Locking the Prop

[PeterGibbs]

Re: Locking the Prop - thanks!

Dave White,

Congratulations on making the shortest and most acceptable assessment of the argument. Helicopters have been used for so long to (falsly) lead the argument for locking - it was time to kick it into the long weed.

My Volvo manual (120 S leg) does not recommend locking - the contrary in fact, but with a caveat that the engine is run every 10 hours of free wheeling to ensure good gearbox lubrication.

Some gearboxes do not appreciate free wheeling, and they should be locked, if the manual insists.

Wear is another factor that is greatly overstated as justification for locking, but the whine on some boats can be penetrating - at this point I give in and accept just this case for locking (perhaps just at night!!)

PWG

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[Rowana]

Do you do much sailing backwards, then??

The fourm needs to know ! ! !

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[Rowana]

Only if . . .

. . you're making a lot of leeway or sailing sideways.

Shouldn't think it would be a problem with a long keeler.

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[john_morris_uk]

Re: Only if . . .

The helicopter argument is complete b***s. Having trusted my life to autorotate landings I speak with feeling! The helicopter uses the momentum or inertia of the rotating blades to flare out the descent as you approach the ground.

Furthermore stalled blades offer much less resistance than rotating ones.

Science over intuition please!

And stalled blades and non rotating shafts are better for your gearbox and cutlass bearing!

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[MainlySteam]

Think you may have misread my post, as I say autorotation uses the case of most drag, which I think you are calling lift (not incorrectly), and that is with a freely rotating rotor - and you seem to be saying the same.

However, I would be intrigued to know why it is you think that the basic drag principles (in autorotation lift is drag) are different between a rotor in air and one in another fluid eg water.

John

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[MainlySteam]

Re: Only if . . .

<<<Furthermore stalled blades offer much less resistance than rotating ones>>>

Is that not what we are saying but some others seem to inferring means one should freewheel ones prop. Unless of course there are different basic rules of hydrodynamics for different fluids - and I suspect there are not.

The helicopter does not stall its rotor, it wants high resistance (call it lift, drag, stickiness, or whatever if one wishes) so freewheels it. It works as I say, and because it has drag, resistance, lift, or whatever you wish to call it it can flare out using your words.

Stalled blades, as you say, offer less resistance and that is why we stall a yacht's prop if we want to reduce its resistance.

John

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[john_morris_uk]

Re: Only if . . .

Apologies - I misread your posts. Must write out one hundred times 'I must not make posts late at night after several glasses of vino collapso'.

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[BigART]

As this is not comparing like with like, this theory is somewhat economical with the truth. For a start, the rotor is providing lift (albeit somewhat less than the aircraft's mass), not drag. Comparing a variable pitch high aspect ratio rotor blade moving through air and designed to autorotate with and a low aspect ratio boat propeller in water is not really valid, there a large differences in tip losses, coefficients of lift/drag, aerofoil sections, etc. Being in the trade, so to speak, I could go on....

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[BigART]

Re: I think you missed the point

I'm with you old chap!

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[Twister_Ken]

Helos vs boats

A helo rotor and a boat prop exert their force in different directions (unless the boat is going astern). The rotor pulls the helo, the prop pushes the boat. Therefore, when the prop is freewheeling it will not 'actively' be pulling the boat backwards, where as when the helo is falling, the autorotating rotor will be trying to pull the helo upwards (or at least slow down its fall).

Does this do something to make the prop/rotor comparison invalid?

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[jimi]

Re: Helos vs boats

Actually I think both a prop and a lock are very useful going to windward as they can sit on the windward rail and help keep the the boat flat .. and if there's a hooker you might get fresh fish for tea ..

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[MainlySteam]

For me to understand your theory you will have to explain to me the difference between lift when a freewheeling rotor is falling (ie pulled by gravitaional force) through the air (a fluid) ie the airflow is through it, and drag from a freewheeling propellor when it is being pulled by a boat through the water (another fluid) ie the water flow is through it, and why you consider they are different.

I will be intrigued if it is discovered that any fundamental difference exists.

Seems I have said the same thing a number of times so far. I guess the earth is flat and my posts are falling off the edge of the world .

John

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[MainlySteam]

Re: Helos vs boats

...and another hooker providing ...... Whoops better not continue, might get excommunicated!

John

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[gtmoore]

With my sailing skills it most probably looks like it to some onlookers!

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[Twister_Ken]

Re: Helos vs boats

Well, at least it will smell like fish.

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[jimi]

Re: Helos vs boats

You trying to pull a flanker .. again?

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[Twister_Ken]

Re: Helos vs boats

If I ever find myself out on a wing I always full back on my appalling sense of humour. After all there's no point in being stand-offish and self centred.

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[MainlySteam]

Re: Helos vs boats

What was your prop drag like when you went sailing on your side in the Channel? That is, is there more prop drag when the boat is turned 90 degrees about its longitudinal axis?

John

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[jimi]

Re: Helos vs boats

Air resistance a smidgeon less than water ;-). For the record, I always lock the prop when sailing because it stops the slight rumble and makes things quieter.

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[BigART]

The densities of the fluids are completely different for a start, different Reynolds numbers/viscosities, the solidities of the discs (the area of metal/composite within the diameter of the disc), etc. In short, there are a large number of factors to take into account, and it is the overall balance of all these factors that will give the correct answer.

This is often very difficult to do accurately, and it is often the case that the simplest method of finding out the answer is to go out and test the set-up under consideration, which is why despite all the computing power available, wind tunnel and tank testing of aircraft/boats often throws up surprises, improving knowledge in the long run.

So, I suppose the answer is it depends on the prop. A fat 3 bladed thing or a skinny two blade racing prop would give different drags when stalled/locked in the waterflow, the answer could be different for both.

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