Efficient electric boat

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

  1. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    your battery is knackered so alll your tests are a waste of time ..should hold 12v + under load thats why its called a 12v battery ...one cell is buggered..
    go to k Mart on a 25% discount day .....!!!!
     
  2. MCDenny
    Joined: Jul 2009
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    Location: SE Michigan USA

    MCDenny Junior Member

    Pist,

    I agree the battery is not much good buy I do not agree the tests are a waste of time. I'm measuring actual amps and volts and calculating watts so the data is relevant and comparable. The knackered battery just means I can't get going as fast at the end of the period.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    MCDenny,

    You are absolutely correct. No wasted time.
     
  4. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    K Mart specials are fine if they are DEEP CYCLE. Car batteries only last about 25 times in deep cycle use. It should be very easy to repeat Denny's 66%+ improvement discovery, knackered or not. Some of us can't wait to try this at home!

    Porta

     
  5. pistnbroke
    Joined: Jan 2009
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    Location: Noosa.Australia where god kissed the earth.

    pistnbroke I try

    Well the motor was designed to run at 12v so when you get down to 8-10 how can that be accurate.... there are only two types of battery starting and traction ....the only other types have different labels.gel electrolyte ..the plates all come down the same production line and get different labels/cases at the end ....been there seen that ...
     
  6. DaveJ
    Joined: Jun 2009
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    Location: Brisbane, Australia

    DaveJ Senior Member

    Have you ever wondered why they tell you not to use a long electrical extention lead, and if you have to, get ones where the gauge of the wire bigger. The reason for this is the voltage drop caused by the resistance of the lead, if you motor is design to run at 12v it needs that 12v.

    Let me try to explain some more, lets say your motor is 120 watt motor design to run with 12v at 10amps. Now if the supply to the motor drops to 10v the motor still tries to pump out 120w so for that to happen the current increases to 12 amps, if the winding in the motor can only take 11 amps, it burns outs.

    Getting back to the extention lead, now there is more current so more heat, which increases resistance of the lead more so more current is required, this is a bad cycle that keeps on going until the smoke escapes, usually in the motor.

    Now you have a regulated control over the motor and its supply, how much this effects your test results are speculative, but i'm sure when you have the throttle wide open (figure of speach) and because current being pulled are 35A your current readings will be high because of not the full 12v getting to the motor.
     
  7. MCDenny
    Joined: Jul 2009
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    Location: SE Michigan USA

    MCDenny Junior Member

    Dave and Pist are on the right track but only half way home. They both forget the fact that the motor's RPM drops as the voltage drops, the slower turning prop doesn't absorb as much power.

    The motor's standard "speed coil" control is no different than using a too small too, long extension cord. It adds resistance which causes a voltage drop which reduces the voltage at the motor. A PWM speed control creates the same effective voltage drop at the motor by tricking it with pulses of 12v.

    The only way you can control the speed of a dc motor is by varying the voltage. It's not built to run on a certain voltage, it's built to turn a certain RPM per volt, its Kv rating. Trolling motors turn about 1200 rpm at 12v so they have a Kv = 100.

    Motors have a certain ability to withstand and dissipate heat - that fixes their current limit. They have a certain mechanical strength to not fly apart at high RPM, that fixes their voltage limit. Probably some brush performance issues here too, but you get the idea.

    Reducing the motor voltage to 8v means the prop now turns about 800 RPM. It's a lot easier (less torque) to turn it 800 RPM instead of 1200 so it pulls less amps, not more. My measurements bear this out. Of course it's no longer producing nearly as much power (watts).

    Dave's example of a 120 watt motor running at lower voltage / higher amperage only works in real life if you change the prop when you change the voltage so the lower voltage, lower RPM prop would absorb the same amount of power as the higher voltage, higher RPM prop.

    The standard 5 step speed control applies full battery voltage to the motor on #5 and only half the battery voltage on #4. Yesterday in the canoe I measured 29 amps on #5; 15 amps on #4. Cutting the voltage in half didn't raise the amperage, it cut it in half too.

    Final testing ended yesterday (with a new battery, BTW). Power to speed relationship with old and new batteries was the same. Of course, top speed was less with the tired battery but X amount of power (volts x amps) gave Y speed regardless. I'll post the results soon.
     
  8. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Denny, is there something special about using 4 mph that makes your brushed customized MK troll so efficient (66% prop + ? %fairing, + ?%connectors, etc)? Would you expect your currren draw (and efficiency) to be better than the Torquedo outrunner motor set to run with lead batteries at 4mph for example?

    Porta






     
  9. bapou
    Joined: Aug 2009
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    Location: Germany

    bapou Junior Member

    McDenny,
    I'm impressed by your work and the results. Your work is very helpful
    for the development of my solar motor pod.

    You said that you added a spinner and a roughly NACA0025 fairing. I would
    be very curious to see some photos of the setup. Do the cheap APC propellers survive the strong torque of 250 Watt input or do you see some fatigue ?

    Thanks
    Thomas
     
  10. MCDenny
    Joined: Jul 2009
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    Location: SE Michigan USA

    MCDenny Junior Member

    Porta, I just picked 4 mph arbitrarily. I'm aiming to drive an 16' waterline sailboat with just one 12v battery with range goal of >10 miles. 4 mph seemed like the fastest I might hope for. Power required goes up exponentially with speed and I can't store much energy in one 50# battery.

    I think the Torqeedo would do better than my set up. It uses a brushless outrunner motor and a nice big prop. The smallest one costs US$1100 here; they make a bit of gearbox whine and the 3 blade makes it hardfer to stow. Besides I could never bring myself to cut the lower unit off a brand new Torqeedo.

    One of my goals is to rig this up in a well about 15'" long by 4" wide. The motor unit would be about 12", 3.25" wide and about 13" high. The motor needs to be small enough to be stored in the well when the plug is in for sailing. The sail boat won't be done until next spring and I won't be able to really work out the details until then. The speed - range - watts tests with the canoe was to give me data to plan the sail boat drive with. I have learned the modified motor will suit my sail boat just fine.

    Bapou, I'm going to sumarize my experiment and post some pics later tonight. The props seem plenty strong. I was pretty careless cutting a slot in the hub for the shear pin but i had zero problems with props. The tips are very thin and prone to getting dinged up. When that happened I just sanded them smooth again. They are so cheap (US$3.00) that anyone could afford several spares.
     
  11. Guest625101138

    Guest625101138 Previous Member

    Denny
    Good stuff. Yes getting rid of apendage drag is very helpful. These are things you really appreciate when you use pedal power. The other factor that will be significant is windage in the Tern.

    I am looking forward to the photos.

    How old is the battery? If it is a deep cycle battery you should find it will improve with use if it has not sat aound for years without being cycled. 20% of rated is very poor. Another advantage of reducing the draw is that the battery efficiency will improve. So this is a win-win outcome.

    Rick
     
  12. MCDenny
    Joined: Jul 2009
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    Location: SE Michigan USA

    MCDenny Junior Member

    Final test results - PWM Controller, APC 10x6 prop, Fairing

    These experiments could go on and on as every piece of new data seems to bring a new question or a new thought about something else to try. Life goes on however so I'm calling the attached graph my final answer. There are a couple of pictures of the motor with fairing, APC prop and spinner.

    I started with a used Minn Kota Endura 36 trolling motor with simple, inefficient resistor speed control. I added a Castle Creations Mamba Max PWM speed control wired directly to the power wires going down the motor tube, bypassing the internal contacts in the resistor speed control. I tried about a dozen different APC model airplane props and learned the 10 x 6 gave the best boat speed output for battery watt input. I also made a NACA 0025 fairing 1.5" thick and 11" high and fitted a 3" diameter spinner meant for a model airplane. Speed data was taken with a 14' waterline plastic canoe. All up weight was about 300 lbs. A garmin GPS was used to measure speed, averaged over 30 second periods. Volts and amps were measured with a "Watts Up" meter connected in series next to the battery. An 80ah 12v battery was used. Range estimates are based on 80% depth of discharge and a battery with a peukert exponent of 1.12 (typical of a quality AGM battery)

    Baseline with unmodified motor:
    -Speed #5 (WOT) gave 4.3 mph at 330 watts; range 1.7 hrs, 7.3 miles.
    -Speed #4 gave 2.9 mph at 178 watts; range 3.4 hrs, 9.9 miles.

    Switching to an APC 10x6 prop:
    -Speed #5 gave 5.1 mph at 410 watts; range 1.3 hrs, 6.6 miles.

    Adding the PWM controller and adjusting 'throttle' to give 4.3 mph speed:
    -Standard prop pulled 330 watts; range 1.7 hrs, 7.3 miles
    -10x6 prop pulled 260 watts; range 2.2 hrs, 9.5 miles
    -10x6 prop plus fairing and spinner pulled 190 watts; range 3.2 hrs, 13.8 miles.

    Conclusions:

    -Adding the PWM control and 10x6 APC prop increased range 29% over standard motor on setting #5 while maintaining same speed.

    -Adding the PWM control and 10x6 APC prop increased range 88% over standard motor on setting #5 while maintaining same speed.

    The prop alone costs $3.00 and takes just a few minutes to modify to fit on the MK drive shaft. It increased speed 19% but the higher boat speed requires more power so the range suffered a bit. With the resistor control it's not possible to slow down efficiently as the second highest speed setting cuts the voltage to the motor in half and wastes half the power in heat.

    Adding a PWM controller costs $80 to $100 but permits efficient operation at any speed.

    The fairing was made from scrap lumber in about an hour. The spinner is also a hobby shop item and cost $6.00.

    This modified motor will suit my needs for powering an 18' sail boat to be built this winter. I'll update the forum when I figure out a good mounting system.
     

    Attached Files:

    1 person likes this.
  13. portacruise
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    portacruise Senior Member

    Thanks for the pictures and final report. I have some friends that that use that very same model MK on their 10' inflatable pontoon fishing boats. They use a heavy battery with the standard setup to power across longer distances. Even without the PWM, but using some of these simple changes, they should be able to cut battery weight in half or so and use cheaper gel cells to get the same range at full speed.

    One interesting thing from your graph is that with your modifications the original no load drag of the motor (50 watts per you post 227) is enough to drive a canoe at 2mph assuming the sketched curve continues along the same line!

    Great posts.

    Porta




     
  14. MCDenny
    Joined: Jul 2009
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    MCDenny Junior Member

    Porta, It's a little dicey using the graph at very low power levels as measurement inaccuracies can be a greater proportion of the total result.

    It is true though that very little power is needed to move a boat slowly. At the WoodenBoat Show last summer I was easily able to row a 19' sail boat with 3 men and all sail up (in zero wind and current) at least 2 mph.

    I wanted to see if I could find a prop that would unload the motor enough to get decent range with the resistor control at WOT and avoid the cost of a PWM controller. A 10x3 showed some promise but I didn't get conclusive data. APC props are not available in pitches lower than 3 in appropriate diameters. If your friends want to try this on their dingy witht the resistor control they should try both a 10x3 and and a 10x6. The 10x6 will produce more speed but at much higher amp draw and commensurate short range. They will want to get a letter V drill (0.377") to bore the prop to fit the .375" MK shaft. Using a 3/8 drill will give a very tight fit - difficult to get on and off.

    Another note about the final graph. These tests were conducted in modest wind, maybe 5 - 8 mph blowing against the current. I assumed the added drag caused by the 3" - 6" chop wouldn't matter very much and the wind effect would be cancelled by taking the speed average over two way runs. Looking at my data from a few days earlier I now see I was wrong on one or both counts. Two days earlier in very calm water I was able to achieve 4.3 mph with 30% less input power to the same motor setup. I now realize that since the wind drag varies with the square of the speed, averaging up and down wind runs is not the same as conducting the test with no wind. In the final test my speed, relative to a 5 mph wind, was -2 mph downwind and +11 mph upwind. That's quite a bit different from the +3 vs. +6 mph apparent headwind I would see at zero true wind speed.

    The conclusion that the prop and controller allow big improvements in range is still valid, but as they say YMMV.

    The weather here is getting steadily worse and I have other things to do so I'm not testing anymore until I get the motor in the final boat next spring/summer. Then, with better weather and a more stable boat so I do not have to return to the dock to switch props, I can repeat the experiments under more controlled conditions.

    Another thought on the fairing. I learned today that the round shaft has a Cd of .47 while the teardrop fairing has a Cd of .09. Even considering the shaft is 1.125 thick and the fairing is 1.5" thick, that means the drag coming from the shaft is reduced by a factor of 4 - tha'ts a lot when working with such small power levels. Also, it think the fairing helps the prop work more efficiently (no proof, though) because the round shaft leaves a turbulent wake and a lot of air right in front of the prop. The fairing smoothes out this flow.
     

  15. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    The fairing may not work as well when turning the motor is used as a form of steering. I have used "shark fin" adapters with trolling motors before and find steering to be very twitchy, requiring constant correction. The cruising speed on most of these inflatable pontoons is probably closer to 2 mph so the fairing may not be such a factor.

    Still it's incredible what you did with the Cd on the shaft and thrust with the APC!

    Porta
     
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