Interesting Technical Question

[Gludy]

A laymans explanation

I am going to try and explain why a left to rotate prop offers greater drag in another way:-

Any prop rotating for any reason in water will generate a force of pulling or pushing on the boat. I driven prop in forward mode generates a push forward to the boat. A prop lef to turn turns depending on the speed of the boat and its design - this will be at a slower speed than the driven prop but this very turning is done at the expense of a drag that equates to the amount of push that prop would give if driven at that speed.

In other words the prop is generating a backward pull of the same amount as if it had been driven forward at that speed.

Just like the drag/propulsion difference between it when stationary and turning is great so is the drag difference between being left fixed and still or turning great. hence a prop left to turn gives greater drag than when prevented from turning.

All the turning is doing when left to rotate is produce a drag equal to whatever push that prop would have given when driven forward at the same speed.

The chopper pilot has to take a downward path inside a descending envelope of angles to ensure that prop keeps turning and hence imposing drag - in this case an upward lift on the chopper - its exactly the same principle. The moment that blade stopped rotating its upward drag would stop and the chopper would fall like a stone.

The principles in both case are the same and that is why a prop designer can state in black and white terms the principles of how a prop operates in these two situations. It matters not if the prop is huge, small or the number of blades etc - the principle is always the same. It matters not if its in water or air - only the viscosity is different, the same principles apply.

I hope that explanation helps. I for one have learnt and understood it better than when I started the thread. Although this was, in itself, not the subject of the thread.

 

[Petrolia]

I just spoke to the Captain of the ship I’m on at this very moment, in the Bay of Bengal off East India. (a 200m drillship)

The Captain says less drag is created with the prop spinning free. In fact he says that once when they needed to work on a shaft they tried to stop it spinning with the disc brakes but they couldn’t as the brakes were not powerful enough ! (While steaming on other engine)

It seems to me that if you have to put energy into stopping a prop there must be an equal & opposite force being applied somewhere else. I say this force is applied to the water in the form of drag, extra drag.

Things always take the path of least resistance. In terms of drag, if the prop wants to spin let it spin. Obviously if you’re worried about gearboxes & shaft seals then do what you think is best.

 

[Petrolia]

I didn't say the engine had momentum, I said the rotors had huge momentum. Although I'm not a pilot as I work offshore I have flown in hundreds & the next time I fly I'll speak to the pilot. To clear it up for myself if nothing else.

 

[Joe_Cole]

Re: A laymans explanation

mmmmmm.

You are assuming that the hydrodynamic effect of a boats prop is the same as the aerodynamic effect of a rotor. They are not necessarily the same. Boat props have much less of an aerofoil built into them than aircraft props; AFAIK they work on more of a "screw" action.

I suspect that it's a mstake to assume that they are as similar as you are assuming. There are too many variables.

 

[ccscott49]

I have a twin screw displacement boat, when motoring with no sails up, I use two engines, as prop walk makes my auto pilot put a set on the rudder with one engine running, so I find two engines more efficient, fuelwise.
However if I can put a mizzen sail up, and allow the wind to counteract the propwalk, I can use one engine, with good fuel savings. Of course dependant on wind direction, on which engine I run.
I wont go into putting all the sails up, as this is not the question.
Lots of tralwer type yachts can rig a mizzen sail and those that can't, can often be retro fitted and a lot are, with the added benefit of steadying purposes.
I allow the other prop to rotate, but as the inner cutlass bearings are fed from the engine, the shaft squeeks like blazes with no lubrication, so I made a water feed crossover system, which allows me to feed both inner cutlasses from either engine. I and my brother have used this system over many thousands of miles of motoring and motorsailing. It works. I hope you find this useful, sorry I cant give speeds and consumption figures, which would be useless anyway, depends on your boat!
By the way, if you do want to lock a shaft, a simple "spanish windlass" of rope wrapeed around the shaft with one end secured, will do it, tie the ignition keys to the rope, then you wont start that engine with the shaft locked.
Or small enough rope to stop the shaft free rotating, but not enough to stop it under power, just in case.

 

[Gludy]

Very interesting and you actually provide the evidence to prove the captain wrong.

"The Captain says less drag is created with the prop spinning free. In fact he says that once when they needed to work on a shaft they tried to stop it spinning with the disc brakes but they couldn’t as the brakes were not powerful enough ! (While steaming on other engine)"

I can understand that because the force they have to apply to the shaft to stop it spinning IS to counter the drag it is creating - it was so big they could not stop it. This force on the shaft is not there helping the prop to propel the boat forward it is the force exerted by the prop dragging the boat - if this was not the case we could all dispense with the second engine and just have a rotating prop left spinning that would provide forward propulsion all on its own - we would have created energy!

"It seems to me that if you have to put energy into stopping a prop there must be an equal & opposite force being applied somewhere else. I say this force is applied to the water in the form of drag, extra drag."

You are right ... the captain is wrong. The force you are applying to the shaft to stop it equates to the drag being exerted by the rotating prop - if this was not true then the force was the other way then we would have this miracle of a feely rotating prop produces large quantities of forward propulsion!

"Things always take the path of least resistance. In terms of drag, if the prop wants to spin let it spin."
You are mixing things up - things obey the laws of physics and in many situations drag is created - a drag that can be reduced.

So part of what you said is right - there is a large force in that rotating shaft - you need to apply and equal and opposite force to counter it - that force in the shaft has to be drag... can you see that now?

 

[Gludy]

The baldes in the chopper have very little momentum - can be considered as almsot zero and what little there is contributes nothing to the autoglide enevelope the pilot uses to land without power - he translates his potential vertical energy into lift by ensuring the props keep rotating as he falls and this is why he has a min and max angle of falling.

When you ask the pilot = he will confirm this.

 

[Gludy]

Re: A laymans explanation

Sorry - I disagree the principles are the same.

Just think about the fact that the ship example given earlier could not brake the shaft as there was too much energy needed to stop it - where is that energy coming from in that rotating shaft? - it is coming from the only energy source - the other engine. it is the drag on the free prop that is dragging the boat.

 

[tcm]

A free-spinning prop is perpetually forced to present a blade behind static air. A fixed prop doesn't.

 

[Gludy]

"so I find two engines more efficient, fuelwise.
However if I can put a mizzen sail up, and allow the wind to counteract the propwalk, I can use one engine, with good fuel savings. Of course dependant on wind direction, on which engine I run."

That makes sense - the example I gave earlier also found that two engines were more efficient - the main reason is the increased drag of a freely rotating prop. This is not always the case as per 'ShipWoofys' example when doing 3.5 knots at tickover on one engine However at all reasonable crusing speeds it is almost certain it is the case unless you fix the free prop.

When you add the sail you are adding extra propulsion - its as simple as that - you would do even better if you could stop the prop rotating.

 

[Gludy]

Then we agree tcm!

 

[Nauti Fox]

Err... bit of a simple answer really, try both ways next time you are out, if its significant it should show up on a GPS.
Oops, should have read all the posts.

 

[Gludy]

I have just had another thought on that ship that could not brake the shaft - had they started off with the brake on the shaft they could almost certainly have held it but because they allowed it to rotate end generate much more drag they no longer had the power in the brake to stop it.

No matter which way you look at it that energy in that rotating shaft has to represent the drag of the prop - the prop is producing a dragging force by its very rotation - a force it would not be producing if it was braked.

 

[Nauti Fox]

Gludy can you actually produce facts to back up what you say, not heresay?
As regards helicopters and autorotation the rotors are kept spinning by the upward flow of air through the blades and by altering the pitch on the rotors you control the helicopter and the speed of the blades. Just before landing you increase the pitch to slow the decent using the stored energy in the blades, so dont see how this relates to ships propellors.
On fixed wing multi engine propellor driven aircraft the blades are feathered before the engine stops so that the edges of the blade are facing the airflow so offering the least amount of drag. So that is of no use either regarding ships.
Like you I can find no actual referance of which way is better, but gut feeling is that letting it freewheel would produce less drag, but I cant back that up... yet.

 

[Vagabond]

I run on one engine most of the time at hull speed of about 8 -9 knots at about 2,500 revs, Volvo AD41's so it's about 1000 down on max.

If I try to go above hull speed the boat doesn't like it but she tracks really well and is very comfortable at hull speed.

I let the other prop spin and have the engine shut down. I just swap engines every hour.

 

[Gludy]

What you say about choppers and aircraft is true but it is not relevant to this issue - what is a fact is that a chopper when losing engine power lands safely because its blades can rotate and produce lift as the air flows through them converting it energy by virtue of it height and any forward momentum into blade rotation so that it can descend with the rotors offering lift - this is the same as dragging a free prop through water.

if we could feather props on boats then doing so would reduce the drag of the free prop as would collapsing the blades.

I am posting some drag calculations on this thread later today - the moment I get the email with them in, so yes I am producing facts.

If you stop and think about that ship that could not brake that freely rotating shaft and ask yourself where was that energy coming from and what direction was it acting in - you will see that the force in the shaft equated to the drag of the prop.

A prop can be still and have water passing over it to make it turn or it can turn and make water pass over it by engine power turning the prop - there is no difference to the prop - its designed to produce thrust in a normally forward direction when power is applied to it, it will equally provide the same feature as drag when it is pulled through the water - that same prop when being dragged through the water by the power of the other engine turns and produces a thrust - water is passing over it but that thrust is acting as a drag not propulsion - it has to be. if it were acting as propulsion then we would have created energy from nowhere.

So application of the simple rules of physics together with observation demonstrates the truth of what I am stating. It is also a known fact to those skilled ship designers within the industry.

 

[tcm]

yep. I can see jfm's "path of least resistance" but the free spinning prop is being permanently upset by its form, so isn't free to find that path of least resistance. I *think* one could say that if the blades were flat instead of twisted but with equivalent drag they'd find the path of least resistance by staying still.

 

[Gludy]

Ok - no argument with that but you would probably use less fuel if you could lock your shaft on the free prop.

The entire purpose of my orignal point on thsi thread was to try and find out if the extent of the drag of the free prop was such that it was more than balanced by the fuel saving of runnign on one engine. This is a function of how ell the engines perform at varying loads. I think at low tick over speeds there is a fuel saving by running on one engine even if one prop is free (Shipwoofys Boat) as soon as that speed strats to rise the drag on the free prop increases so that the most efficient running is either:-
1. Both engines to produce the given speed - an example of this was given early on in this thread.
2. one engine with the other prop locked.

For any given boat this will depend on the efficiency of the engine etc but what never changes is that a fixed prop offers less drag than a rotating prop - its just a question of if this is sufficient to do away with the fuel saving of running on one engine. Certainly it seems that unless you can fix the prop then at reasonable speeds above tick over, it si poinltess running on one prop to save fuel - however running one one prop with the other fixed should prove even more economical.

Even in ShipWoofys case of 3.5 knots speed and one engine, if the shaft on the other could be locked he would do even better.

 

[Vagabond]

The inefficiency of running a diesel engine I think it's something like 20% of the energy from the fuel gets turned into power far out weighs the drag of a prop.

 

[Gludy]

Tcm - I again have to agree with you!