Locking the prop

Wayward_son, with the greatest respect you are wrong. The prop shows less drag when fixed in the water. There has been lots of discussion and research about this, and if you do some research on YBW you will find the arguments to back up my assertion that fixing the prop is less drag.

I know it might seem contrary to your instinct, but the blades stalled in the water produce less drag than blades rotating.

When I confused myself over this a while ago I remember someone pointing out to me, "Which do you want to be in? A helicopter with no engine and blades rotating, or a helicopter with no engine and blades fixed". The answer is the former - as you can successsfully land a helicopter with auto-rotating blades. With fixed blades it's not a landing its called a crash...
 
I fail to see the connection, and respectfully beg to differ about drag/resistance on a rotating prop.

/forums/images/graemlins/cool.gif
 
hehe... here we go again!

Well you see, when the prop is allowed to freewheel, the angle of attack of the blades is reduced and they therefore do not stall, just provide drag.

When the prop is locked, the angle of attack is extreme, the blades are stalled, and the laminar flow of fluid (be it air, water or custard) breaks up, and provides very little drag.

It is I that is the purveyor of the aforementioned helicopter sales pitch, but now let's move onto fixed wing aircraft: Stalled wing produces no lift, ergo aircraft falls. Airflow over the wing (or prop blade) produces lift and therefore permits a glide.

I speak with authority on this matter, even though other subjects less to do with fluid dynamics and more to do with boats may be beyond me!

From a drag perspective: Stopped prop good, freewheeling prop bad.

If your gearbox is prone to jamming when you use it to stop the prop, maybe the noise/wear/drag is preferable to a jammed 'box.
 
If you can't see the connection, and assuming that you have a fixed bladed prop try sailing in light airs with prop rotating and prop fixed, You will soon see the difference. It needs to be light airs and below hull speed, because as soon as you approach hull speed, the drag from the hull is significant and masks the effect of the extra drag of the prop.

I obviously can't make you believe what I believe the evidence to show. I can suggest that you open your mind to going against your intuition.
 
John ,with the greatest respect your analogy with a free spinning helicopter rotor is flawed,

A helicopter pilot with a dead engine allows his main rotor to spin freely as the helicopter falls(similar to a yacht prop freewheeling)just before hitting the ground he reverses the pitch of the blades so the momentum produces some lift to reduce the impact.

Therefore I cant agree with you that a locked prop produces less drag.The helicopter rotors rotate freely to reduce drag and then only increase drag(lift)when reversed.

In practice on a small cruising yacht I cant notice any appreciable difference in boat speed one way or the other but hate the noise of a rotating prop.
 
How many angels can dance on the end of a pin!Bumblebees cant fly!Gordon Brown won't raise taxes!
Don't get involved in the theory,try it on your boat,and measure the effect locked and unlocked,if you can't measure a difference then its not worth worrying about,if you can,go with your results.
FWIW after many discussions and trials over the years I would always lock the prop I know it improves boat speed and it stops that irritating rumbling noise,and probably cuts down wear on shaft and bearing.

PS never ask a service chopper driver what happens if the motor fails as he might demonstrate and an autorotate descent is not for the fainthearted
 
One of my previous boats was a Hurley 24 bilge keeler which had a 20 hp engine and a 15 inch 3 bladed prop(large prop for the size of boat. I tried it both ways and probably due to it being a slow old thing anyway couldnt notice any difference.

If you have proved otherwise with a more responsive boat and probably more accurate instruments then Ill take your word for it.

My Present boat has an outboard which produces very little drag when stowed in a locker /forums/images/graemlins/smile.gif.
 
I did a test on this recently, while on a 40 mile passage.... in pretty much unchanging conditions, I did 5 miles with the prop rotating, and 5 miles with it locked.... (did a fair distance to even the effect out of minor wind variations)... did 0.19 Knots faster on average with prop locked (3.76kts versus 3.95kts measured through the water to avoid tide effect).... didn't change tack, didn't alter sail trim and didn't have significant change of tidal direction, so no lee bowing effects and all that) ....now you could argue that the wind changed, or sea state changed.... well, the wind didn't, at least not according to my instruments which also measured average wind speed over the period.... sea state... well you'll have to take my word for that one.... I know its not ultra scientific, and there is room for error...but I did my best to remove as many variables as possible...hadn't thought about the hull speed argument, but the wind was quite light, so I guess I got that right by being below hull speed by accident.... so... me.. I'm convinced that the boat is quicker with the prop locked...
 
As a heli pilot, I can promise you faithfully that the freewheeling rotors provide lift in autorotation. I can actually achieve a glide angle of 50 degrees in autorotation, and touch down reasonably gently WITHOUT USING POSITIVE PITCH as long as I have the forward motion that I get from the 50 degree approach path. IT IS NOT FREEFALL.

To the non-believers: I give up. Please continue to disbelieve the theory from those that know it, and the practical results of those who gave it a try. I will say nothing further on the subject.
 
It's not quite as clear cut as that. Several people, myself included, have posted results of someone that has actually tried the experiment with differnt props. It depends on the prob and size is only actual answer.
 
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Wayward_son, with the greatest respect you are wrong.

[/ QUOTE ]

A fixed prop in the water being dragged has the forward force of the water being asserted over its entire surface area. If the blade is allowed to rotate then the initial flow moves to the front edge of the blade which makes an apparent profile that is less because the blades path is now in a corkscrew.

In practicallity, as someone else pointed out, you probably couldn't stop the shaft with your hands, because it is as if you are putting on the brakes. So, if a locked shaft produces less drag (as you so pointed out) then surely a reversing prop would also help the yacht's forward speed. This is of course rediculous.

In airplanes with variable pitch props, in the event of an engine failure, the prop is adjusted to full-feather (blades almost flat) because the freewheeling prop produces far less drag. It would take alot of force to prevent a propellor from rotating, which again is like applying brakes. I am a pilot by the way.

In your helicopter analogy you missed the point that the craft is held aloft in forward flight by a rotary wing, which must keep rotating to maintain controlled flight. A gyrocoptor doesn't even have a motor driving the rotor. For either to travel forward without allowing the rotor to rotate would result in drag overpowering the collective forces keeping the craft aloft and they would fall like a greased anvil.

Lastly, imagine a stern-wheeler with sails. If the engine ran out of steam and you had to continue under sail, would you lock the paddlewheel in an attemp to gain a couple knots? Well as ludicrous as it may sound, a paddlewheel and a prop are the same thing except that the former has a steeper angle of attack, so like the paddlewheel, the prop produces less drag when allowed to rotate.

With all due respect, am I still wrong?
 
Re: hehe... here we go again!

[ QUOTE ]
When the prop is locked, the angle of attack is extreme, the blades are stalled, and the laminar flow of fluid (be it air, water or custard) breaks up, and provides very little drag.

Stalled wing produces no lift, ergo aircraft falls. Airflow over the wing (or prop blade) produces lift and therefore permits a glide.

[/ QUOTE ]

Your two statements are contradictory. "the laminar flow of fluid (be it air, water or custard) breaks up, and provides very little drag." The breaking up of the flow is the product of drag! The greater the angle of attack - the greater the disruption of flow - the greater the drag.

Stalling a wing is a result of excessive drag (from angle of attack, icing, a seized motor resulting in stopped prop) as it is with a prop. You simply cannot get you propellor any closer to running in reverse than by stopping it.

Does your car roll better down hill with the wheels locked up?
 
In airplanes with variable pitch props, in the event of an engine failure, the prop is adjusted to full-feather (blades almost flat) because the freewheeling prop produces far less drag.

That may very well be true for feathering props, and as you will realise is actually done to stop windmilling drag. Windmilling (ie the spinning of a pitched prop) creates higher drag than a prop that is not windmilling. So your example defeats your own argument.

Furthermore, for fixed pitch props on aircraft they are never allowed to freewheel in the case of engine failure. A lesson learnt very early on in the history of flight.

Your comparisons with paddle wheels and car wheels would seem to be entirely irrelevant.

John
 
Darn it - I was saving starting this argument again for my 1000th post.

Believe what you like guys, It appears that nothing will persuade the unbelievers either way.

I still come back to my simple statement. Try it both ways but only when sailing in light airs because if you are near hull speed, other drag factors are playing a far bigger part in the equation and will mask the difference.
 
As has been said by many this is a regularly aired discussion that often (apparently) gets very technical still without conclusive proof it seems since you either believe or don't!

FWIW over more than 30 years and many boats/engines I have ALWAYS locked the prop and have never had a failed gearbox.

On some boats with 2 bladed props I used to line the prop up behind the skeg if there was one (marked shaft) if racing or going any distance. I even did this with a 3 bladed so that one blade at least was 'masked' and only two in the full flow.

I re-engined our last boat with a Volvo MD22L which used to stick in reverse after being locked under sail. All you needed to do was (as Ships_Cat said) give a quick (like one second) blip of the starter with the throttle as close to neutral as possible and bingo it is possible to engage neutral instantly and start normally, I didn't bother with pulling the 'stop' cable either.

On our current boat we have a Yanmar 4JHE 44hp and a Brunton Autoprop. The Brunton is an excellent if pricey mod but the method of use is to stop the engine whilst still in FORWARD gear and still moving forwards, then leave it locked in forward position, the Brunton blades the feather automatically to present the least resistance to flow and give lowest drag. With this set up the stick in gear syndrome doesn't happen. However engaging gears is 'clunky', I am told by Yanmar this is normal and especially so with a folding or feathering prop where the load suddenly increases as the blades open and bite. If we didn't have the Brunton, I would do as I've always done and lock it in reverse, IMO less drag and certainly less wear and tear on the gearbox, shaft seals and bearings and no noise.

Unbelievers can do as they wish! /forums/images/graemlins/tongue.gif

Robin
 
Wayward Son wrote:
In airplanes with variable pitch props, in the event of an engine failure, the prop is adjusted to full-feather (blades almost flat) because the freewheeling prop produces far less drag. It would take alot of force to prevent a propellor from rotating, which again is like applying brakes. I am a pilot by the way.
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Dude, if your boat has a feathering prop, I would agree that you should feather it. When you learnt to fly, you were doubtless told how you would glide a sh!tload faster with a stationary prop than you would with a freewheeling prop, even if it was slowly rotating due to the engine running at tickover. So yes, an aircraft fitted with a fixed pitch prop and it's engine running at tickover in a dive will be travelling slower than an identical aircraft performing the exact same maneuvre deadstick (prop and engine stopped). I'm sorry if that seems ridiculous to you!

I find it quite amazing that you were never taught that during your PPL/CPL training.
 
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Having lived with a spinning prop for a while I will change to a folding prop asap

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Don't get your hopes up. Folding props also spin if left in neutral. I may not be as vigorous as a fixed-blade prop but the grinding noise is still there!
 
I have always put the diesel forward gear haven’t jump started it yet, and if I forget to select neutral before starting at least its already in gear and starts with minimum battery loss.
 
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