Locking the prop

The reason that this subject raises so much discussion is that both answers are right according to the speed of rotation of the prop.

When stationary the blades are stalled and have a relatively low drag.

Allowing the blades to rotate causes an initial increase in drag, peaking at 50-70% of the theoretical freewheeling speed. In a low-friction setup or where some power is applied to the shaft, the drag falls and by 90% speed it is lower than the stationary drag, falling to close to zero at 100%.

So if there is very little friction the speed will be higher and drag low but in most typical installations the friction will reduce the speed of rotation nearer to the maximum-drag condition.

A bench test will only give realistic results if the friction is at a realistic level.
 
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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.

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How do they stop the prop from freewheeling, pray tell?

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

John

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Why? Please explain?

Again, you simply can't get any closer to being in reverse than by binding the prop.
 
http://www.djaerotech.com/dj_askjd/dj_questions/freewheeling.html
Quote:
OTOH, for a full-scale piston aircraft, the prop is attached to what amounts to a giant air pump. Pumping lots of air at high pressure can involve massive amounts of power, so that engine being driven by the windmilling prop can absorb gobs of power and make huge amounts of drag while doing so.

For a turbine engine, things get more complicated. There are two general types of turboprop engines, "fixed shaft" and "free turbine".

The "fixed shaft" type such as the Alison T-56 in the C-130 Hercules, P-3C Orion, etc., or the Garrett TPE-331 in the Fairchild Metro, OV-10 Bronco and others, has the power turbine that drives the prop mounted on the same shaft as the compressor. This means that for the prop to windmill, it also has to drive the compressor. Since the compressor can absorb astonishing amounts of power, typically far in excess of even the engine's max rated output power, the windmilling drag of a fixed shaft engine can be astronomical, typically more than sufficient to make the airplane fall out of the sky like a stove. Such engines and their propellers typically have a whole army of backup systems to make sure this can't happen, even with some system failures.

The "free shaft" or "free turbine" type, such as the Pratt & Whitney PT-6 and PW-100 series, has two or more separate, concentric shafts. The outer ones carry the compressor stages and the turbines that drive them. The innermost shaft contains the power turbine that drives the propeller gearbox. If you shut down the engine, the only thing the windmilling propeller can drive directly is the power turbine, which in this situation typically absorbs very little power. However, if the plane is flying at fairly high airspeeds and the variable-pitch prop is at a fairly flat pitch (causing it to windmill at a very high rpm), the profile losses in the blades alone can absorb enough power to make a significant amount of drag. The EMB-120 crash that killed Senator John Tower was caused by a truly freakish propeller control system failure that allowed the prop to go to an abnormally flat pitch. The resulting massive windmilling drag on just one side of the airplane caused the airplane to lose control.

In a gas-powered model, the pumping ability of the engine would create a lot of windmilling drag. However, in most cases the torque required to drive the engine is sufficient to stop the prop. With the prop stopped, the drag is approximately equal to the flat-plate drag of the area of the blades themselves.
 
The force that you are applying to the shaft to stop it from rotating is drag. You are fighting the rotation which is a byproduct of the forward motion of the boat where the water is taking the path of least resistance over the prop resulting in rotation. Yes, the rotating prop creates its own drag, however to eliminate it you would have to remove the prop from the water completely.
 
an extract from a reliable source (I am sure) .... there are other references, (e.g. also for fixed pitch prop not just a variable pitched prop) I could quote but to my mind this sums it up nicely.

I hope you dont feel this is confrontational - the points put for and against have been well put, but when I did hydrodynamics in nautical science in the late 1960's it was considered a free wheeling prop offered less drag than a locked prop.

maybe things have changed but I cant imagine why /forums/images/graemlins/confused.gif

U.S. Navy Towing Manual
5-10
Controllable pitch propellers may be left installed
if set in “maximum forward” pitch,
where they offer the least resistance to towing.
They may also be set in a “zero pitch”
condition for added drag if desired.
5-7.1.9 Locking Propellers
When propellers remain in place and are not
allowed to free-wheel, lock the shafts by an
installed shaft-locking device or by another
suitable method as illustrated in Figure 5-3.
5-7.1.10 Allowing Propellers to Free-Wheel
If any type of propeller must be allowed to
free-wheel due to the condition of the towed
vessel’s propulsion train, propulsion machinery
must be disconnected from the shafts or
adequate lubrication provided.
A means for lubricating the shaft bearings
must be provided. The stern gland on the
shaft will normally be water-lubricated. Provision
for this must be made while at the
same time ensuring that the water does not
.....blah blah blah ........

/forums/images/graemlins/cool.gif
 
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Quote:
so that engine being driven by the windmilling prop can absorb gobs of power and make huge amounts of drag while doing so.


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Read: If the prop is being forced to try and turn the engine (such as if you put it in gear) it absorbs GOBS of power and makes HUGE drag.

Unfortunately, you appear to be misinterpreting the article. I had to read it twice as well, no worries.
 
OK everyone here is another analogy. First of all a rotating prop DOES create more drag than a non rotating prop. If the principle behind this fact did not work then gyroplanes would not fly, nor would natures spinning tree seeds (is it sycamore?) float gently down to the ground. Not only is the discussion on angle of attack pertinent, but also you need to bear in mind that a rotating prop presents a complete disk of resistance to the water (gyro mechanics again).

Just go out in light airs and try it out. It is more marked if you have a decent sized 3 bladed prop than if you only have a 2 bladed prop.

However to answer the original question, a shaft brake is required. Depending on the design, it does resemble a disk brake affair on the prop shaft. You could try Vetus to see if they do one.
 
Huh? You asked how they stopped the props freewheeling. That article tells you they have various safeguards to ensure the prop doesn't. I havent interpreted it in anyway whatsoever, I simply quoted it.
 
If you read it again you will see that they are refering to a turbo prop with a fixed shaft and that the "windmilling drag of a fixed shaft engine can be astronomical". This is more of a direct correlation between a prop shaft being engaged or left free. Note: they are referring to the drag of the ENGINE when allowed to windmill as a result of it's connection to the prop. In a boat the drag of the engine (especially diesel) overpowers the rotation. To go into detail about the differences between turboprop and normally asperated engines would be way off topic.

By the way, this is the best forum site ever! All contributor's responses, though in disagreement, are intelligent and well thought out.
 
"How do they stop the prop from freewheeling, pray tell?" was your question in relation to airplanes. The article I quoted explains, not just for turbo props, and if you want more info, there are other related articles on the same site, explaining in some detail
 
Our airplane, along with any other piper, cessna or beach I've flown has no means of stopping the propellor. In fact, the procedure to recover from an engine failure involves troubleshooting the engine controls (mxture, throttle, carb heat, fuel selector, mags, primer) to find if the engine will return to life or not. The propellor is happily spinning the engine at a safe rate when the proper glide slope is maintained and instantly restarts when the problem is found. If the problem isn't found then about 600' of unoccupied earth is required imediately!

I will talk to my father tonight. He has been a comercial pilot and flight instructor for 30 years. We recently had this discussion, due to an event at the flight school, and I will have him clarify in case I misunderstood his explaination. Another noteable mention was my great uncle was the cheif test pilot for Britten Norman Aircraft on the Isle of Wight (sp?).
 
Wayward Son

It's not that you're wrong that bothers me; it's that you talk as though you are right. Those new to sailing who read these forums to learn, and I think they deserve more than that.
<soapbox>
Give it some thought.
</soapbox>
 
Thought I'd chip in!. I've got an outboard in a well, which I usually leave in place when I sail.

If I lock the prop then the engine tries to kick up when sailing fast. If I let it spin then the engine doesn't. To me that says there is more drag when it's locked.

Dave
 
Re: Wayward Son

You better speak towards landaftaf as well since he too believes a freewheeling prop produces less drag (as well as others). Singleing me out doesn't seem quite fair as I have read and re-read the responses here, giving credit to their posters whether I agree or dissagree. Their views are well thought out, intelligent, and are based on what they have interpreted from materials that they have provided. My views are formed the same way and I have already said that I may have misunderstood some of the information I had been given awhile back and would further investigate before commenting further. To me that doesn't sound unreasonable, steadfast, or uncooperative.
 
I was having lunch with some marine engineers today (who are also yachtsmen) and we discussed the whole business of drag and locking shafts.

The conclusion we came to was that it depends....

Before you think I'm recanting - I'm not, because I still believe that most yachts with fixed props have less drag with the prop fixed than allowing it to rotate.

However not all props are equal. On a warship which has a particularly efficient prop (some are variable pitch and some are fixed pitch) the engineers claimed that it is sometimes better to 'trail' one shaft and allow it to rotate if you are running on one engine/shaft for fuel economy. But this is with a prop that has more than 100% blades - in other words they overlap. However they then admitted that you can actually run generators off the shaft as it rotates, so there was a bit of a scratched head syndrome when the engineers realised what they were saying.... Might not be less drag after all...

Conversely, there was a general agreement that on a yacht, the blades do not usually overlap and locking the shaft and allowing the blades to be stalled in the water is less drag.

If you don't believe me - try it for yourself. See my posts above.
 
An earlier post on this gave a link to some (very complex) research done into props (air) and the drag characteristics - it seemed to be inconclusive as the variables depended on the size, pitch and number of blades.
In your case - (I'll assume) a 3 bladed outboard prop has a far greater & flatter surface area than most "normal" shaft/saildrive props and so a different drag characteristic may well be found. (can't you lift the engine up anyway?)

For those that what more to think about - it takes quite a bit of energy to stop the wheels on a car - but once your skidding the stopping power is not so great - hence the need to cadance brake or anti-lock brakes in wet/icy conditions.
Apply this to a shaft - Is this not because there is a lot of power transfered through the shaft when the prop is being turned, but once it is stopped it takes far less effort to keep it that way.

The power transfered through a freewheeling propshaft is changed into something (power doesn't just dissapear right?) - so in most cases it is turned into heat and noise, hence the need to service the gearbox more often. So where does the power go from a locked prop? Hmm ... /forums/images/graemlins/crazy.gif /forums/images/graemlins/confused.gif

Me? I lock my shaft - reverse gear with a Yanny 2gm20 - no noise! Stuff the drag ...
 
Re: Wayward Son

This is all piss and wind.
Take your boat out on a calm day power it up to 6 knots,stop propulsion and time how long it takes to drop back to 1 knot,prop locked and prop unlocked,repeat as many times as you can to to eliminate test error and if you get a meaningful answer then thats a eureka moment for your boat.
Diverting as all these discussions around sycamore seeds,aeroplanes,helicopters,and other assorted analogues may be there is no substitute for empirical data.

PS Well done John you got to a 1000!
 
I think most of these guys have inboards. We outboarders are a rare breed! Last time this thread came by I did some tests on my outboard and some inboards ashore at Strides. If you flick the prop round with a finger, an outboard will do several revs before it stops so has little friction and doesn't drag when spinning. Inboards stop dead when you spin the prop so have lots more friction and drag.

Geoff
 
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