Electric Hull Designs

Discussion in 'Boat Design' started by matt76, Sep 22, 2007.

  1. Guest625101138

    Guest625101138 Previous Member

    More on the 800lb foil boat:

    The catamaran hulls would have WL beam of 1ft and draft of 7". They would be set 8ft apart.

    Using a modified NACA foil (4512-64) you should be able to sustain flight with 650W power to the prop. The main foil would be 12ft (yes 12ft) wide with a chord of 100mm. This foil would be placed just in front of the CoG and would need to be mounted at least 1ft below the hulls. You would need a "T" foil for rudder and lift at the rear. Have not determined if stable flight is possible without control on the rear wing but I think it should be possible.

    The speed is higher than I have been considering so the higher Reynolds number gets the foil into a more efficient regime.


    The lift to drag for the foil alone is 48:1 so similar to a high performance glider. I have made allowance for strut drag and windage so the end result is somewhat less.

    I have not attempted to determine if there is a power hump as this takes a lot of iterative analysis. My bet is that it will be noticeable because the hulls are so short.

    The foil would need to be quite strong. I would look at solid aluminium to start with.

    Rick W.
     
  2. alan white
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    alan white Senior Member

    I gather Decavitator represents the state of the art. 600 watts is substantial, and I'm assuming that's foiling speed and not initiation which would be somewhat higher?
    The weight of batteries seems to be a matter of cost. Lithium Ions (perhaps as sold for use with cordless skilsaws at Home Depot) would reduce weight a lot at some high cost. I'm guessing the LI batteries would be one third the weight of lead acid. I don't know. But this would certainly reduce power requirements. Funny thing about batteries. As they get lighter, they not only reduce drag, they also can be reduced in number because of the reduced power requirement. Gains are therefore expotential when one uses lighter batteries.
     
  3. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    A lot of interesting thoughts here.
    With batteries you can't plane.
    So you must go long and slender.
    This leads towards the rowing shell style.
    But you want some more deck space and stability?
    Make a trimaran :)
     
  4. Guest625101138

    Guest625101138 Previous Member

    There would be plenty of people to dispute you cannot plane with batteries. Here are some samples:

    http://www.youtube.com/watch?v=p6ban1MMgUY
    http://www.youtube.com/watch?v=MR-6dwFVlLs

    And if you think it can only be done with RC models have a look at this:
    http://agnimotors.com/video2.html

    I guees it is a matter of how far you want to go and how fast but there is no doubt electric powered vehicles can do wonderous things. Here is a recent example of a land record vehicle. Watch till the end:
    http://www.tgdaily.com/content/view/33853/113/
    This is really interesting about the batteries!!

    Rick W.
     
  5. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    I agree, you can plane, but not for a very long time:
    (from www.Agnimotors.com)
    On November 1st 2005, 24 year old Helen Loney piloted ‘An Stradag’, Henry Engelen's 15ft electrically powered hydroplane to a new World Unlimited Electric Water Speed Record average of 68.09mph.

    I think at 12 knots a long, slender tri will be more efficient.
     
  6. Guest625101138

    Guest625101138 Previous Member

    If you read through the previous posts you will see there is a length constraint of 16ft.

    With constraint of 16ft and speed of 12kts I am betting a foil borne boat would be close to giving the best range. Maybe a displacement catamaran. Gets down to the load to carry and weight of batteries to which would come out on top.

    It would be an interesting challenging to see what comes out best. Need more detail on expected load to do numbers.

    Rick W.
     
  7. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    ermhf, yes, sorry ... :(
    Didn't see that.
    I once saw a foil borne bicycle craft with one guy cruising around the harbour at maybe 12 knots :)
     
  8. lewisboats
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    lewisboats Obsessed Member

    Long and slim is what I suggested on Woodenboat where there is the same post. Foiling is a possibility but he wants to carry up to 2 people on a (VERY) small lake...maybe up to 140 acres. At 12 kts you are talking what?...two minutes of planning time minus acceleration and deceleration? I suggested a Sneakeasy (which has seen 10.4 mph with electric) but at 26 ft it was too long. I am trying to get a realization that it is a matter of Physics VS $$$ VS length and beam but so far I haven't had a response to the latest post.

    Steve
     
  9. keith66
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    keith66 Senior Member

    Interesting thread, i have worked on one electric boat as detailed somewhere else on here "contender hull" I am rebuilding a 1905 SE Saunders launch 28ft long x 5' 8" beam she is a lovely example of the early displacement launches of the time, long and slender with a fine entry rising into a flat run with barely inches draft at the stern. Though mine was powered by a petrol engine some of her sisters on the Thames were electric powered and are reported to have acheived good speeds.
    Some of the Saunders launches used to umpire rowing races, namely the "Consuta" (51') were capable of speeds around 13 knots with virtually no wash, the hull having a prounounced tunnel aft to reduce draft. Consuta has been rebuilt and is now in commision fitted with steam plant.
    If you are operating on a small lake whats the rush!
     
  10. matt76
    Joined: Jun 2007
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    matt76 Junior Member

    I'm still here, i'm reading everything everyone has to say and researching the advice.

    * My over all length can't be over 16 feet (because of the size of my shop)
    * Up to 500lbs of batteries if needed (at least 300lbs)
    * Two passengers (no more)
    * Maybe 50 lbs for the motor 72 V Performance
    Peak efficiency: 88.6%
    Peak power: 34.3 hp
    Cont. power: 8hp
    No-load speed: 3590 rpm
    Stall current: 1440A
    Stall torque: 200 ft-lb
    * Cedar Strip and fiberglass construction (i don't like plywood, i'm a stripper, i know the cost)
    * Small lake usage only
    * I want to be able to go as fast as the hull and motor combination will allow
    * I want to be able to have her completely decked except the cockpit, batteries and motor will be under a hatch with cooling fan

    Thank you for all the advice, I'm learning more and more. I'm aware i'll need to spend maybe $5000.00 in motor, batteries, controllers, chargers, prop, ect. And probably another $2000.00 in wood, glass, and hardware. sounds like fun to me.
     
  11. Guest625101138

    Guest625101138 Previous Member

    We are now getting more information. It would make life simpler if these details were stated up front.

    So we now have set the total weight of 400 to 450kg, length of 16ft, design speed of 12kts, range not really important, cruising power of 8HP and peak of 34HP. This is a lot of power for a small boat.

    You can go to this site and play around with the parameters:
    http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
    It only goes down to 15kts but that will be easy with 8HP. You still need to build light though. The sort of numbers to get started are weight of 420kg, lcg of 1.2m (you can place the batteries where you like), beam of 2m, beta of 10 degrees (or 0) and leave others at 0 until you get the least power combination that is realistic. If you go too wide with weight too far aft then it will have a big power hump and make large wake trying to get on plane. You want a static trim using the full waterline length.

    A typical outboard motor and prop will give an efficiency of 70% but if draft is not a constraint then you can make a high aspect prop that will have an efficiency around 85%.

    Another option is to use a stepped hull. This is more complex than planing but less complicated than foil. I doubt that it is worth the effort as you really have power to burn for planing.

    Rick W.
     
  12. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Nice :)

    I use this as a rule of thumb: calculate hp/ton, tae the square root, multiply with 3.5 plus minus, really 3 to 4 depending on hull efficiency.

    So, 8hp/500kg is 16hp/ton, square root is 4, multiply with 3.5 and you have 14 knots. Estimated!
     
  13. alan white
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    alan white Senior Member

    At eight horsepower, input wattage will be approximately 6.8 kw at 15% loss.
    At 72 volts, six 12 v batteries of 1.133 kwh each, 94 ah, total of 564 ah, allows a one hour run.
    I don't know how much those batteries would weigh. If they weighed 60 lbs each, the weight would be 360 lbs.
    Sounds like this is about what you're saying. At top speed (25 kts?), the batteries would deplete in a bit under fifteen minutes. Recharge at 10 cents per kwh, cost, sixty-eight cents. Not too shabby.
    Using a displacement hull with foils would allow greater efficiency at sub displacement speeds, maybe 1500 watts to maintain the 5.36 kt approx. monohull speed, with running time of 4 1/2 hrs, and far longer at 4 kts, maybe 9 hrs, for a range of about 40 miles. Top speed would get you only about 6 miles.
    At 10 cents a kw recharge cost, and a 68 cent full recharge cost, and a range of 40 miles, this is the same as getting 160 mpg.
    That's not bad for a boat that can also scream across a lake in a few seconds.
    Every bit of this is pure didn't-leave-my-seat semi-hokum, based on memory more than anything else.

    Alan
     
  14. matt76
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    matt76 Junior Member

    that sounds good to me alan, thanks for all the replies, more advice or more ideas the better.
    I'm sure i can make her light and strong and look even better.
     

  15. Raggi_Thor
    Joined: Jan 2004
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    Raggi_Thor Nav.arch/Designer/Builder

    I think it's a very interesting project.
    Please post some pictures and more info later!
     
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