Prop locked or not

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Sailfree

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From another thread


"You might want to review some work Peter Mackenzie did at Strathclyde University which suggests....... well have a read.

http://strathprints.strath.ac.uk/5670/1/strathprints005670.pdf"

If you look up reference it clearly states in what appears to be an authoritive work that a freewheeling prop has less resistance than a locked prop when sailing.

Q1. Have I understood this correctly

Q2. I thought previous threads concluded the contary

Q3. Is there any other research into this question

I am looking for facts rather than opinions.

Just when I thought I understood the right answer someone goes and changes it /forums/images/graemlins/grin.gif
 
Hmm, interesting and it runs contrary to my own findings when we used to have a fixed prop. Locked prop gave best speed but nothing like as good as an autoprop /forums/images/graemlins/smile.gif
 
Regardless of any debate on efficiency, I would opt for locked, for the simple reason that the noise of the prop whirring away when under sail disturbs the peacefulness of sailing.
 
The work I have seen on this implied that it makes a big difference how free the prob was to freewheel. A prop driving the back end of a gearbox and a rubber cutless bearing is probably not 'free' in this context.
I also agree about the noise!
Don't forget that the trad wisdom for racing was to have a 2 blade prop locked in line with the skeg/keel.
Personally I would not want a fixed prop, a folding or feathering prop makes such a difference.
 
On one of our first trips last year with a new Autoprop, which must be locked when under sail, the steering suddenly developed an apparently serious problem. I could turn to port, but only with great difficulty to starboard. To cut a long story short, it turned out that the prop had somehow started to turn and the prop wash was exerting a great force on the partially balanced rudder. On stopping the prop with the gear lever, all was well again. Phew!
 
Interesting article.
The locked prop approach was popular with long keeled yachts with two bladed props. and there was a great ritual about locking the prop so that it "hid" behind the keel with the blades vertical.
As you might expect , when the propellor revolves freely with little or no braking torque then there is little dissipation of energy ie low drag. In most boats however, the stern gear and the gearbox do exert a significant torque so ther is a significant loss of energy with consequent drag.
In any event, most cruising sailors lock off the prop to avoid wear and noise and most racing boats have folding props. So in the middle ground , the decision to lock or not will probably make less difference in a club race than one bad tack!!!!
To respond to Salfree's questions
Q1 YES
Q2 PROBABLY
Q3 ALMOST CERTAINLY
 
[ QUOTE ]
A freely rotating prop must cause less drag than a locked one otherwise it wouldn't rotate - think about it!

[/ QUOTE ]

I did and it doesn't!

Try shutting off the motor of a helicopter and see if you would rather drop like a stone with a locked rotor or a bit slower with one that is rotating from the airlow.

Anyway there are other considerations like wear on gearbox, shaft seals and bearings, lack of lubrication of the gearbox and plain old irritating noise.
 
Whatever the theory it often depends on the gearbox if you should or shouldn't as Lady in bed has said. If put some gearboxes in gear with a fixed prop trying to turn it can lock up the gearbox. I've done it!
 
Volvo gearboxes tend to stick in gear with a locked prop but a quick blip of the starter before putting it into neutral and actually starting does the trick. I had to do that on our last boat where we fitted a Volvo MD22L, but the same engine in Perkins blue with a Hurth gearbox doesn't lock.
 
But Robin .... in that situation a Chopper is in Autorotate.

But still - I was always told a rotating prop has more drag.

My PRM box - you can put in reverse or fwd - prop still rotates. Anyway PRM manual says no need to lock prop - it's fine to spin on its own.
 
Thanks, that's that really, isn't it? Thank goodness. No more interminable arguments and analogies to batmobiles, helicopters, and other unrelated vehicles.

FWIW I lose half a knot when I stop the shaft. It wasn't quite as clear-cut on my Centaur but I'd wager that there was extra drag with a locked shaft -- well, that's proven now, anyway.
 
Sorry lemain, I think this could run on!, I've read the document now, and tried to de-cypher some of the graphs.
The graphs show drag increases as a function of torque. The biggest torque they consider is less than 2Nm.
I think the drag in the gearbox could be a lot more than this, so the real drag may approach or overtake the locked case.
2Nm at 1000 rpm is less than one third of a HP. Gearbox losses are probably way more than that?
I can see that the answer may vary from boat to boat, in line with observations.
It does back the concept of a folding prop though!
 
A very persuasive paper. My only complaint is the use of the term "myth" to describe a theory, albeit incorrect, for which there was at least some theoretical backing.

It provides an excellent example of the limitations of untested mathematical models.

Like Refueler, I have a gearbox which allows my prop to rotate in both forward and reverse. The combination of a Yanmar YM30 and a Brunton Autoprop provides me with something of a puzzle.

I have to lock the shaft to prevent the Brunton spinning it at much higher revs than the engine ever does. Evidently the Autoprop can not only repitch to maximize the conversion of a driven shaft's energy into forward thrust, but also repitch to maximise the conversion of forward motion into rotation of a freewheeling shaft. At hull speed, a high-pitched whining can be heard, and the rubber on the shaft seal was on one occasion too hot to touch. (No, I hadn't forgotten to purge the air from the shaft seal.) I can only guess what happens in the gearbox, but it can't be good for it.

My shaft stopping routine is: (1) stop engine in gear; (2) try moving control lever to neutral; (3a) if it's stuck in gear, the prop is locked by the gearbox - shut down procedure complete; (3b) if it does shift to neutral, shift into the opposite gear, then repeat (2) and (3); (4) if this fails to stop the shaft rotating, put on a shoe, lift the floor above the shaft and gently brake the outer rim of the shaft coupling with the sole of the shoe until the shaft stops, then shift to the opposite gear.

A bit of a palaver, isn't it?
 
One other factor that concerns me is the potential wear on the gearbox.

My VP Saildrive manual states that a fixed prop should be left to freewheel. Aside from the noise, I've always been concerned about the wear on the gears, seals etc on long trips. As such, I've always locked it by selecting reverse and I know the previous owner did so too.

HOWEVER... I've just had to strip it down to replace the input shaft and flywheel carrier after they had worn down to almost nothing. This may be a result of the oft discussed VP 2000 series weakness, but I can't help but think that the back-pressure from the locked prop didn't help.

I've now invested in a folding prop and hope this will keep the wear down. Oh... and perhaps I'll follow the manufacturer's instructions in future /forums/images/graemlins/ooo.gif
 
This one comes up every few months!! Work I did a couple of years ago clearly showed that the drag from a freely spinning prop was much less than for a fixed prop.

But then "freely spinning" is the crux. An outboard in neutral needs a torque of less than 10g cm whereas a typical gearbox in neutral takes over 250g cm to get it turning (or whatever that is in newton thurbligs).

So if you have an outboard in a well or wherever go into neutral and it's hardly worth lifting it out of the water. If you have an inboard the friction in the works says lock it up or buy a folder!
 
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