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

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

  1. Charlie R
    Joined: Jul 2019
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    Location: River Thames

    Charlie R Junior Member

    I have a 50' x 13'5'' steel houseboat/barge. We are year round liveaboard and cruising (on pure solar electricity) for 8 month of the year. Canals and rivers in the UK.

    I'm considering what upgrades we can carry out to get a bit more speed out of the boat. The current drivetrain is a Lynch motor (Agni 143, specs here: AGNI - ASMO Engineering - The Expert of Electric Drivetrains http://asmokarts.com/index.cfm?pageID=24), driving a 23"x27" prop via a 6:1 reduction drive. We are doing about 2mph on still water, on around 1950 rpm at the motor.

    Not the most hydrodynamic hull, but I'm not sure how much that matters at these speeds. I think the delivery of water to the prop could be improved, also the prop itself seems not the most efficient for low speeds. I'd like to get 4mph if possible.

    Realistic options are repropping, remotoring or regearing. We could up the power - 8.5kW is about the most the battery will deliver, but we do get 2-4kW from the panels to add to that on a sunny day. Possibly extending the propshaft to get the prop away from the swim and into slightly clearer water would also help. I'd love some input from someone with experience in prop/drivetrain design if possible!

    Thanks very much
    IMG_17.jpg IMG_21.jpg IMG_23.jpg
     

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  2. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Charlie R,

    Welcome to the forum.

    You're only turning 325 RPM at the prop.
    I'd be tempted to go larger diameter, finer aspect ratio on the blades and look at two blades instead of three.
    It appears you've got the clearance for a larger diameter.

    My other thought is to water cool the motor and up the voltage a bit.
    I couldn't get the motor "Performance Graph" to open.

    2 MPH to 4 MPH low numbers but still double!

    Have you considered contacting the motor designer?
     
  3. KeithO
    Joined: Jul 2019
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    Location: Michigan

    KeithO Junior Member

    A prop shroud closely fitted to the prop should help a lot at such low speed.
     
  4. jehardiman
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    jehardiman Senior Member

    Realistically, you need the largest Kaplan prop and 19a Kort nozzle you can fit, looks like you could fit a 33-36" wheel and duct between the horn timbers. You have to look at something that hull shape as a push boat, which generally are overpowered. Someone would have to run all the numbers for your exact hull shape, but a quick estimate says you'll need between 9.6-17.5 kw to make 4 knots.
     
  5. gonzo
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    gonzo Senior Member

    Kort nozzles are only efficient with a propeller with blades designed to fit with close tolerances. The "dog ear" propeller he has is not the right blade shape. 6kW (8HP) is a tiny amount of power for a large barge. A much smaller blade area will increase efficiency, since he only needs enough area to absorb the power input to the propeller. Consider the typical propeller of a small outboard for comparison.
     
  6. Artem Klochko
    Joined: Dec 2018
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    Location: Ukraine, Dnipro river

    Artem Klochko Junior Member

    Smaller prop will definetly decrease efficiency. Remember, big and slowly rotating prop have higher efficiency than small and fast one for displacement move of the hull. That's why 6 kW enough to move 33 tons.

    Charlie R, awesome project!
     
  7. DCockey
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    DCockey Senior Member

    Larger, slower rotating props are more efficient up to a certain size and rotational speed. Beyond that size and speed a larger and slower rotating prop will be less efficient. For this boat the maximum size which fits may be smaller than the size for maximum efficiency.
     
  8. DCockey
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    DCockey Senior Member

    What is the actual voltage and current to the motor with the current prop at maximum speed?

    How was the current prop and reduction gear ratio selected?

    Do you have a copy of the "performance graph" for the motor which you could post. It does not appear to be currently available on the website.
     
  9. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    As already noted, a bigger slower rotating prop is the ideal goal. However not all designs are able to achieve this. But any improvement, such as a Kort Nozzle etc will improve the efficiency of the prop you have. It may not improve the speed beyond what you could measure though! Since you do have a brick in the water, let's be honest. A 10% improvement in the prop efficiency may sound good, but that is just one part of the whole "efficiency" of the propulsion system. Since in this mixture of interactions you have the: hull efficiency, appendage efficiency, prop efficiency shafting efficiency and the RRE all having an influence. Hence improving your prop efficiency may be great, but you may not see much difference in the speed, especially with a brick of a hydrodynamic hull form!

    However, the simplest way is the easiest, more power. What is the current displacement of your barge?
     
  10. Charlie R
    Joined: Jul 2019
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    Location: River Thames

    Charlie R Junior Member

    Thanks very much for the input everyone, food for thought there.

    I have played about with various gear ratios, measuring speed, current, rpm, etc. as much as is practical while also navigating - variations in depth, width, traffic ect. make it a bit tricky. I can load the motor up a bit more without exceeding current handling, but thermal runaway sets in fairly quickly. We gained about 1/4 mph from another 1.5kW. I think doubling the power would give us another mph, and 4mph would, as jehardiman estimates, take around 4x our current power.

    But I think there is room for improvement with our existing power train. At 6kW we're sitting at a sweet spot in terms of electrical efficiency, and also our current battery and panel arrangement means we can cruise all day even under cloud, and recharge in a couple of fairly overcast days (or under good sun, go all day and barely touch the battery), so I'm keen to extract what gains I can before increasing power.

    Hull shape would obviously help, but not much we can do about that. Plus it is pretty great in terms of living space and available area for panels.

    I think we could go a bit bigger on the prop, I don't know what the minimum clearance ratio to the skeg/uxter plate ought to be, but we could probably go up to 29" without actually hitting anything.

    I do feel that the prop would gain from being further from the swim section and closer to the rudder though - not sure by how much the 1.5" propshaft could be lengthened before the unsupported length protruding from the stern gland would be an issue.

    Water flow from the prop is not great once up to speed, though pretty reasonable while accelerating (and acceleration not too bad, we hit top speed in a couple of boat lengths...).

    So I reckon that's the bit of the system it makes most sense to work on next. I want to get the base efficiency as high as possible, given the other limitations. Kort nozzles + appropriate props do seem an interesting option, though the possibility of a mattress/sleepingbag/carpet getting stuck in would fill me with dread. Sadly quite possible on the canals!

    Thanks very much. We could up the system voltage, but that would mean an expensive battery change - 60V on LiFePO4 would be ideal, but 40kWh of that does not come cheap (yet). We could also change the motor for a water cooled brushless, but may as well wait until the battery upgrade as with lithium a 40kW motor upgrade would be possible as well. I've been in touch with the motor designer (Cedric Lynch) , he did in fact come down to the boat to check out the setup the winter before last, but I think we're close to the limit on 48V.


    I don't have the performance graph, the IP of the motor has done some shifting around! They don't make them any more, though the Lynch Motor Company motors are essentially the same. Actual voltage to the motor varies, but with panels switched off and a full battery we are at ~48.8V/135A/325 rpm tied up. The current drops by 10-15A once under way, prop rpm comes up a little with the higher voltage under reduced battery load.

    Thanks - I measure the speed down to a few cm/sec so any gain will be noticed! I don't know at what sort of speed the bricklike nature of the boat will start to dominate the performance completely, and I'm not sure what the relationship between prop thrust/RPM/power is? Power increases expontially with linear RPM, but how does thrust increase?

    The drive train is very simple, toothed belt pully from motor to shaft, which is suported by a single bearing and the stern gland. Hence changing gearing only takes half an hour or so... What is RRE?

    I've done a bit of measuring of the system, with various ratios. See below. Figures are somewhat approximate as testing consistency not easy! ratio testing.jpg
     
  11. Artem Klochko
    Joined: Dec 2018
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    Location: Ukraine, Dnipro river

    Artem Klochko Junior Member

    Large cargo ships have prop RPM in range 100-300. Commercial guys count every penny spent for fuel - that's why I think slow RPM is more effective than fast one. Commercial ships built to have maximum fuel efficiency.
     
  12. Barry
    Joined: Mar 2002
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    Barry Senior Member

    With the 23 inch pitch, the props theoretical rate of travel at 325 rpm is 623 fpm but your boat is only travelling 176 feet per minute.
    I am not a 2 mph knowledgeable guy but this seems to be about 176/623= 28% Prop travel efficiency.
    Just from the look of the prop, and the picture angle might impact my comments, it visually appears that the pitch is very high for the speed.

    Generally speaking, one of the parameters of a prop or vane thrust efficiency design is the inlet water speed, ie boat speed, into the prop.
    That is why when you see an open cross section of say a multi stage, turbine, jet pump, as the speed increases from inlet to outlet, the angle of the
    vanes increase because the inlet speed to each stage is increasing. The concept of multistage is to only accelerate the fluid in small increments to keep at least one element, cavitation, to a minimum
    but to also increase the thrust efficiency of the unit

    I would suggest to try a lower pitched prop. Perhaps you can find something used at a prop shop, or chandler or borrow someones spare from another boat.

    So perhaps if your prop is exactly matched for your existing power input and the boat speed, you only need a new prop
     
    Last edited: Jul 20, 2019
    Artem Klochko likes this.
  13. Artem Klochko
    Joined: Dec 2018
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    Location: Ukraine, Dnipro river

    Artem Klochko Junior Member

    Barry,
    I think it is a good point. I think need to increase prop diameter to maintain same power at prop with decreased pitch without change RPM (and reduction)
     
  14. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Read my post carefully. I said he needs a smaller blade area, not smaller diameter.
     

  15. Barry
    Joined: Mar 2002
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    Barry Senior Member

    You may not even have to increase the diameter, just get the prop to it absolute optimum efficiency at the low speeds and high thrust value that you need.
    You could PM Baeckmo to get his comments
     
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