Help me design the "Poorqeedo", another efficient electric boat

Discussion in 'Boat Design' started by mental_boy, Jun 2, 2010.

  1. mental_boy
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    mental_boy Junior Member

    Good point, Porta.

    I think wetsuits would be a reasonable thing to use on the bay, at least until I am more familiar with the boat.

    One thing the catamaran has going for it is raft like stability. I sincerely doubt catamaran is going to flip, unless I make the beam too narrow and/or the platform too high. If I run out of juice or suffer a double drive failure I'll just break out the paddles. If I'm not near anything I can paddle to I'll just radio the coast guard and get a tow.

    Fyi I intend to make the motors and motor control systems completely independent, with the exception of the shared batteries.

     
  2. mental_boy
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    mental_boy Junior Member

    Rick - The poly pipe is a great idea, I'm definitely going to go that route and extend the tube above the waterline inside the hull. That seems simple, economical, robust and efficient. 5/16" is the 8mm equivalent here and just happens to be the inside diameter of the reamed hole in the prop. Now I just have to find some 5/16 ID poly pipe....

    If I put protection around the prop I'll add it after I launch the boat and do some performance tests. That way I can quantify the performance losses and decide if I want to keep it.

    As far as the hull goes, I think I'm just going to use the original hull you posted and just make it taller (like you suggested). The shape underwater will be unchanged. I'm just going to take the offsets from the dxf you gave me, re-draw the hull in the "hulls" program and export the dxfs of the flat panels. I probably wont start that until monday or tuesday night if you have any other suggestions.

    Back to motors and propulsion:

    I just did some battery shopping and I've found that it is not economical to buy medium sized deep cycle batteries here. So if I want 72v, I'll either have to pay twice as much per watt hour for my batteries or carry 240-300lb of batteries. In addition I'd still have to scrounge 72v controllers, so this is starting to look like a high cost option (relatively).

    If I run my current motors at 36v I only have 403 W and I will probably be underpowered. So I've decided I need different motors that have more power at lower voltage.

    I have another pair of motors that do not match, but I think they are close enough for this application:

    Motor #1
    Electrocraft E712
    dimensions 4" dia x 5.75"
    Kv = 29V/1000 rpm
    Kt = 329 Nm/A
    9 amps continuous
    60V max

    Motor #2
    Electrocraft E713
    dimensions 4" dia x 6.75"
    Kv = 30.5V/1000 rpm
    Kt = 313 Nm/A
    10 amps continuous
    24V max

    I'm thinking I'll just run both these motors at 36V, overvolting the E713. Since these motors have very similar Kv I suspect their rpm and amp draw will be similar for a given voltage (Jeremy, do you have an opinion on this?). Even though the E713 will be exceeding its power rating by 50%, i think it won't be a problem since it should have similar (or better since it's longer) dissipation properties to the E712. Both motors have the same frame/castings. I'll have to do some load tests.

    These motors will give me about 700W to play with and will allow me to use large, economical trolling batteries. I'll also be able to use the nice 36v curtis controllers I have:

    http://www.globalepower.com/Datasheets/WideRange/MotorController/1218.pdf

    If one of the motors is stronger than the other I'll just throttle that one back. Better than using junky chinese scooter motors...

    These motors should run at ~1100 rpm under load at 36v, which has me thinking about direct drive again. 16x6 APC prop would give me 5.4 knots, while a 16x8 would do 7.24 knots assuming there is enough power on hand. Thinking I'll just start with the 16x6.

    Thoughts, comments?







     
  3. mental_boy
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    mental_boy Junior Member

    Went to Java Prop to try and simulate the 16x6 and 16x8 props, but I have no idea what the foil shape/angle is at various radii. I assume I have to enter info under design, foils and change the density and viscosity under options. Being half awake probably doesn't help.
     
  4. Jeremy Harris
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    Jeremy Harris Senior Member

    You may be able to stick to your original plan with the motors if you hunt around for some ex-UPS (uninterruptable power supply) batteries as used in data centres, telecommunications companies and critical computing facilities. There is a dealer near me (unfortunately a few thousand miles away from you.........) who sells second hand ex-UPS sealed absorbed glass mat (AGM) batteries for a bargain price via eBay (here's his eBay shop: http://shop.ebay.co.uk/battery-power-shop/m.html?_nkw=&_armrs=1&_from=&_ipg=&_trksid=p4340 ). These batteries often have a 5 to 8 year (or even longer) guarantee from the manufacturer, but seem to be routinely replaced after around 2 to 3 years by organisations who place great value on data integrity (government, banks etc).

    A few weeks ago the local seller had a stock of 38Ah 12V AGM batteries (The SBS 40 ones on this data sheet: http://www.dmstech.co.uk/pdfs/battery_sbs_01.pdf ) for just £25 each (around $37 each) that would have been near-ideal for you. 6 of them would have given you well over 2000 Wh of usable capacity for a total weight of about 168lbs and a total cost of around $220.

    I'd do some looking around to see if you can find someone in the battery disposal business in your area. I'm pretty sure the chap local to me is operating a disposal service, but sells on any good batteries he gets in. A look around for battery disposal companies might be worthwhile.

    Finally, if you want to go light weight and are prepared for some work, you can look at using lithium batteries from power tools. DeWalt 36V power tool packs contain A123 LiFePO4 cells that are around 2.2Ah each in capacity. They have an intrinsic fault that means that one or possibly two cells (in the same location in the pack) fail fairly frequently. DeWalt offer a recycling/warranty exchange service through their service centres and a few people over on the Endless Sphere EV forum have found that they can get literally crate loads of used, faulty, packs for free from these service centres. You need to strip the packs, remove all the cells, test them and only keep the good ones, but you generally get around 8 or 9 good cells per pack. It's a lot of work, and you need to then add some stuff to manage battery charge and discharge, but it would give you a very light power source (probably around half the weight of the lead acid batteries) for a pretty reasonable cost.

    Jeremy
     
  5. mental_boy
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    mental_boy Junior Member

    1. I'd still have to find a reasonably priced 60v or 72v controller. They don't seem to come cheap, especially with reverse.

    The 60v chinese ones on ebay are reasonable, but who knows how well they perform. And they still don't have reverse (I like the idea of having tank style controls).

    2. I don't see what's wrong with the (hopefully slightly) mismatched electrocraft motors. They'll probably function nearly identically in operation, and if they don't I can adjust the throttle. I think the idea of asymmetry bothers people in general (it bugs me). Again, I'll have to test them. I can still use a reduction drive to get better efficiency as opposed to direct drive.

    3. I can get 4140 wh of new batteries at walmart for $210, weighing in at 150 lbs. Certainly not the best batteries, but they're a known quantity and they're reasonably priced.

    I like the idea of the lithium ion batteries, but there's no way I'm going to do all of that work. I'd also have to figure out how to charge them without lighting the catamaran on fire. I know some people who would be interested in that kind of thing, I'll pass it on to them.

     
  6. portacruise
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    portacruise Senior Member

    Porta
     
  7. Jeremy Harris
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    Jeremy Harris Senior Member

    Don't worry about needing a reversing, four quadrant, controller, as there's no need at this sort of current. Just use a double pole changeover switch, wired between the motor and the controller to give reverse. That way you can use a really cheap scooter or ebike controller. The cheap controllers are usually easy enough to mod to run at high voltage, just a metter of changing the output FETs, changing the capacitors and changing the value of a resistor or two on the low voltage regulator circuit. If you really want a pair of levers like a tank, then you could look at using changeover relays for reversing, coupled with a centre-connected pot on each lever for throttle control. Alternatively you could use the switches just mechanically hooked up so that they switch at the lever centre position.


    You may be right, and it may be worth trying.

    If you're reasonably confident that the Walmart batteries are OK, then go with them; I don't know anything about them, but your discharge rates are modest so you have no need of really high discharge capable batteries.

    The A123 LiFePO4 cells don't present a fire risk, even when abused, unlike some other lithium polymer chemistries. They are over-kill for what you're after, though.

    Jeremy
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Paddling a cat single-handed might get a bit frustrating, rowing will require very long oars, and single-paddle sculling requires some skill. The yuloh is easy to make and operate and has been used in Asia for hundreds of years for manually propelling large craft. See this link:

    http://www.simplicityboats.com/yulohpage2.html
     
  9. mental_boy
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    mental_boy Junior Member

    That's pretty cool, reminds me of a giant diving flipper. Don't think I'd build one like on that page, maybe something more like this:

    http://www.youtube.com/watch?v=u7MQsdLpWaM

    That way I can have emergency steering if I don't build a steering system and steer the boat like a tank under normal operation. Assuming the tank method is actually practical.

     
  10. mental_boy
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    mental_boy Junior Member

    Isn't this true for a motor operating at it's rated voltage too?

     
  11. mental_boy
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    mental_boy Junior Member

    For the center connected pot to work the controller needs to be wired so high resistance is low throttle, right? Not sure how those things are set up, and besides this all sounds complicated. You were thinking of using the pot wiper for one connection and wiring the other 2 leads together, right? Using a 20k pot would get you 0k-5k-0k with non linear taper.

    The curtis controller will accept an alternate throttle input where 5k is full forward, 2.5k is 0 throttle and 0k is full reverse, with no need for a reversing switch built into the lever. That makes the controls easy to build.

    Again, I'll have to do some motor tests to see how viable the motors are. I.e. at a given voltage and load do they run at a similar rpm and draw similar current? If not I'll explore other options.


    I think the walmart batteries are like most other marine/Hybrid Deep cycle batteries, sponge like lead instead of plates. I could also buy Costco marine batteries or Sears, etc. and they'd be very similar.

    I just read somewhere a good rule of thumb is to not discharge the hybrid batteries more than 50% and they'll give good service. That gives me 2070 watt hours at 50% discharge, which is fine since I don't think I'll be hurting for range. The same source said the rule of thumb for plate type batteries is 80% dod.

    You guys are right though: cost, durability, usable AH, weight and easy of installation are all important and I haven't done much investigating. I guess I could be persuaded to switch to used AGM batteries if they were clearly a more durable, economical choice.

     
  12. Guest625101138

    Guest625101138 Previous Member

    You can use larger diameter pipe. It does not need to be a tight fit. You can fit PTFE or nylon bushes. There are even small stainless bearings that will fit inside standard poly pipe.
     
  13. mental_boy
    Joined: Jun 2010
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    mental_boy Junior Member

    Ok,

    I've been fiddling around with java prop to compare different pitch props. I couldn't get the file from this page to load:

    http://www.recumbents.com/wisil/hpb/prop/default.htm

    but I was able to get the viscosity/density info from the text file and see that the angle of attack for the foil was 3 degrees for all radii (at least that's what I think is listed), same as the default.

    I didn't know what to use for the foils, so I just went with the E193 at all radii since it was mentioned on the page above. The numbers are definitely a little rough, so it's probably not so good in terms of absolute accuracy, but may be ok for comparing props.

    I changed the diameter of the lower pitch props to optimize efficiency at 3 m/s (or 5.8 kts)

    (EDIT: This data is pretty much worthless. Please scroll down to see more of my dodgy prop simulations.)

    Results:

    [​IMG]

    If the image hosting bails here's the same data scrunched up:

    Propeller 50N thrust 80N thrust
    ----------------------------------------
    12x6 prop 81.0% efficient 78.9%
    14x8 prop 83.0% efficient 81.2%
    16x16 prop 86.0% efficient 83.7%
    16x16 prop, 3% belt loss 83.4% efficient 81.2%

    50N is the number Rick chose for his hull in calm conditions, 6kts with a 400kg load. I added 30N to simulate extra load and/or a head wind. I picked 80N because I believe that is roughly the most thrust that can be produced at 3 m/s with my motors. 700W total, 350w per prop, 245W per prop @ 70% motor and prop efficiency. 245W/ (3 m/s) = 81.66N thrust per prop max.

    I think the lower pitch props look pretty attractive when you factor in belt reduction losses for the 16x16 prop. It's possible the motor efficiency changes as rpm changes, but I don't have that data.

    If I choose an 14x8 inch pitch prop I estimate I'll have about 600w available at 6kts, 29V, 10 amps direct drive. This is 50% more than Rick estimated would be required for a 400kg load and calm conditions. With that load and calm conditions, top speed could exceed 7 kts @36V.

    From Rick's numbers, his hulls with a 400kg load have the following power requirements (assuming 70% efficient prop/motor combo):

    4 knots 135W
    6 knots 414W
    8 knots 1114W (is this right? I took the 780W number from the other cat forum and divided it by .7 to correct for prop/motor eff)

    If I chose a 12x6 prop, the maximum power available at 5.5 kts would be ~700W and the top speed would be ~5.5 kts. With a roughly 350W power requirement at 5.5 kts, the boat would chug along at 1/2 the motor's rated amperage. If there is a headwind or a greater payload, they'll just draw more amperage and I will maintain speed up to 2x power.

    Also worth noting, an advantage of a smaller prop is less curvature in the prop shaft or a better angle on a straight shaft.

    Questions, comments? Errors?

    I should probably go load test those motors....
     
  14. portacruise
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    portacruise Senior Member

    Yes, it's also true below rated voltage under overloaded above rated current conditions. Overvolting can improve efficiency of a motor and be a source of emergency surge power to get you out of a jam. Years back, I worked out a quick change prop on a 12v rated minn kota troll and used a series-parallel switch on 2 batteries to run at 24v when I had unexpected headwinds to fight. This overvolting cannot be done with the more efficient electronic commutated motors to my knowledge. Unless the switching components are rated for higher than design voltages you fry the electronics even staying below rated motor amps.

    Porta


     

  15. mental_boy
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    mental_boy Junior Member

    EDIT: this is also garbage

    I forgot there are 7 inch pitch propellers in the smaller sizes, java prop tells me that 13 inches is the optimal diameter for 7 inch pitch.

    13x7 seems like a good size: at 36v and 1100rpm gives a speed of 6.34 kts.
     
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