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

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

  1. Guest625101138

    Guest625101138 Previous Member

    If you have a reasonable multimeter it is not hard to determine the efficiency you will get from an electric motor.

    1. Get a 1.5V battery and connect directly to the motor terminals with the multimeter in circuit to measure amps. Clamp the shaft so it will not turn. Meter needs to be on the high current range. Take the meter out of circuit and measure the voltage across the motor terminals. If you have two meters then do the voltage and current readings at the same time.

    2. Connect the motor in circuit with a multimeter set to read current and a battery with the full voltage you intend to use. Take the current reading and check the battery voltage with the motor running at full rpm.

    If you post the values you get I will do the efficiency calculation for the full load condition.

    Rick
     
  2. mental_boy
    Joined: Jun 2010
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    mental_boy Junior Member

    Great, I'll definitely post those values when I figure out what I'm doing. I guess I'll get back to the hull while I meditate on the motors.


     
  3. Guest625101138

    Guest625101138 Previous Member

    I do not know if you have looked around this site very much but there are some good threads on stabilised monohulls or faux-tri configurations.

    This one has quite a few pictures:
    http://www.boatdesign.net/forums/boat-design/trimaran-motorboat-stabilized-monohull-29665-2.html

    This is the way to get the efficiency of a slender monohull but with the stability closer to a catamaran.

    Something else to meditate on if you have not already seen the idea. You will get much better range at a given speed than the cat you are considering.

    Rick W
     
  4. mental_boy
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    mental_boy Junior Member

    I hadn't actually considered that. When you mentioned it before I was envisioning something like an oxford rowing shell with stabilizers, not exactly the thing to carry 4 people with.

    One thing I don't like about the catamaran is the entire occupied area is elevated and contributes a lot of windage no matter how it is done. Building a deck structure that is both strong and light weight is also challenging.

    I would certainly prefer a regular monohull to a stabilized monohull from a emotional standpoint. I like the way some of the stabilized monohulls look, but generally not the small ones with good load carrying ability. It might look good to my eye with the right classic double ended hull. hmm.

    I really like classic narrow monohulls like Winsome (the one Jeremy is building), Whio, and the Bolger sneakeasy (with the exception of the stern).
    Not the ugly sneakeasy, the nice one:

    http://www.boattest.com/images-gallery/News/bolger7.jpg

    I imagine to get good stability I would end up with a wide hull that has high drag. I don't think I'd want to be choppy water with a sneakeasy.

    I think the maximum length I can legally move is about 6.5 m or so, but I still have to get it on the rack.

    I'm thinking about rewinding my motors for lower voltage. It looks pretty easy, it's very inexpensive, and it sounds like the efficiency can be much better with a hand wound motor due to the higher fill of the wires in the slots in the rotor.

    I'm still a little wary of the chinese scooter motors, but I may go that route too. Unite (the manufacturer) lists the efficiency at ~75% for most models.

    [QUOTEI do not know if you have looked around this site very much but there are some good threads on stabilised monohulls or faux-tri configurations.

    This one has quite a few pictures:
    http://www.boatdesign.net/forums/boat-design/trimaran-motorboat-stabilized-monohull-29665-2.html

    This is the way to get the efficiency of a slender monohull but with the stability closer to a catamaran.

    Something else to meditate on if you have not already seen the idea. You will get much better range at a given speed than the cat you are considering.
    [/QUOTE]
     
  5. Guest625101138

    Guest625101138 Previous Member

    When you mount three 105Ah batteries down the bottom of a hull you can get the CoG down fairly low. It means you can keep it a little narrower and still get acceptable stability.

    The box keel arrangement is between the stabilised monohull and a conventional monohull. This may be a variant worth considering. Mount the batteries and motors in the keel.

    I have transported 7.5m on the car roof. The sort of boat you want to build really needs a trailer.

    Rick W
     
  6. mental_boy
    Joined: Jun 2010
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    mental_boy Junior Member

    I like the box keel idea a lot, although I'm worried about the weight of the hull. One huge advantage of the catamaran is it can be broken into manageable parts.

    I could always pull a Phil Bolger and build a monohull in two pieces, like the folding schooner =)

    Ignore the previous comment about rewinding the motors. Even though I can get more windings in the gap, I think with an adequate gauge wire the Kv will go way up and I'll need a reduction drive. Still looking for my free lunch...


     
  7. mb2778
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    mb2778 Junior Member

    Hey mental_boy, just stumbled across your post and figured I'd throw in some points for consideration regarding weight of materials... Granted, this may have already been stated, but I don't have time to read through all of the past posts just yet. Anyway, you estimate 10 pounds of epoxy and 5 pounds of glass for the hull, but I think this is seriously underestimating the materials needed for the hull. I'm not sure if you're considering S&G or strip-building or some other wood-cored construction and just didn't post weights for the wood required, but either way you will definitely need more than that in epoxy and glass. With those values, you have 66% resin and 33% fiber, which won't let you get much in terms of strength out of your materials. To give you an idea of what you're likely to come across, I build vacuum-bagged kayaks that are 17' long and 22" wide with the hull weighing in at a minimum of 9 pounds when built with Kevlar/Coremat/Carbon. This would DEFINITELY not be strong enough to sustain anywhere near the loads you're looking at, and is not a cheap way to build. Even with infusion, you won't be able to do much better than these numbers since any core you use will absorb some excess resin. Anyway, I'd start by calculating the square footage of your hull then deciding on what kind of laminating schedule you'll implement. By multiplying the weight of fabric+core+epoxy (assuming a 50% resin content at absolute best if you're hand laminating) by the number of square feet you should get a rough estimate. What kind of reinforcement fabrics are you thinking of using? You'll definitely need more than the 25 feet you currently have, since this would be barely enough to give you a single skin of glass which will be about as flimsy as a sheet of thick acetate. Anyway, best of luck, this should be fun!
     
  8. mental_boy
    Joined: Jun 2010
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    mental_boy Junior Member

    Hey, thanks mb.

    Those numbers were just for guessing the hull weight. I had a gallon of epoxy, so I weighed it. I made up the number for the fiberglass. The weight of the hulls was based on the plywood only.

    The hull will be based on the lines Rick uploaded in post #15 and post #40. I will try to increase the freeboard a little to make best use of the plywood.

    My rough plan for building the hull a is just making it out of 6mm marine meranti plywood. I'm thinking 5 bulkheads of the same material and long gussets made of triangular pine strips or something in all the corners. On the outside there will be fiberglass below the waterline and on the deck (draped over the side by about 2") and there will be glass tape on the bow, stern and all the seams where the plywood is butted.

    The reason I'm glassing the bottom is because it will get dragged on things and will be submerged. The deck is getting glassed since the glass is there, but I'm not certain it needs it. I don't think I'll glass anything inside the hull except where the plywood butts lengthwise and for reinforcing the bulkheads that attach to the crossbeams.

    This is just off the top of my head, I need to find some similar craft online and see what they do for materials.

    I've only built one boat ever, a sea flea. It looked like the one in the link, only not as ugly =)

    http://www.hydrostream.org/GuestArchive/MinimaxPic1.jpg

    It was made of 6mm construction grade plywood and 19mm pine on the sides, no fiberglass required. It was pretty fun, went 18mph with 4hp....

     
  9. mental_boy
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    mental_boy Junior Member

  10. Guest625101138

    Guest625101138 Previous Member

    You can get glass tape for doing internal seams. You can use an epoxy filler in the corners that does not slump. I mix up the epoxy paste, spread into the corners then lay tape while it is curing. I wet out the tape with straight epoxy. For your hulls I would use a single run of 400gsm triaxial tape.

    Blue foam makes solid bulkheads if the hull is going to be fully sealed. Use ply where you want to make motor mounts.

    I have found water curable polyurethane glue to be the best for joining foam to timber and glass. It expands as it cures so seals any gaps and is firm enough to glass over.

    Rick W
     
  11. Guest625101138

    Guest625101138 Previous Member

    I would not go thinner than 6mm on your hulls. The curve in the sides is not much so you will get unfair shape if you use thin ply. The bottom is flat so will flex too much if 1/8" ply.

    Rick
     
  12. mental_boy
    Joined: Jun 2010
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    mental_boy Junior Member

    Thanks. I'll go with the 1/4 inch, like you suggested. I was just thinking in terms of strength, not fairness of the hulls. They certainly wont be the lightest hulls, but I should be able to move them around.

    Interesting idea of the foam bulkheads. I'd consider it if I was cutting the parts by hand, but it's less guesswork for me to cut wood bulkheads on the cnc machine.

    You mentioned a round over on the hull improves efficiency by only 3%. I have a 1.5"/38mm round over bit, think I should bother?

    Still working on the motors. Another option is some 20V 5.7A servos that I could try at 36V/5.7A to get 200w, but I'm not so sure they'd survive! They're sealed, so I'd have mill holes and cool them with a small blower. One nice thing is the Kv is only 42rpm/v. I opened on up and it had fairly beefy 21ga wire...

     
  13. Guest625101138

    Guest625101138 Previous Member

    Rounding the edges is simply to avoid a sharp corner. The glass cloth will sit neatly over a rounded edge whereas it bubbles in a tught corner unless you bag it.

    Rick
     
  14. portacruise
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    portacruise Senior Member

    Those servos should survive so long as they don't bog down and stall in weed or such. The 1500 rpm gives lots of top headway on the rpm. May not need cooling if you pick a less aggressive prop to keep the currrent just under 5.7A.

    Porta

     

  15. mental_boy
    Joined: Jun 2010
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    mental_boy Junior Member

    Thanks Porta,

    I asked about the motors over at endless-sphere and no one else seemed to think overvolting them would be a problem provided the amperage stays the same. If I do use them I'll still probably cool them for insurance, since there will be more heat due to core losses.

    I said in my last post the Kv was 42, it's actually 29rpm/V. So at 36V the rpm would be 1044 and maybe more like 950 under load.

    Still, the motors are only total about 400w overvolted. Photo of disassembled motor attached.

    I'm thinking no matter what motors I use I'll make a simple alarm that goes off when the current exceeds a certain threshold. That way I won't cook my motors. I built something similar before for a different application. Just a voltage regulator plus a voltage divider (two resistors) for reference voltage, and a comparator (lm339) that compares voltage between the shunt and the reference.


     

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