotate freely then there is very little drag or lift on the blades. If you have a draggy gearbox or friction brake one the shaft then drag and lift on the blades increase as the angle of attack increases. This is, of course, precisely what propellers are there to do - though in normal use they convert torque into thrust and not vice versa.
Then, as you say, if you lock the prop it stall and you get a marked reduction in drag ... but that's from conditions immediately below stall, which are by definition high torque / high drag. Under low torque conditions that drag of a spinning propeller is usually substantially less than that of a locked one.
capnsensible
Well-Known Member
In the past I've sailed racing yachts that had a viewing port above the prop so you could line the blades up vertically and put the shaft brake on.
capnsensible
Well-Known Member
Don't forget Yanmar engine owners, it's £3k for a new gearbox if you keep putting it in astern whilst you are sailing. It will most definitely knacker it.
MM5AHO
Well-Known Member
I will scan and reproduce the paper that purports to show that a locked 3 blade fixed prop produces less drag than the same when free-wheeling.
What I do know is that I can detect no difference in boat speed locked to spinning on my fixed 3 blade prop, but a noticeable difference in noise.
I doubt that any system is sufficiently low friction to allow the prop to spin at it's ideal rate to pitch such that it slices the water without drag. Mine starts spinning (if unlocked) at about 3 knots.
Putting my gearlever in Fwd does not lock the prop, only if it's put in Rev does the spinning stop. (MD2030 with MS2Ld gearbox.)
What I do know is that I can detect no difference in boat speed locked to spinning on my fixed 3 blade prop, but a noticeable difference in noise.
I doubt that any system is sufficiently low friction to allow the prop to spin at it's ideal rate to pitch such that it slices the water without drag. Mine starts spinning (if unlocked) at about 3 knots.
Putting my gearlever in Fwd does not lock the prop, only if it's put in Rev does the spinning stop. (MD2030 with MS2Ld gearbox.)
sailaboutvic
Well-Known Member
Geoff mine the opposite my MD20 with a 120 E sail drive , one would think it would lock both ways , I must be missing some thing although Volvo say it will lock both ways .
vyv_cox
Well-Known Member
I have been locking my Yanmar gearbox in astern for 15 years now, but it is with a Bruntons Autoprop. It rotates in neutral despite being a feathering type.
vyv_cox
Well-Known Member
Maine Sail did some good work on this, mounted a prop on a towed catamaran hull and measured the drag turning and fixed. Turning was far lower.
Adrian
Well-Known Member
I cant get my head round this, why is it any different, can you provide sources? I have a Yanmar engine so would like to know!
MM5AHO
Well-Known Member
A reason often mentioned as to why a gearbox should not be allowed to freewheel, is that some gearboxes have an oil pump powered off the input shaft that lubricates the box. Without engine power there's no lubrication. Freespinning driven by the prop is turning without lubrication they say.
On other more mechanical gearboxes (like my Volvo one), the oil is just a bath that gears sit in, so lubricated without pumped oil.
But there could be other differences between standard and saildrive options?
On other more mechanical gearboxes (like my Volvo one), the oil is just a bath that gears sit in, so lubricated without pumped oil.
But there could be other differences between standard and saildrive options?
CAPTAIN FANTASTIC
Well-Known Member
To reduce drag, (for fixed pitch propeller) put it in nutrual to freewheel. To reduce wear on the shaft and gearbox, put it in gear.
dolabriform
Well-Known Member
so 50 / 50 then
KAL
Well-Known Member
After a near miss with a submarine between Les Minquiers and St. Malo, I tend to leave the prop spinning so they can hear the rumble and know I’m there, even if I don’t know they are!
MM5AHO
Well-Known Member
I tried to scan a paper I have on this topic, but when using enough resolution it produces a file size too big to upload to this site. But summarizing...
By Nigel Warren, published in Practical Boat Owner, December 1972, pages 114, 115.
It says shaft friction is the problem, along with output stage of the gearbox, shaft seal etc, the rotational friction causes a freewheeling prop to rotate slower that if frictionless, and that the reduction in speed creates drag.
But the relationship isn't linear, in fact in his graphs of measured friction against prop rpm, the friction rises to a peak then tails off. At 6 knots for example, a certain measured prop has a drag peaking at 200 rpm and of over 100 lbs drag.
He states.. Taking the situation at 6.5 knots with the largest tested propellor (3:1 reduction) the loss of speed due to the locked prop would be around 0.3 knots (4.5%). If that propellor were freewheeling under the restraint of the shaft friction the speed loss could be 0.8 knots (12%).
This was written in 1972, when yacht speeds were typically 6kn max, and inboard engines had large fixed three bladed props. Give the ratio of drag to boatspeed, much higher drag would occur making the locked prop assertion probably invalid. But for old boats (like mine) surely what was true then is still true?
This peak in the graphs in the article asserts that in effect a locked prop has less drag than a slow rotating one, but a fast rotating one has slightly less drag than a than locked one. So the key to solving the mystery for a specifi boat is what amount of shaft friction there is, resulting in what shaft rpm. My guess on my own boat is about 200 rpm, but I'm sure that under power, the shaft rotates much faster, closer to 600?
By Nigel Warren, published in Practical Boat Owner, December 1972, pages 114, 115.
It says shaft friction is the problem, along with output stage of the gearbox, shaft seal etc, the rotational friction causes a freewheeling prop to rotate slower that if frictionless, and that the reduction in speed creates drag.
But the relationship isn't linear, in fact in his graphs of measured friction against prop rpm, the friction rises to a peak then tails off. At 6 knots for example, a certain measured prop has a drag peaking at 200 rpm and of over 100 lbs drag.
He states.. Taking the situation at 6.5 knots with the largest tested propellor (3:1 reduction) the loss of speed due to the locked prop would be around 0.3 knots (4.5%). If that propellor were freewheeling under the restraint of the shaft friction the speed loss could be 0.8 knots (12%).
This was written in 1972, when yacht speeds were typically 6kn max, and inboard engines had large fixed three bladed props. Give the ratio of drag to boatspeed, much higher drag would occur making the locked prop assertion probably invalid. But for old boats (like mine) surely what was true then is still true?
This peak in the graphs in the article asserts that in effect a locked prop has less drag than a slow rotating one, but a fast rotating one has slightly less drag than a than locked one. So the key to solving the mystery for a specifi boat is what amount of shaft friction there is, resulting in what shaft rpm. My guess on my own boat is about 200 rpm, but I'm sure that under power, the shaft rotates much faster, closer to 600?
Kylora
Well-Known Member
My Brunton Autoprop is on a shaft drive, connected to Mitsubishi based Thorneycroft.
I leave it in forward gear at tickover, and then strangle the engine.
I'm sure that I read that advice in the Brunton literature.
It seems to stop the spinning.
I found that if I stop the engine in neutral, and then engage a gear, then it won't stop the shaft from turning, though I might only have tried that technique in reverse as that was my previous custom with a fixed 3 blade on a Beta.
Cheers
Ash
DipperToo
Well-Known Member
I have recently fitted an Autoprop with a Yanmar 3YM30 shaft drive. I found the statement below on an older piece of Bruntons literature. I think that Yanmar generally discourage any sort of action leaving the engine in gear and the more recent Bruntons literature simply directs you to the engine manufacturer. I would have thought that once the prop was feathered that there would be minimal load on the engine/gearbox anyway?
Feathering your Autoprop under sail
All you need to do, is to stop the engine whilst motoring in ahead, and thereafter leave the
engine engaged in ahead. With engines fitted with some hydraulic gearboxes, engage your
shaft lock. Your Autoprop distributor will be able to advise you if this is necessary. Details of
the Autolock manufactured by Brunton’s Propellers are also available on request
sailaboutvic
Well-Known Member
You quite right and that's what we do now ,
although when we had a chat with Bruntons before we brought it we where toldto put it in rev if used on a sail drive and Volvo said the same , we now also find that by stopping the engine in gear that it very rearly locks in gear .