Ideas for a Fast Efficient Electric Hull

Discussion in 'Boat Design' started by papawoodie, Jun 22, 2010.

  1. papawoodie
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    papawoodie Junior Member


    Hi,

    Thanks for the feedback....

    Yeah, I'm inclined to agree with you. I never wanted to 'fly' or skim above the water, was just hoping to take advantage of some free lift...

    Probably too much trouble for this project...

    Still, its such an attractive concept. But I'll save it for another day.

    Top speed is my primary target, with minimal power requirements as the guiding principle, using 'backyard shade tree' technology...
     
  2. papawoodie
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    papawoodie Junior Member

    Many thanks for your contributions.

    I've got the 3 files, and am working on learning DelftShip sufficiently to view what they represent.
    In just looking at the numbers though, it seems they are closely related to your most successful pedal-powered craft.

    I could easily be persuaded to sit-on-top as opposed to being in a wet sloshing cockpit.

    For the sake of minimizing watts /maximizing speed and range, let's target the narrowest practical beam.

    Hmmm... I'm picturing a nice leather BarcoLounger Recliner with "The Magic Fingers"
    and a 24 quart beer cooler all bungee corded....

    Ouch! I think Reality just knocked!

    Thanks, again,

    David
     
  3. papawoodie
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    papawoodie Junior Member

    Thanks for the links!

    As I eventually get a much more refined set of specific parameters, I'm hoping, collectively, we can pick the most appropriate motor(s).

    It looks like we might even be able to specify our needs to Astro -- as far as power, RPM, kV -- and they'll custom built for the job at hand.

    Hopefully, the price will still be somewhere between Competitive and Affordable...

    Appreciate the leads,

    David
     
  4. mental_boy
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    mental_boy Junior Member

    You might want to rule out a planing hull depending on the water conditions.

    I built a 2.5m x 1.5m boat that did 18 mph (measured on the gps) with an unmodified 4hp suzuki sailboat outboard motor. This was in calm conditions in the oakland, ca estuary:

    http://en.wikipedia.org/wiki/Oakland_Estuary

    The one time I took the boat out on the San francisco bay in a mild swell, the best speed I could get was 6mph because I couldn't get up on a plane.

    So what is the water like where the race is held?


     
  5. kroberts
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    kroberts Senior Member

    Hmm... Have you thought about a hydrofoil? There are a few sailing hydrofoils and even a couple human powered versions. One is a kayak and one is a sort of hop up and down arrangement. Put 6 kW on what would normally be a human powered hydrofoil kayak and you might get somewhere fast.
     
  6. papawoodie
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    papawoodie Junior Member


    We're here in the glorious LowCountry of South Carolina -- surrounded by islands and rivers and salt marsh along the oceanside. Lots of potential for use around here in these tidal waters.

    The race is held in a convergence of two tidal rivers and involves circling a sizable island. Its held mid-autumn, so weather can be a factor. Tidal currents and mild breezes are the norm for that time of the year. When the weather threatens to go foul, if the race isn't postponed, there is hardly any interest -- either from participants or curious spectators. All of which sort of defeats the purpose, in my opinion....

    On the other hand, I'm more interested in a 'fair-weather' craft. Don't think I'll be tempted to venture out in rain or winds or rough water. The others can have their fun in those conditions... I'll wait 'til another day for sun and gentle breezes!
     
  7. portacruise
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    portacruise Senior Member

    Wonder what happens when you hit something of minor mass or something like weed tangles in a foil. Seems like you might drop out of warp pretty fast....

    Porta
     
  8. Bahama
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    Bahama Junior Member

    Sorry I didn't have time to draw up a picture and didn't see how to do it; but now I see the little (very little) paperclip icon and so I drew up what I was thinking of here:

    Winged Hull.gif

    The top one shows just the single wing and the double is below. For the double hull I basically envision a shell that just extends outward to emulate an asymetrical hull as best as you can and then the second shell of the same shape extends further out. Notice that both shells drop backwards a bit as well; it would be possible to flare out the aft portion a bit as well if you like. I don't know how much they should flare out so I exagerate them outward to show the effect.

    Anyway, this is what I was pondering, I would thing that it would create more lift just like two sails do in the air.
     
  9. papawoodie
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    papawoodie Junior Member


    Something like that might have some potential, but I feel it would require very still water to operate well in. Any waves, or motion would cause the hull to pitch and/or roll, which would disturb and readjust airflow continuously. Any gains would be quickly offset by irregular motion.

    Perhaps you could try rigging a pair of sails horizontally, from the bow aft to the mast, to achieve a similar effect.
     
  10. papawoodie
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    papawoodie Junior Member

    So I want to start some discussion on the advantages/disadvantages of using one vs. two electric motors.

    With one, it would obviously be setup along the centerline, with the prop beyond or under the stern.

    With two, the stabilizer/amas could serve as either motor mounts or shaft housings. The props would extend to aft the sponson tips.

    It would seem to me, on a rudimentary level, that two moderately sized motors, set up to turn in opposition (ie. towards the centerline), would be potentially faster. Would it be faster for the power available and/or how would 2 affect the range, for a given battery bank?

    Would 2 props, at say 75% of the overall size of a single larger prop, be more or less efficient?

    Could they deliver sufficient speed, yet turn at slower RPM's?



    Perhaps I'm putting the cart in front of the horsie... Do we need to determine prop size first?

    I've played with JavaProp, but have no clue what I'm doing. I get a 90+% efficiency with a .75m 2 blade prop at 300 to 400 RPMs with 1475W power for 7m/sec... Whatever all that means...?



    The overall general parameters, so far, are:
    --- Speed of 12 to 15 knots over a range of 35 miles (56km)
    --- Length of 24 feet (7.3m)
    --- Beam of 1 foot (.3m)
    --- Hard chine (single or multi-chined, to be decided)
    --- Overall displacement at 400 pounds (180kg) - or less, if possible with selected components
    --- Rough estimate of power capacity required: 5kWh
    --- Initial calculation of power for speed: 1450 to 1470W at 7m/sec (25 kph or 13.6 knots)


    Where do we go from here?

    Thanks, again,

    David
     
  11. portacruise
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    portacruise Senior Member

    Good question, I await to see what the experts say. Two motor setups would mean more total weight and more water friction plus the need for constantly balancing outputs, would seem to me.

    Porta

     
  12. Guest625101138

    Guest625101138 Previous Member

    David
    JavaProp is probably giving you close to the right value but if you look at the geometry you will find they are incredibly narrow blades and will have no chance of handling the thrust. To get a suitable size blade you need to reduce the rpm and reduce the AoA (it can be negative) to get the chord width that gives adequate strength. If they are simple flatbar blades then 50mm wide would be absolute minium I expect.

    As a first estimate I took 60mm wide blades with 10% thickness. It could be heavier than you need but I did not have time to look at the strength.

    Anyhow you will get 86% efficiency from a 600mm diameter prop. It does not cavitate, which could be an issue as you push the speed. The shaft power was 1785W at 7m/s. I am still working on 200kg displacement. If lighter you will use less power but keep in mind there has been no allowance for windage and appendage drag.

    The problem with the two props in the stabilisers is that they are not well planted. The aim is to keep them out of the water most of the time. You run the risk of frequaent aeration. A single big prop behind the hull will be well planted. At speed the hull will trim bow up and bury the prop deeper so no risk of aeration unless you are going slow in waves around the length of the hull.
     
  13. Bahama
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    Bahama Junior Member

    Thanks. The only other variation that I could think on the subject was to have a fiberglass outer shell, but then place a strong blow up bag that is pressurized with water so that it doesn't actually have any wings, it's simply an expansion of the hull. Again, it creates a slightly asymetrical wing of the entire hull in the proper shape to simply create more lift--which I think would mean that you could steer more directly upwind if you want.
     
  14. Guest625101138

    Guest625101138 Previous Member

    I was certain I had already posted the attached Delftship file but cannot find it - maybe I put it on the wrong thread. This is a very quick version of the hull with some fairing on top to reduce windage.

    I would deck the hull to seal it completely and mount the gear on that deck. There is only a small gain by placing batteries into the hull and this would complicate the structure. The fairing can be made quite light as it does not play a role in the hull structure. Sit in a semi-recumbent position to keep weight and windage low.

    The fairing is on its own layer so you can play with both elements separately.

    Repeating the prop data - the required shaft power is 1785W for 7m/s at a shaft speed of 284rpm. It has a very aggressive pitch for the 600mm diameter. This has no allowance for windage and appendage drag. I expect you would want a continuously submerged rudder so it will chew up maybe 20W.

    Allowing for the motor and controller efficiency you will be around the 2kW mark if you have something good.

    A single Mars motor would handle this with margin on 36V. It has plenty of torque so you do not need to be as careful in selecting the reduction. Going to a lighter outrunner motor requires more care with the reduction. Also the efficiency needs a close look at the actual operating conditions.
     

    Attached Files:


  15. papawoodie
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    papawoodie Junior Member

    Rick:

    The design looks fabulous!

    No doubt, if I can ever get a grasp on DelftShip, someday in the distant future, I'll be able to produce a rendering, though much more rudimentary. After spending most of yesterday, merely trying (unsuccessfully) to get the program to acknowledge a hard chined flat bottom hull (and finally surrendering), I'm convinced that paper and pencil and 'back of an envelope calculations', though primitive, are adequate to get me to the 'working with lumber' stage.

    So, I can readily, and deeply, appreciate the time and talent you invest in this forum, helping so many.... May your praises be sung loud and long!

    I like the boat! If I build the hull as a Dacron covered 'skin-on-frame' the fairing ought to be a snap. Somewhere, the recumbent seat will have a sturdy attachment into the hull frame, protruding above deck --- and I'll fashion the fairing to secure to these same fastenings. A small cleat at the bow and two at the stern will serve to 'tie' the leading and trailing surfaces.

    It looks quite 'do-able'....

    Many thanks!

    And, I'm coming to better understand the rationale for a single motor (somewhat begrudgingly). I've been unable to locate any info online regarding any benefits of two over one -- except for maneuverability, redundancy for breakdown, or having twice as many opportunities to spend money.

    So far, it seems the only 'touted' rationale for dual motors is quicker 'out of the hole' or 'off-the-line' speeds -- neither of which is relevant to my goals.

    Certainly, I can support the notion of buying fewer items! I, too, like the idea of mounting the 'power box' on deck. That should make rigging up easier (from off the trailer). The batteries can be housed and placed to best affect balancing the hull (keeping the bow down). And I think that the section amidships could be built with the deck lowered within the hull sides, with self draining, to put weight as low as possible. Batteries, motor, and seat brackets could all be incorporated therein.

    Quite likely, I could scheme the seat, batteries, and motors into a single transportable unit. Using the recessed deck area as a sort of tray, the seat could be fashioned into a 'drop-in' frame that is secured by pins into the tray. The motor could be housed in a water safe cover to fit under my raised knees, and the battery box could be a sort of foot rest at the fore end of the frame. The drive shaft could slip into a aft slanted tube running between my legs and under the seat, along the centerline.

    I'm up for the challenge of fabricating my own props, which keeps open the avenues of getting the most powerful and efficient blades for the overall speed objectives. In fact, a second 'spare' prop might as well be made, both for back-up and for the many times that going fast isn't the big interest -- more moderately proportioned screw for leisurely cruising or fishing or a beautiful day on the water...

    Rick, I again offer my sincerest appreciations.

    Your participation is invaluable!

    David
     
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