Prop rotation when sailing

I've got an outboard, which I usually leave in neutral, with the leg / prop in the water, when sailing.

Took a friend out, who strongly advised me to put it in reverse as we sailed, which advice I followed.

Outboard always starts within 3 pulls... not this day:mad: 5 minutes of pulling , choke on, choke off, etc etc. Don't know if there's any logic that caused the difficulty, but I'm not doing that again.
 
http://www.catamaransite.com/files/propeller.pdf

Links to the original work.
This suggest a drag reduction from about 40 pounds to about 21 pounds, allowing a 3 blade prop to rotate with low friction.
I question whether the friction in a typical p-bracket, stuffing box and gearbox is really that low, I would like to see some numbers justified for that.

If you guessed the output side of the gearbox was soaking up 1 hp, that's 350W or so.
At 7knots or 3.5m/s, that would imply 100N of drag or 22lb, if the propellor was 100% efficient as a turbine. At a typical 50 to 60% efficiency, it might add 40llb?

Any mechies know what the real power to spin a gearbox output shaft is?
I guess it depends on the gearbox, and how much oil it is sloshing around.
 
parsifal said:
When I stop motoring I always leave the gearbox lever in ahead. There must be a reason why everyone else seems to prefer astern but I can't think of one.
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Think you will find this does not stop the prop turning, whereas putting it in reverse does.
 
noelex said:
I did not read the YM report, but I have seen some figures, from what looked like good experimental design, that showed the opposite.
I suspect as others have said that it varies with many factors, but there is probably not much in it.
Be careful with your gearbox though. Some are very particular.
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Probably useful if you do actually read it, then you can make informed comment about the contents instead of just guessing.
 
Tranona said:
Probably useful if you do actually read it, then you can make informed comment about the contents instead of just guessing.
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Bit harsh.. He's just highlighting other reports suggest the opposite....

Unless you believe that everything written in YM is gospel......:D
 
BAtoo said:
Bit harsh.. He's just highlighting other reports suggest the opposite....

Unless you believe that everything written in YM is gospel......:D
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Not harsh at all. Not a question of believing what it says either, but the report does cover the issues the poster is trying to comment on, and in a far more meaningful way.
 
webcraft said:
With a folder you can engage astern briefly to fold it then put it back in neutral -water flow over the prop will usually keep it folded.

- W
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Yes. My folder continues spinning (and remains open) unless I put it in reverse for a moment after stopping the engine. I fully sympathize with those who say they wouldn't leave it rotating whether or not there was a benefit; the noise is quite intrusive.

I too am not convinced that there is always a benefit in letting the prop auto-rotate. As others have pointed out, an auto-rotating helicopter rotor acts as a parachute; a fixed helicopter rotor does not.
 
its another one of those "it depends" questions.

The tank tests I have seen show conclusivley that a spinning prop has more drag than a stationary one - BUT I think blade area may something to do with this - but then aero props are not very fat are they and they act as brakes if they spin implying they have more drag when moving.

I have to stop my folding prop from spinning by selecting reverse, then into neutral after the prop has folded. I had a max prop once and was convinced it did not feather - but I did nt dive down to look.
 
noelex said:
I found this online that supports, with experimentation, the notion that a free-wheeling prop has less drag.

http://www.cruisersforum.com/forums/f114/prop-shaft-spinning-when-sailing-7815-2.html
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Links to the same thing I linked to.
Does not alter the question of whether a prop on your average marine gearbox is realistically considered 'freely spinning' or whether the work in turning the gearbox is significant. I don't know what the work done in turning the gearbox is, but it isn't zero.
 
There is no issue anymore: MIT, Strathclyde University and a plethora of individuals, including the chap who did the YM tests, all conclude the same thing - rotating prop equals less drag, and by a significant amount.

Whether or not you lock your prop and how you do it are determined by other factors such as gearbox manufacturers recommendation, wear & tear, noise, personal preference.
 
Salty John said:
There is no issue anymore: MIT, Strathclyde University and a plethora of individuals, including the chap who did the YM tests, all conclude the same thing - rotating prop equals less drag, and by a significant amount.

Whether or not you lock your prop and how you do it are determined by other factors such as gearbox manufacturers recommendation, wear & tear, noise, personal preference.
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Only if the torque output in their tests is an accurate representation of the input of a gearbox. I don't see any evidence of that.
I will ask my mech eng mates what it might be.

Unless anybody here has any data?

The interesting conclusion is that the drag in both cases is quite high.
 
Marsupial said:
aero props are not very fat are they and they act as brakes if they spin implying they have more drag when moving.
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I don't believe this is a relevant comparison.

If an aircraft prop is turning in the glide it is rotating the engine against considerable resistance and so doing substantial work which it takes from the aircraft's energy, i.e. you have induced drag as well as form and profile drag. If it's stopped from rotating the blades stall. This is a high form drag condition but you have lost (most of) the induced drag and the total drag is less (but still very noticeable). Motor gliders usually have feathering props (some have folding props or retracting engines) and it makes a big difference.

In the case of a boat propeller freewheeling with low resistance, the total propeller drag should be less than if the prop is locked and stalled - unless the part of your transmission that is still rotating has very high frictional losses (as already pointed out by some other posters).

Everyone should have folding or feathering props so that the price can plummet with mass production :rolleyes:
 
noelex said:
The only link I can see from you is on post 23

which takes me to completely different paper.

Anyway Mainsail article is certainly worth reading for anyone interested in the topic
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Sorry, my google search took me there via somewhere else. Can't find it right now, gotta find my wetsuit and go sailing. With no prop at all.
Cheers.
 
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