Getting the most from electric drive - 33 tonnes on 6kW

Discussion in 'Electric Propulsion' started by Charlie R, Jul 17, 2019.

  1. Artem Klochko
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    Artem Klochko Junior Member

    Ok, sorry.
     
  2. Charlie R
    Joined: Jul 2019
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    Location: River Thames

    Charlie R Junior Member

    Those are the lines I have been thinking along. Reduced pitch (but by how much? What is a reasonable pitch speed/prop slip?) and maintain thrust by either increasing prop diameter or blade count (or rpm). However I have no idea how those principles would translate into the acutal specification on the prop. I've contacted the prop suppliers here in the UK who often come up recommeded (Crowthers Marine, Norris, et. al.) but they haven't been much help.

    I know its a bit of a moving target, in that a prop optimised to get as much speed out of our existing setup as possible, might not be well optimised if we up the motor power at a later date, so still looking for enough theoretical understanding to take a sensible next step.

    I seem to remember coming across a write up of a dutch barge with a prop optimised for low speed electric propulsion, and it had a 5 or 6 blade narrow chord low pitch propellor, more like an aircraft turbofan design, though google doesn't seem to be throwing anything up at the moment...
     
  3. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Regardless, increasing prop efficiency isn't going to give you more than 1/4 knot added speed.
    Realistically, more power is needed in combination with an efficient prop.
     
  4. Charlie R
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    Location: River Thames

    Charlie R Junior Member

    More power is on the agenda. But a 12.5% increase in speed for zero extra energy input would be good as well...
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    It is Relative Rotative Efficiency (RRE).
    It is a ratio of the measure of the efficiency of the prop behind the boat versus the prop in open water.

    Do you have plots of power versus speed ?
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Funny, it's almost as if the hull "designer" has had the goal to make life ugly for the poor propeller..... .
    -The hull resistance is far greater than necessary, due to bad shape.
    -The propeller inflow is terribly disturbed due to bad hull shape.
    -The rudder is operating in the propeller jet, which in this case is something like 8 mph and is designed to cause considerable drag.
    Now let's look at the propeller first. If there ever was a case for a Kort nozzle, this is it. The determining factors are two; maximum propeller diameter and available power. Pitch, shaft speed, blade area aso can be calculated from the two, no guessing needed. You have already sought out the sweet spot in terms of motor power and its rpms (asfo voltage, current and efficiency). So Charlie, what is the distance from hull to the bottom base line or whatever lower limit?
     
  7. Charlie R
    Joined: Jul 2019
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    Charlie R Junior Member

    I do have a bit of data on this, but not enough points to be much use. The speed is so low that variation in current, wind, battery voltage, etc. make it very hard to separate out the signal from the noise.
    It is 30" from the base plate to the top of the skeg. Though the stern gland is not centred in the gap, a little higher than the centre point unfortunately so the actual maximum prop size would be a bit lower.

    I know the hull shape is not ideal - I would be open to suggestions for improving it but not sure there is much to be done without considerable steelwork. We do need more outside space, I would like to install a proper generator at some point. Keen to hear any ideas though!
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Would be good if you could get some form of speed v power. As this will tell you about this:

    The shape/trend of the curve will indicate if, if any, mods to the hull will assist matters much. Since the hull is ostensibly a brick, as its MO is a box and makes maximum use of the volume/space.

    I wouldn't take a sticking plaster approach or focusing on just one item at a time.
    You have several issues you would like to be addressed, thus don't do them each in isolation as they are all linked together. So I would suggest, if you have future plans note them all down so you can plan how to address each one at the same tiem and estimate its effects.

    And that's the issue at hand. Whatever changes you would like to improve performance (one of many no doubt), the solutions will be indicated by this - the amount of rework on the hull and its subsequent cost implications too.
    Hence listed down all the changes/mods/tweaks you would like and address them all at the same time - as a design study and cost analysis approach - to understand the implications and attempt to minimise any costs and see which items are a nice to have versus those which you can afford.

    Otherwise you could be spending a lot of money for not much, if any, real gain.
     
  9. baeckmo
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    baeckmo Hydrodynamics

    To give you an idea of the propellers operating environment, and what it could have been like ........ See typical results of CFD simulation in enclosed pdf.

    The shape of the triangular "pod" is really bad for the propeller flow. All inflow is taken from below, and the pod is effectively blocking the main flow, creating large scale vortices in front of the propeller. In "real life" there is a horizontal flange to make things worse.

    I concur with Ad Hoc that the hull issues should be adressed before digging into propulsion details. The modifications I made for the CFD simulations are simple jobs with straight plating and constant radii, yet quite effective.
     

    Attached Files:

    Dejay, Ad Hoc and Artem Klochko like this.
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Nice little plots Bodo.
    Just emphases the need for a more suitable/compliant hull - if the desire is for an increase in speed. Hence what is the power v speed plot like - and unless this is addressed is the rest worth doing.
     

  11. Charlie R
    Joined: Jul 2019
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    Charlie R Junior Member

    Wow, that is incredibly helpful, thank you very much. I intended to take enough images to create a photogrametric model of the hull when we were last out of the water, but time did not allow unfortunately. Not had much time to get online this week, we have had good sun so heading upriver.

    I have been trying to get some decent speed vs power data today, but the variations in river flow, depth, width, battery etc. make it pretty meaningless. I think we need to take some drag readings under tow to get anything worthwhile. Might have a chance to do that next week, cruising with another boat.

    Really great food for thought there. Just from watching the propwash it's clear that water delivery to it is very far from ideal. Very lumpy, you can see great chunks of air getting pulled back under around the stern.

    Lock opening so heading off now, thanks again!
     
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