Interesting Technical Question

[Its_Only_Money]

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The reason the locked prop offers less drag than a spinning prop, is that a locked prop is effectively a stalled hydrofoil and the spinning prop is generating lift because it is acting hydrodynamically.

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But isn´t "lift" on a boat prop = thrust

>bang< as my head explodes!

On a mobo a stopped prop equates to a high % of a flat plate being dragged through the water, on a sailboat the % is much lower (empirical eyeball evidence) hence most of the flow through the swept area of a locked sailboat prop is clean(ish), on the mobo there is simply no clean water flow through the swept area. Consequently I´d expect a sailboat prop to be better locked and a mobo prop to be better running free.

 

[BrendanS]

The real answer to whether a prop has more or less drag when freewheeling or fixed is that it depends - on a whole bunch of variables. A PhD student tried it as an experiment, and width of blade, pitch etc etc all contributed, and was fairly inconclusive, other than to say it either needed more research, or a whole bunch more sizes and types of props - otherwise each prop would have to be empirically measured.

 

[Its_Only_Money]

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Interesting BUT what is meant by a free wheeling prop>

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Good point, for all we know the section in the manual on "preparing props for towing" may have included the section on removing the couplings twixt shaft and gearbox etc so the prop would truly freewheel....

 

[Gludy]

I have just phoned a prop designer and asked him the question.

The answer is that the fixed prop offers less drag - always.

He explained it by saying that pretend the prop and shaft are an umbrella - imagine jumping out of a helicopter holding the ‘umbrella’ – if the prop was free to spin you would gyroscope down to earth - drag would be increased. If it was locked so that it was a fixed prop you would drop like a stone = less drag..

Its nothing to do with anything other than the prop is acting like a gyroscope hence increasing drag – turning the gear box is nothing to do with the drag.

This chap designs props and I totally forget I knew him – so a call to him has at least settled that in my mind.

rb_stretch was right!

 

[Joe_Cole]

I think he meant "Gyro Glide", not "Gyroscope".

The gyroscopic effect is altogether different; a gyroscopic "umbrella" would plummet to earth whether fixed or rotating!

 

[jfm]

I might be being thick but I just dont get it. I mean, I dont see how a fixed prop creates less drag than a free spinning one. The umbrella analogy from Mr Propellor expert gets us nowhere. Are you saying a round parachute would make you descend more slowly if it were spinning? That doesn't sound right.

But, still happy to be proved worng. I'm only running on intuition here...

I know many boats do lock the undriven props, but that is often to deal with gearbox lubrication etc.

Gludy re your other question our boat does about 3.5kts on one engine in gear at idle rpm, and 6.5 knots with both in gear at idle rpm. According to the computer the mpg is better on one engine at the 3.5kts. The extra speed given by the second engine doesn't fully make up for the extra drag of 6.5kts versus 3.5 kts

 

[Gludy]

You are of course right ... I stand corrected

 

[Gludy]

JFm
I totally accept that the case you quote with your boat means that the boat is more efficient when run on the one engine ...... I think this may be right with other low speeds on other boats as well.

My friend can actually calculate the drag of the free prop and even suggested having a small electric motor to drive the shaft enough to counter the drag. However he also stated that its almost certainly better to run both engines at low revs than even enter into the question of stopping one engine.

He stated that the drag is considerable - I will post some calculations tommorrow after he has emailed me.

If a helicopter has a frozen rotor blade it will drop like a stone to earth and crash.
What they do is they free the blade and gyro glide into land. My son flies them and has done over 40 such practice gyro glides - its part of pilot training.

So here a freely rotating blade offers negative drag as it where and slows the helicopter down - the prop freely rotating through the water offers the same drag - it would offer less (always) if it was stopped from rotating.

In running a boat at very low speeds we have an example here where it still better mpg with one engine stopped - we have another at 7 knots where less fuel is used with both engines. A factor has to be how efficient the engines are at different speeds but nothing changes the fact that a prop left to rotate offers more drag - even if its in neutral than a prop that is clamped to stop it rotating. Further as the speed rises the gyro gliding prop offers significantly larger amounts of drag. Calcs for this will be posted soon.

 

[Gludy]

"Are you saying a round parachute would make you descend more slowly if it were spinning?"

No - I am saying that a prop above your head would make you descend slower if it was rotating than a fixed one and the difference is large.

 

[hlb]

Snore.... What are go going to do with this information once compiled. Why not just take the prop off and have done with it!!

 

[BrendanS]

ermmm, I can't believe a prop designer would give such a simplistic answer for a variety of reasons. Does he design helicopter props or boat props, and in which case mobo or raggie.

Reason for asking, is that drag varies according to design, so no prop designer would ever give a cut and dried answer, in much the same way a scientist like me would add bits onto any answer like, yes, no, but.

A helicopter has huge diameter blades, with not a lot of total surface area, but huge loading on each blade. A mobo has far lower diameter blades, but with massively increased % of area covered for the diameter. Many raggie boats have slim chorded 2 blade props etc etc.

Each have different characteristics. The drag of each, will have similarly disparate drag co-efficients. Hence my question about what sort of prop your friend designs, as the statement that fixed props equates to less drag always, is just not true

 

[BrendanS]

Lets put this in laymans terms

Downsize a helicopter blades to a size you can put on the back of a boat. How effective do you thing that would be.

Upsize a typical duoprop to helicopter size. Do you think it would fly?

Now do you think they are comparable?

 

[Petrolia]

Sorry to butt in cause I'm only new here but I don't get the locked prop offering less drag either.
If a helicopter losses power it can auto-gyro down safely because the rotor has huge momentum so is effectively still powered and can therefore still create lift.
I'm afraid I have to side with JFM and the US Navy.
I believe a free rotating prop creates less drag.

 

[Joe_Cole]

The auto-gyro effect is NOT due to momentum of the rotor. If it were then the blade would stop quite quickly and the helicopter would fall out of the sky. They don't.

 

[BrendanS]

Helicopters can land, for exactly the same reason an autogyro can fly. Nothing to do with momentum, purely down to the aerofoil of the blades. Try putting them behind a boat in water tho?

 

[Gludy]

Brendan

"ermmm, I can't believe a prop designer would give such a simplistic answer for a variety of reasons. Does he design helicopter props or boat props, and in which case mobo or raggie. "

His answer is that for all props (non collapse types) the drag when not fixed will always be greater than the drag when fixed - that is an understanbable answer because of the nature of what is causing the drag.
He is a boat prop designer.

"Reason for asking, is that drag varies according to design, so no prop designer would ever give a cut and dried answer, in much the same way a scientist like me would add bits onto any answer like, yes, no, but."

I am a chartered engineer and I can give you equally simplitic answers to equally simp;otic questions. This prop designer is stating a clear principle and is providing figures to me by emailo. He requested the dimensions of my props so he could be specific to mine but that does not change the fact that when rotating without power they offer greater drag than when fixed.

"A helicopter has huge diameter blades, with not a lot of total surface area, but huge loading on each blade. A mobo has far lower diameter blades, but with massively increased % of area covered for the diameter. Many raggie boats have slim chorded 2 blade props etc etc."

Makes no difference to the principle at all - you are getting lost in the fog of detail. sure the exact prop etc matters when you calculating the exact effect, the exact drag but to state that for a given prop when fixed is a lower drag situation is understandable and logical.

"Each have different characteristics. The drag of each, will have similarly disparate drag co-efficients. Hence my question about what sort of prop your friend designs, as the statement that fixed props equates to less drag always, is just not true "

You are right each prop is different but taht has no effect at all on the principle that it offers greater drag when left to rotate - gain I will be posting further details. I am not inventing this ..... anyone with skill in the subkect would tell you the same.

 

[Gludy]

"Lets put this in laymans terms

Downsize a helicopter blades to a size you can put on the back of a boat. How effective do you thing that would be.

Upsize a typical duoprop to helicopter size. Do you think it would fly?

Now do you think they are comparable? "

clearly YES because its the principle - the eact drag values make no difference to that at all. Explain why if a helicopter freezes it baldes it drops like a stone - if the engine stops it can glide to the ground - that is the principle we are talking about here. It makes no difference wether it is air or water, large or small prop.

 

[Gludy]

When a helcopter loses power the engine has no momentum at all - it can gyroglide to earth only if the prop is rotating - ask any helecopter pilot. Sorry but you are wrong on this.

 

[Gludy]

Totally true.

 

[Gludy]

Spot on brendon, its down to the gyro effect of the blades and the same principle applies to any prop in any medium. The actual size/shape make no difference to that principle.