Efficient electric boat

Discussion in 'Boat Design' started by Jeremy Harris, Jun 22, 2009.

  1. bapou
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    bapou Junior Member

    Update on progress

    Hi Rick,

    I'm still working on the motor, controller and battery unit and it will still take a while until I can make real world measurements on the boat. I fitted the Hall sensors on the motor and rebuild the same chinese ebike controller as Jeremy is using. I also got some carbon Bolly props... but there are still many steps to go.

    Thomas
     
  2. MCDenny
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    MCDenny Junior Member

    Trolling motor test update: After three days of wind we finally had a beautiful sunny calm fall day yesterday. Alas, its windy and rainy again today.

    I tried 11 different props and compared connecting the PWM controller to the motor input leads vs hooking the PWM controller output directly to the wires going down the motor tube to the brushes.

    At 30 amps the factory input wires and speed controller (set on 100%) absorbed 7% of the power - about 25 watts. Also, the no load (no prop, in water) power consumption is 50 watts so there is a good bit of energy "overhead" to pay for. The prop shaft has two back-to-back rubber lip seals and I don't know what condition the brushes/commutator is in.

    Of the props, a 9 x 6 gave the best boat speed per watt out of the battery.
    To make a relevant comparison, all at 4 mph with a 12v 80ah battery with a peukert exponent of 1.12 and using the PWM controller:

    Driving the stock prop through the stock motor input leads gives 9.2 miles range.

    Driving the stock prop with a direct connection to the brush wires gives 10 miles range, an 8% improvement.

    Driving the APC 9x6 prop with a direct connection to the brush wires gives 16 miles range, a 60% improvement!

    My new connectors did not get warm but the battery power still dropped off noticibly over the two hours of intermittant testing. It is an old deep cycle battery of unknown capacity. It shows about 12.4v when fully charged but drops quickly to 11.2 with load. At the start of the test the motor will pull 30 amps but only about 22 by the end. I had assumed this was due to the bad connectors generating more resistance as they heated up but now suspect the battery is tired out. Anyone have any other ideas why this might be dropping?

    Using the GPS trip computer function to give average speed over a 30 second run worked well.

    Next steps are to:

    Get more data points with the standard, 9x6 and 10x6 (a very close second place) props to make the conclusions more reliable.

    Get a better battery of known capacity.

    Measure the effect of adding a spinner to reduce drag.

    Measure the effect of adding a fairing around the motor tube to reduce drag.
     
  3. Guest625101138

    Guest625101138 Previous Member

    Denny
    That is useful comparison data.

    Fairing will give some gain - maybe 5%.

    If the battery has sat around for a long time then cycling through full charge to about 25% will improve storage capacity if it is not too old and designed for deep cycle. The attached shows the voltage level for various state of discharge. This voltage should be read with only a slight load on the battery of a say 1A.

    You will not get 80Ah from a lead/acid battery rated at 80Ah when pulling 20 to 30 amps so range estimates need to allow for this. I would expect about 60Ah before it starts to droop badly. This is one of the advantages of lithium. They give closer to rated capacity at high rates of discharge. Also as you noticed the voltage drop is significant so if you think in terms of watt-hours it will be much less than 960Wh. Realistically about 700Wh. As the battery size increases you should get closer to the rated output.

    Can you confirm you are talking about speed in statute miles per hour and not knots?

    Rick W



    the lead acid is inferior.. Was you range estit will
     

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  4. pistnbroke
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    pistnbroke I try

    Re your battery ...if you mean you charged it to 14.4v and then let it stand over night and it was then 12.4v then its in the suspect range ..should be over 12.5v ....Unless you are using the battery every day there is no substitute for a normal car typle starting battery ...all the other so called leisure .gel . etc are really starting batteries ..only a traction battery ( fork lift ) is any improvement on a standard car battery ...and get the cheapest you can ..K mart etc ...recharge at 10.8v minimum. what make was the trolling motor with what sort of controller ( electronic or switched) and what size cables did you use to the brushes..??

    Your 80 ah battery will only give that at 8A at 16 it will only give 84 % of 80ah and at 32A 84% of that figure ...etc etc
     
  5. portacruise
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    portacruise Senior Member

    HOPE THESE OBSERVATIONS ARE OF SOME USE AND ARE TRANSFERABLE TO YOUR SETUP.

    PORTA
     
  6. MCDenny
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    MCDenny Junior Member

    Thanks for the responses.

    Rick, I am definitely talking about speed in statute miles per hour and distance in statute miles (5280 feet). Sounds faster than quoting speed in knots ;)

    I am projecting range to 80% depth of discharge using a peukert exponent of 1.12 in the calculation. This allows for the lower amp hours available at large discharge currents.(1.12 is the correct one for good AGM batteries, not the old wet SLA I had laying around.) I will use an AGM in my new boat. A typical test session is only taking 10 to 15 ah out of the battery. My automatic 10a "smart" charger brings it up to full in just a few hours.


    Piston, Its an Minn Kota Endura 36, it comes with a resistor speed control but I am using a PWM controller meant for an RC model car and capable of 100 amps continuous current. I have 8' loing 6 ga wires going from the battery in the bow to the motor head. The wires going down the tube look like maybe 10 or 8 gage. I've not tried to take apart the actual motor yet.

    Porta, The tests were done in light winds and a steady 2 mph current with the up current and down current speeds averaged. Speed was the average over a 30 second period.

    4 mph is about the zero slip speed for a 4" pitch prop so the standard prop plus 3 or 4 diameters of 4" pitch APC props all performed about the same. I have more detailed data for the standard, 9x6 and 10x6 props which I will post later tonight. The bottom line from those tests is still a 60% plus range improvement with the APC prop.

    So far, in five test sessions, with increasingly more controlled proceedures, I'm seeing the 60% improvement every time.
     
  7. pistnbroke
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    pistnbroke I try

    Thanks for that ....Minn Kota tip .....to dismantle motor remove prop then the two through bolts and pull it apart ....You will find a cardboard tube loose behind the commutator ....whats that for I thought ??? Well you put it between the brushes to hold them out and as you push in the armature it pushes the tube out ......and it just sits there rolling about ,,,, Have the 86ib thrust minn kota 24v dont use the resistive controller just 12 or 24v
     
  8. MCDenny
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    MCDenny Junior Member

    Me again. Here's the results of the most careful speed vs. watts test to date. This measures the efficiency of the standard prop, an APC 9x6 and an APC 10x6. This was with speed measurements at 5, 10, 15, 20 and WOT amps for each prop, taken up current and down current, light wind and far enough from shore to insure the current was reasonably constant. The actual data points are very close to the trend lines.

    Following the 4 mph line across the graph, you can see that the 10 x 6 prop requires 170 watts while the standard prop takes 270. This translates into 14.6 amps vs 22.5 amps, which translates into 14 miles vs 8.4 miles range ( a 66% improvement) with a healthy 80 ah AGM battery.

    The next nice day I am going to try a 10 x 7 prop, and to determine the effect of a spinner and a fairing around the motor tube.
     

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  9. portacruise
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    portacruise Senior Member

    I USED THE BIGGER MINN KOTA WITH THE 1/2" SHAFT 20 YEARS AGO SO WAS ONLY RAN THE 14" AND 16" APC BECAUSE THERE WASN'T ENOUGH HUB WITH THE SMALLER PROPS. ALSO THERE WERE SOME NON-WEEDLESS ALUMINUM PROPS FROM SHORTY EVANS THAT I EXPERIMENTED WITH. I GOT SOME IMPROVEMENT IN THE 15% RANGE WITH THE ALUMINUM AS I RECALL.

    ANYWAY, THANKS FOR PASSING ON THIS INFORMATION! I HAVE SOME FRIENDS THAT CAN USE THE INFO FROM YOUR EXPERIMENTS WITH TROLLS.

    HOPE THIS HELPS.

    PORTA
     
  10. MCDenny
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    MCDenny Junior Member

    Porta, Range comparisons are done at the same speed. Read across the graph and note the watts required at 4 mph for the three different props.

    Holding the watts constant instead and reading up gives three different speeds for the three different props. The 10x6 indeed drives the boat faster so you could go further in the same amount of time. But because it takes a lot more power to go a little bit faster the extra prop efficiency is mostly taken up going faster with not much benefit left to go further.

    In the previous test the 9x6 prop showed slightly better performance than the 10x6. In the latest test, graphed above, the 10x6 was slightly better. I think this is the more reliable conclusion as this was more rigorous; 5 data points instead of three and I was further offshore to make the current effect more constant.

    I have the fairing (an eyeball NACA 0025 1.5" thick, 6" long, 11" high) ready to go. Weather is supposed to be nice Monday so I hope to have another report tomorrow.
     
  11. portacruise
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    portacruise Senior Member

    Thanks Denny, I see what you mean now. I was comparing at constant current to see distance with each prop rather than using speed. The current would have to go up gradually as the voltage drops to maintain 4mph, but you seem to have it covered by assuming 47 ah which is considerably below 80 ah rating. The stock prop seems to be built for lower top speed as there isn't so much difference in current for the two props to maintain 2 mph. That fits the main application of troll motors- designed to move heavy boats at low speeds while cutting back on weed interference. It appears you have found a clever way around that design limitation.

    I think Rick recently posted a diagram of an ideal fairing he worked out in regards to someone's question. It was sharply pointed at the two ends with gradual tapering in toward the body.

    Looking forward to seeing how your fairing works out.

    Porta







     
  12. Guest625101138

    Guest625101138 Previous Member

    Denney
    Your power data for the 10X6 is very close to the prediction. The rpm must be lower under power than your provided.

    The efficiency gain over the standard prop is quite impressive. Anyone who thinks a typical boat prop can match the efficiency of high aspect blades only need to look at your data.

    Rick
     
  13. portacruise
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    portacruise Senior Member

    Rick, how much would your custom designed props improve on the APC 10X6, if you designed for the same 4 knots and setup?

    Denney, at least some of the torquedo props are designed to run in the range of 1200 rpm and might be interesting to see how much more improvement can be had from them. You can order them from the replacement props section, but they are VERY expensive compared to APC. May not be worth the price if there is not another doubling of efficiency.

    Vic

    Porta

     
  14. MCDenny
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    MCDenny Junior Member

    Rick, I was told the MK motor did 1200 rpm at 12v. I have no way to directly measure rpm. Plus the battery sags quickly to 11.2 volts and it just can't put out the amps after it is used a bit. Today I started with 12.38v and the motor pulled 31 amps at WOT, after about 2 hours WOT = 15.3 amps. Power usage for the session was only 17 ah and the battery is rated at C20 = 85 ah. Obviously the prop is not spinning at 1200 rpm all through the testing.
     

  15. MCDenny
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    MCDenny Junior Member

    Test results for 10x7 prop, fairing and spinner

    Got back on the water today to try out another prop and a 1.5" thick NACA0025 fairing plus a 3" dia spinner (motor body is 3.25" dia).

    Check out the attached graph!

    10x7 prop: 4 mph took 180 w; 15a; 3.4 hr; 13.6 miles
    Add fairing: 4 mph took 128 w; 10.7a; 5.0 hr; 20 miles
    Add fairing and spinner: 4 mph took 110 w; 9.2 a; 6.0 hr; 24 miles

    The 10x6 performed a little better: 150 w; 12.5a; 4.2 hr; 16.8 miles
    10 x 6 plus fairing (no spinner): 112 w; 9.3a; 5.9 hr; 23.6 miles

    Reducing drag and improving electrical and prop efficiency is paying big dividends in lowering amp draw. The lead acid battery's peukert effect means lowering current disproportionately increases duration. So far I have more than doubled range @ 4 mph over the stock motor by adding about $100 and zero weight.

    All the results today were a little better than the prior session as I raised the motor about 2" to get the top of the fairing up to the waterline.

    I'll post a picture of the motor with fairing and spinner tomorrow.

    Absolute final test will start with a box stock motor and progress through PWM through motor leads, PWM wired direct, plus 10 x 6 prop, plus fairing and spinner. All with a strong battery to keep the span of data points roughly equal. Stay tuned.
     

    Attached Files:

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