Most efficient hull design

Discussion in 'Propulsion' started by Sundevil, Sep 8, 2013.

  1. Sundevil
    Joined: Jan 2012
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    Sundevil Junior Member


    If you were designing a boat with a 5kW electric motor, what shape would you pick to get the best speed? Would you use foils or any other futuristic designs?

    I wouldn't need to have access to them or to have anything in them except maybe some water tanks?

    I would also like it to be stable while at anchor... :p

    Assume a 3000-3500lbs boat that would need to fit in the Erie Canal. :)
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    You mean there is a question as to whether a 3500 lb boat would fit the Erie Canal :confused: How long is your extension lead to power the motor ? ;)
  3. Sundevil
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    Sundevil Junior Member

  4. micah719
    Joined: Jul 2012
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    micah719 Plotting Dreamer

    I've been kicking around an idea for a cruising sailboat capable of navigating the UK canals. Not an easy task, since the maximum beam limit is 6'10", and in totally the wrong place: near the waterline. Also, add in maximum airdraft of 6' and a draft of 2'.

    A sharpie hull was the only possibility to get useful room since sideways and vertical were limited. I'd have had the prop in a half tunnel, an e-motor and the ballast consisting of batteries, a diesel genset and the various tanks. Composting head. Still nutting out how to mount the masts, since I want a junk rig; can't see any way to make tabernacles work. Perhaps a detachable hinged a-frame to lift the sticks out of their wells, with guy-ropes and tackles led to a winch.

    Summed up....narrow and shallow were the benefits of the hull shape, and would be relatively fast compared to other boats of the same length (still vacillating between various configurations in the 26' to 36' range). Standing headroom only through hatches with awnings, which isn't so bad since when it gets rough you wouldn't be standing about anyway. Think of a supersized Paradox. Because of the dimensions, it wasn't possible to make the chine-runner idea work because the overall length and the aforementioned beam/draft constraints messed up the proportions between beam/draft needed to provide the necessary hull foil shape when heeled, so a pair of canted lifting bilge keels (although I also considered external bilgeboards).
  5. DCockey
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    DCockey Senior Member

    As long and as narrow as possible. No foils, just smooth and fair.

    Now you're complicating the question. In reality boats designed around maximizing a single characteristic are rarely good boats.

    Required stability will be one of the determinates of minimum beam. And then there are accommodation needs, etc. And there may be other constraints on size such as trailering limitations or marina berth expense.
  6. DCockey
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    DCockey Senior Member

    Given the very different constraints of the Erie Canal and the British waterways a separate thread might be appropriate for micah719's project.

    The Erie Canal has very different size limitations than British waterways. Length and beam are not limiting factors for most recreational boats on the Eire Canal.

    All Canal System lock dimensions are 328 feet long and 45 feet wide. The area available for vessels within a lock is 300 feet long, 43.5 feet wide From

    Also most British waterways have a speed limit of 4 mph over ground. Speed limits vary on the Erie Canal from 5 mph to 45 mph with some sections not having speed limits.
  7. PAR
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    PAR Yacht Designer/Builder

    A 7.5 HP electric motor and you're thinking about "best speed" on a 35' LWL craft - really? Anything over 10 MPH would be generous, but in all honesty it's available power verses weight, with drag penalties reduced to practical minimums. Your dimensions for a cat hull suggests the hulls will be each others wakes, worse for a tri and a 1.5 - 1.75 ton 50' vessel, would be a spectacular engineering undertaking, likely of considerable cost in design and forced material choices. Let me guess, you're on a budget too.

    I'm not trying to make fun of you, but you do realize the extreme nature of your SOR, makes taking this seriously difficult?

    The logical route would be to establish a practical SOR, within the budget you have, unless this is a theoretical exercise. If this is the case, then some reasonable restraints need to be employed, as 1.75 ton 50' yachts just aren't, even if you take the human ergonomics out of the loop (assumed).

    Lets look at reality - a double ended box section form, with a 24" beam on a 40' length will have about 1,500 pounds (again assumed 2x in a cat) of displacement with 5 1/4" of immersion. Seems simple right - now try to make this so it doesn't fold in half, bridging it's first encounter with a wave train. Making the freeboard 24" to keep the math simple, that's 5 sheets of plywood for the sides and about 6 for the bottom and deck. Assuming epoxy, tape and fillers don't weigh anything and using a scant 1/2" sheet, just the planking is 550 - 600 pounds per hull. No stringers, no sheer clamp, no beams, no hull connections, no batteries, no engine, but just the planking is 1,100 to 1,200 pounds. This is just 1/2" plywood, which in this shape would need considerable longitudinal reinforcement, to say the least. Obviously, we'll need to use carbon, spectra and Kevlar throughout, particularly given the 7.5 propulsion envisioned. Good luck with this . . .
  8. tomas
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    tomas Senior Member

    That is a SWATH configuration, i.e., the hulls are submerged, which will feature more drag for your electric motor than hulls that float on the surface. The resistance will be even higher if you add the third hull that you mentioned.

  9. philSweet
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    philSweet Senior Member

    7.5 hp, 3000lb, and best speed don't belong in the same requirement. Clearly speed is the least concern out of all the craft's requirements. Got a 5MW unit? Or can you get the hull weight down to 70 pounds? For a 3500 pound boat - a 30' waterline and sub-6' beam would be about right for mill pond conditions. 25' wl and 7' beam might actually work as a boat if the freeboard was kept at a minimum. Open boat, no cuddy or house. The powering is at the high end of the canoe and sharpie range. Both types operate at Speed Ratio = 1 or so. So you may get over the second wave hump and up to speeds of SR =1.2 or so with the right hull.

    Something like this, I imagine.
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