Pedal boat rebuild?

Discussion in 'Boat Design' started by Tiny Turnip, Apr 12, 2015.

  1. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    I have been running my pedal boat, based on a Dart 18 catamaran, for 6 or 7 years now, and have been very pleased with it.


    There are two things that bug me, and I'm toying with the idea of a new build to address them.

    If I do tackle it, it will have to be as simple as possible, as my available time is very limited.

    I would like the boat to be

    a) lighter - manoeuvering it out of the water is a pig. the Dart 18 hulls weigh around 40kg each, plus the weight of the Dart beams, my 2x6 longitudinal beams, the drives and bracketry, and the 15mm ply deck.

    b) easier to assemble. relying on nuts and bolts, even stainless / galv in a salty and sandy environment is less than ideal, and not conducive to a quick assembly/breakdown.

    I would like to keep the overall dimensions pretty much the same - it is a good compromise for speed, (4knots cruising, 5-6 knots flat out) function (day pottering, swimming, picnics, fishing, creels) and conditions. (beach, inshore, sheltered, lakes, lochs)

    I am thinking about rebuilding with a flat bottomed single chine hull, made from either
    4mm ply, fibreglass taped joints, or xps panels/slabs, fibreglass skinned, (single layer of cloth) with ply strengthening where appropriate, similar to surfboard construction.
    the decks could be say 3 stressed skin panels perhaps 4 inches thick.

    So, questions.

    1) just sticking a finger in the air, (I know, I will do the sums!) what sort of weight savings might I achieve by either method?

    2) do the suggested constructions sound robust enough overall? (a beach cat is probably a good model for how it will be handled on land, stresses will be very much lower on water, I think.)

    3) I'm concerned about point loads on the top skin of the deck panels. how much (if at all) do you think they should be beefed up? I'm aware that there will have to be a fair amount of beefing up to make the little cantilever beams that the pedal drives attach to.

    4) I would be looking for a really quick and robust way to lock the deck panels to the hulls, that is simple to build, and obviously is up to the job structurally. I am imagining the deck would run across to the outer edges of the hulls. I'd appreciate any thoughts for this - I've gone round in circles with ideas for locating bosses and holes, ties, wedges, lashings...

    5) at these relatively low speeds, and the dart hulls are only a foot wide, will I be compromising the cruising speed at all? Would I gain much by going to a double chine, bearing in mind that speed/ease of construction is most important.

    6) can anyone explain how to resize the image to get the post to format sensibly?

    Many thanks for any thoughts.

    Last edited: Apr 12, 2015
  2. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Here's a (very) notional sketch:

    Attached Files:

  3. Trent hink
    Joined: Sep 2013
    Posts: 26
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    Location: Sarasota fl

    Trent hink Junior Member

    Hi, I think it's a cool looking boat. I'm surprised no one has answered your post yet, perhaps it's because of the annoyingly large picture you posted.

    I don't have and real design knowledge so I probably can't help much but I have a few ideas you might consider:

    Your catamaran hulls and cross beams are way overbuilt, especially if you are using the boat in protected waters. A beach cat has tremendous loads from the rig.

    Do you need a solid platform between the hulls? A lightweight frame with a trampoline or netting will probably be much lighter than any platform. I built a frame for my sailing canoe and wove my own trampoline using old spectra kitesurf line and a book on how to make cast nets. It saved about 30 lbs compared to the crude wood platform I was using previously.

    You might consider a design where the crossbeams are lashed to the hulls, it's a valid method and allows lighter design because the lashings allow a lslight bit of movement.

    The strongest part needs to be the connection between the seats and the pedal drive, that's where all the forces are. Everything else can be very lightly built.

    If you go with plywood hulls you could get pretty lightweight, very sleek hulls using tortured plywood construction. I built my 16' sailing outrigger canoe using 2.7mm plywood covered with 4oz fiberglass.

    If you want to consider that type of construction check out "gougeon brothers on boat construction" which has a whole chapter on tortured plywood construction design and techniques. You can download the whole book here for free:

    As for what weight is possible, Sea Cycle is a similar design. They claim a weight of 175 pounds with the heaviest component (I'm guessing the hulls) at 41 pounds. That boat is made of roto-moulded plastic which is heavy so if you are willing to put some effort into better design and materials you should be able to get the weight a bit lighter.

    If you coplywood that design exactly and just use plywood/glass hulls you should be able to get similar strength with a lighter weight.

    Good luck. Maybe my ideas help, otherwise sorry for wasting your time!
  4. Tiny Turnip
    Joined: Mar 2008
    Posts: 819
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Hi Trent

    Thanks for your response and your supportive words.

    Yes, the picture size is annoying, isn't it? I've tried hacking about in the forum code, looking for clues to resizing on line, but to no avail, so if anyone has an idea for my question 6, please let me know.

    I am painfully aware of my pathological need to over engineer stuff! some back of envelope calcs show I should easily be able to get to considerably less than half the weight of the dart hulls building in 4mm ply, allowing weight for strengthening at deck joints etc.

    I did consider using the original dart tramp, but I was working with the beams as a quick an easy way to get rigidity between the seacycle drives, the seat and the dart cross beams, and I didn't like the prospect of the beams and seats interfering with the tramp. Plus the solid deck really is a boon for activities and even a tent...

    I'll have a think/experiment about with 4mm ply for hulls - maybe 6mm for the deck, with ply beams on edge under - though I'll be limited to how thin I can go if I want a screwed and glued connection - maybe just epoxied?

    I'll consider some little ply box beams to interface with the seacycle yokes, which could integrate nicely with a the deck structure...

    lashing is definitely in the frame, but my Solway Dory sailing canoe/trimaran uses lashings, and the assembly time is a bit of a drag... (they have a lovely slick new way now, with aluminium tubes for the outrigger beams and keyways to lock it all in place, which is *really* sweet!)

    I'm favouring getting a big chunky peg and eye of some sort where the inside gunwhale of the hulls meets the deck, and then some holes and toggles on the outer gunwhale, where its reachable. Something like that.

    Many thanks for your help and time in replying


  5. WindRaf
    Joined: Oct 2014
    Posts: 343
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    Location: Italy

    WindRaf Senior Member

    'as simple as possible'

    so why a new one in plywood?
    Why do not cut from the Dart that whitch is useless and rebuild it?

    the same that make a big repair

    is only an idea
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