Energy used by a propeller/motor underwater

[maryden]

There is an electrically operated propeller combined with a motor in a light housing laying sideways in a pool of water (not conventionally upright as on a boat) - think of a helicopter in the water.
The propeller/motor will be 8 metres diameter by 1 metre depth and will weigh 22 metric tonnes.
I would like to find out how much power (energy) the propeller/motor would use if the only thing it had to do was to raise itself upwards by only 10 metres - I know that when it has risen by 10 metres and the motor stops the unit will sink again – the sinking action is immaterial, it is just the energy used in the lift that I am interested in.
The journey time upwards would be fifteen minutes.
The propeller/motor will be stabilized so that it does not rip itself away from it’s moorings. The propeller/motor will be powered by a self supporting external cable so that will not be part of the equation.
Thank you.
Dendalo.

 

[zeusfaber]

Suppose the question came from a couple of chapters earlier and asked you to suggest power and overall energy with a crane doing the lift (ah, you already asked about that in February).

A.

 

[maryden]

A, yes, that is true. Previous advice was taken and it has taken this long for another angle. The information gathered here would be a great help. Thank you.

 

[zeusfaber]

So once again, it's a potential energy problem.

Two or three real-world effects to take into account:

Remember to allow for the buoyancy of your thingy.

Remember to allow for the efficiency of your thruster and associated drive/shafting - this is unlikely to be good

Work out whether you need to worry about energy loss due to drag (at under a metre per second, you may not be too bothered by this).

A.

 

[maryden]

I have to admit that this is a strange area for me. I have thought on this a bit further and have thought that maybe the draft of the propeller is to shallow to be of any effective use (if the whole thing is possible). So let me propose another size where the propeller blade can make a purchase. The diameter is the same at 8 metres but the depth of the propeller motor will now be 10 metres and the weight is still 22 metric tonnes in a light housing. I did wonder if it would travel to slowly so I have shortened the time for the lift to fifteen minutes which is a bit faster. It still only has to rise 10 metres so I would imagine the pool would have to be a bit deeper to accommodate that fact. The prop/motor unit will be stabilized so that there is no wild spin off.
I cannot be any more informative than that because the use to which this will be put has never been done and my engineering discipline is in another area which is why I am here. It is just a rough idea from a rough outline that I have given. If the outline is too flimsy in detail then I will understand and can only thank you for your time.

 

[zeusfaber]

Is the objective merely to raise this thing through the water column, or is there some other essential benefit to having a massive propeller churning away?

If you ballast properly, you could quickly get to the point where the propeller has to do very little work (or none). As well as making the design simpler, a near-neutrally buoyant device is going to be a lot more energy efficient - and you won't have to bother about how to react the propeller torque. Since you're already committed to substantial moorings, you might consider making the platform half a tonne light and then just winching it down to the pool floor. If thrashing about is a compulsory part of the brief, then this sort of arrangement lets you optimise your whisks for thrashing without having to make them generate lift too.

A.

 

[maryden]

There will be no ballast other then the device itself (it simply has to weigh that amount. It will not be buoyant either. Think of it as a deadweight that has to lift itself the ten metres, that is all - this makes me think it may use more than 'just a little' power. When it has reached the mark the motor will stop and the device will sink back to the bottom of the pool. The sinking process is something that will happen after the lift and is not part of the equation.

 

[IRstuff]

I don't think that it's a straight potential energy problem, because the fan(propeller) efficiency is a function of the viscosity of the fluid and the drag on the blades. Therefore, in addition to the pure potential energy aspect, there would be a power draw due to the rotor efficiency or "disk loading," q.v.

http://en.wikipedia.org/wiki/Disk_loading#Power_required

A pure potential energy problem does not require the object to expend energy to stay at a fixed altitude, but your object does, which is more like a helicopter than moving an object vertically with a rope and a motor. You may need to peruse this article to find out if you are operating OGE or IGE.

TTFN
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[IRstuff]

Oh, submitted too early. I'm not sure if the disk loading equations account for the drag of the blades through the fluid, nor the drag of the object itself, so that's also energy expended that would not be the case if you simply hoisting the object with a canonical "rope." You don't say how big your 22 tonne object is, and it may be large enough to interfere with the disk loading, although 8 meters sounds pretty gigantic, but 22 tonnes is quite a bit. Is the 1-m height the rotor and the object, or just the rotor. In any case, you should look to doing searches for helicopter related equations and apply water density and viscosity to them and you should get closer to your final answer.



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[maryden]

I do not know if this helps. Everything automotive in engineering is down to how much power it uses no matter how good the construction. For example, the Emma Maersk uses 80 M/W (plus another 30, I believe) to push her through the sea using a slightly larger propeller. So, my thinking is this - if a propeller/ motor of the dimensions of 8metres by 10 metres (that is all it is) only had to push itself up by a distance of 10 metres it is not going to require anything like 80 megawatts to do that. So, my question still remains the same, really, how many kilowatts (megawatts) would it take - rough guess - to do that? I do thank everyone for their interest even if I am a bit lost with it.
D.

 

[IRstuff]

There is a GROSS difference between a ship and whatever it is you are describing, just as there is a gross difference between a helicopter and a fixed-wing airplane, as I tried to describe. The equations are all there for ROM estimate, you just have to fill in the numbers.

http://auvac.org/uploads/publication_pdf/AUV Verticle Thrusters.pdf

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[chicopee]

My advice is to study ship and submarine propeller propulsion.