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

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

  1. Jeremy Harris
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    Jeremy Harris Senior Member

    If your Curtis controller has a reversing contactor drive (which I think some do) then that's the easy way to do it, just use that drive connection to operate the double pole changeover relay.

    I was thinking of wiring the pot as you say, with the two ends connected together. It wouldn't be very linear, but would have been cheap! I'd thought that you could have the centre position of each lever set with a means of flipping the forward/reverse changeover switch, so moving the lever forward flicked the switch and then applied throttle, with the same happening when you pull the lever back to go astern.

    Having sailed a catamaran with a single outboard mounted on one hull, though, I'd have to say that I don't think that differential thrust will give you much in the way of steering control, unless you can reverse the motors. I was surprised at just how little the catamaran tended to turn when only being driven by power on one hull.

    Jeremy
     
  2. Guest625101138

    Guest625101138 Previous Member

    If you look at the blade geometry with 3 degrees angle of attack with any of those props the balde chord will be ridiculoudly small. Once you set the AofA to a value that results in the realistic chord you will get lower efficiencies apart from the 16X16.
     
  3. mental_boy
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    mental_boy Junior Member

    Aha, thanks Rick. Looking in the geometry view the chord of the props was ludicrous. I adjusted the angle of attack until the proportions as looked similar to the photos of the apc props, so the chord at various radii should be somewhat realistic. I also changed the Reynolds # to 300,000, I was using 100,000 before. Same E193 foil.

    For the APC 16x16 going 3m/s I get 85.2% eff, similar to what you got. I attached the geometry view and an image of the APC prop so you can compare them.
     

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  4. mental_boy
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    mental_boy Junior Member

    Here's the APC 10x7 prop, also at 3m/s or 5.8 kts. Efficiency is only down about ~3%, so I'm thinking with a belt reduction on the 16x16 the differences start to become negligible. There will be more friction (?) on the prop shaft with the 10x7 since it will be operating at a higher RPM, but the shaft will also be curved less. (Edit: more friction on the poly bearings due to higher rpm, but lower friction due to less curvature on the shaft, no idea how this balances out)

    10x7 seems to have the best efficiency for the 7 inch pitch with this load at 3m/s. When I tried the 13x7 I mentioned before, I was getting efficiencies in the low mid 70's.

    Anyway the 10x7 seems like a good choice. Any Doubters? Suggestions? etc? =)
     

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  5. mental_boy
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    mental_boy Junior Member

    Ok, I changed the 16x16 prop to look more like the 10x7, now I think the comparison is more fair. Efficiency went up 0.6%.
     

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  6. mental_boy
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    mental_boy Junior Member

    Just to illustrate how far off I was yesterday, here is an example of the aspect ratios I was simulating. Rick, if I'm still messing up these prop simulations please let me know.
     

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  7. mental_boy
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    mental_boy Junior Member

    Construction questions.

    Some things that need to be worked out:

    1. Not sure how thick to make the hulls. I think each hull needs roughly 3 sheets of 4x8 plywood. I believe I have enough ~8oz cloth to glass below the waterline, cover the deck and tape the seams.

    I have 3 sheets of 6mm 1/4 hydrotek brand meranti plywood, all or some of which could go in the hull. I also have a sheet of 9mm.

    2. Bulkheads. Any thoughts on how many?

    3. deck clearance over the water. 1 foot / 0.32m????

    4. Beam? 8 feet / 2.5 m?? This is the maximum width for a car or trailer in some areas of California. In other parts it's 8.5 feet.

    5. Deck construction? I'm currently thinking of 3 crossbeams with 2 4x4 foot decks between them. The 4x4 decks could be made with two sheets spaced a few inches apart to make them rigid and light.

    Thoughts on beam type (alu tube? wood?) size?

    6. Prop mounting? I think the stainless shaft is usually pretty hard, so I'm thinking somehow employing stainless shaft clamps like these:

    http://www.ondrives.com/images/CC-CT.gif

    As far as construction goes, I have access to a 4x8 CNC router, a milling machine, a lathe and a tig welder among other tools. I'm good at welding stainless to very thin gauges but worthless for aluminum...
     
  8. Motivator-1
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    Motivator-1 Junior Member

    I think I can help here!

    Hi Mental,
    Having already built several utility Catamarans, and going through the same contiplation as you are, I can offer you what has worked for me.

    I made molds for my boats,v and then bonded the decks to the Fiberglass molded Hulls securely to where I could simply fasten the Bridge Deck to the Deck only, as the hulls were, in this case, only considered as Floats. If building from wood, you need bulkheads for the Hull construction, but at least at the points where your bridge deck attaches, so that your Bridge Deck is driving the Hulls and is attached to the Hulls as well as the deck. For your information, my Fiberglass lamination schedule was first Gelcoat-One 3/4 oz check Mat- One 1708 Biaxial Glass to finish. This gave a hull of about 5/32" thickness (Resin-thick Hand Lay-up) that allowed you to beach the Hulls and stand on the boat with just some deflection. I thought this was enough until on one boat trip, we crept up to some unsuspecting Manatees in shallow water, and in their panicking exodus, one Manatee Reared-up violently, and put a 10" crack into one hull. Duct Tape fixed it quickly and kept us fishing.

    My last Cat had 10" of clearance over the Water, which felt good as to the closeness to the water for the pleasurable experience, as well as what just looked right for this size Boat. My decks were made of 2" Teak wood slats separated by 3/4", that made for a comfortable feel on your feet, a very fine look for finish, and when "Splashing down into the water on large waves, the 3/4" reliefs let the deck give to the sea quite well. You had to raise your feet to not get them wet. It was part of the Novelty of this boat.

    I'm out of order with questions, but as for the Bridge Deck, you cannot go wrong with Aluminum Round Tube construction for the Torsional Stability and minimum weight. Round Tube seems to be the top choice for any small structural construction, like in Car roll-cages, etc. You could do Box- Constructed Epoxy Beams, but with much more work. Believe me, If you can Tig weld SS, you'll be doing Aluminum in no time, and you'll like it.

    I have more to offer. I hope this helps you.

    John
     
  9. mental_boy
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    mental_boy Junior Member

    I think the way it's set up the controller reverses the contactor automatically below 2.5k so I don't even have to think about it.

    The non linearity of the cheap pot setup might not be all bad. There would be a nice big dead spot for zero throttle and you could make fine low speed adjustments when docking. Adjusting between 5 and 6 kts might be twitchy?

    What kind of cat was that? Seems like there would be a big difference between something with a V hull and a rounded hull (say the classic hobie 16 and the glass hobie 18)

     
  10. mental_boy
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    mental_boy Junior Member

    Thanks John,

    I'm curious about a couple things:

    1. Have you ever taken your boat out in less protected waters?

    2. How much does your bridgedeck weigh?

     
  11. Motivator-1
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    Motivator-1 Junior Member

    Hi Mental,
    I can't really answer your Question #2, but it is easy to get weights on Aluminum Pipes, etc. off of the net.

    As for my boats operation in questionable waters, well yes I have. The basic concern here is whether you can whether some rough conditions. My experiences have been in Intracoastal Waters, where an excessive Chop is what you need to deal with. It becomes a wet ride, but quite doable, though I would never want to be in the condition of exposed seas, as they can sink or capsize any boat.

    There was one time that I had fun testing the boat during what was called a 100 year weather pattern. It was interesting to say the least, and what occurred was that we had a 3 day period of 50 MPH winds with gusts above 65 MPH with clear conditions of little rain. I took advantage of this to test out my boat, which I believe, is a testament to it's relative lack of windage. The waves consistantly crashed over the hulls, essentially keeping them partially submerged at all times (which I think aided in Windage), but I was able to make way at about 1/4th usual speed. and at the moments that the wind blew at 65 plus, the boat came to a total stand-still, but began forward movement immediately after the gust. Interestingly enough, I was the only craft out on the waters that day and it was a fun ride. The downhill ride - Downwind - was spectacular as I felt I was flying and felt that I would drive the boat right up the Boat Ramp without the Trailer, and the Steering Yaw was a real challenge going downwind at that speed. Also, on that run, the propeller very often sucked air even with the Cavatation Plate that I had installed.

    I had become quite comfortable with this boat, as well as my Pedalcats, as long as I knew that the hulls remained watertight, and I believe that is Key, as well as being able to steer into the Chop and "Heave To" if needed. My drive unit was a steerable Trolling Motor with a HandleBar control that allowed me to easily keep things facing forward. If I lost this system, though, I'd probably be screwed, but not capsized in Bay Waters, just going wherever the wind wanted unless anchoring or putting out a Sea type anchor.

    I would still suggest avoiding nasty conditions.
    John
     
  12. portacruise
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    portacruise Senior Member



    See above.

    Porta
     
  13. Guest625101138

    Guest625101138 Previous Member

    I cannot recall the Re# but JavaProp does the calculation in one of the Analysis pages. You need to make sure your foil selection is reasonably close. From memory the Re# was closer to 100,000 than 300,000 for most of the blade in your case.
     
  14. mental_boy
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    mental_boy Junior Member

    Thanks again, Rick.

    I Changed the Re from 300k to 100k and the efficiencies of the props dropped significantly, but the relationship between them was still similar:

    APC 16x16_83% eff
    APC 10x7__79% eff

    This is using an Identical shape to the simulations yesterday.

    Then I went to APC's site and found this:

    "The dominant basis for the primary airfoil shape used in most APC propellers is similar to the NACA 4412 and Clark-Y airfoils, except the leading edge is somewhat lower. Also, the aft region is somewhat thicker. "

    Clark-Y is a foil option in Java props, so I switched the foil to Clark-Y from E193. E193 is similar in shape to Clark-Y, but thinner.

    With Clark-Y @ 3 m/s, 50N:

    APC 16x16__79.5%___@423rpm
    APC 11x7___72.7%___@969rpm
    APC 10x7___77.7%___@969rpm
    APC 9X7____80.8%___@969rpm
    APC 8x7____79.8%___@969rpm (loading goes from low to med)

    I tried to match the same shape as closely as possible for each test. It's certainly not a perfect representation of APC's props, but at least the shape is consistent for each size prop in the simulation. The target shape for each prop in the 6 simulations above is attached below.

    I think one big problem with these numbers is that they are optimized for a specific set of conditions. When I up the thrust to 80N, the 10 inch prop appears ideal for the 7 inch pitch. When I lower the speed to 4 kts and 2/3 rpm, the optimal prop without reduction becomes larger still. With 4 kts, 2/3 rpm and high drag, the small props start to look even more unfavorable.

    I suspect the 16x16 prop has a wider efficiency envelope than the smaller props with direct drive, so it could retain the upper hand both at higher speeds and at lower speeds with high drag (as in a headwind)

    Seems like optimizing for one set of conditions is easy, but optimizing for multiple conditions is time consuming...

    Tempted at the moment to say $%#! it and use the 10x7.

    help?
     

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  15. mental_boy
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    mental_boy Junior Member

    I totally agree.
     
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