Ultralight Planing dinghy/tender

Discussion in 'Boat Design' started by deepsix, Feb 1, 2008.

  1. deepsix
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    deepsix Senior Member

    I am looking for a small ultralight planing dinghy roughly along the lines of Jacque Mertens GV13


    That design is for a fiberglass sandwich with plywood core, and a hull weight of 48kg is indicated on the website. My question is could significant weight reductions be expected if a carbon/kevlar foam sandwich construction was used? I am thinking along the lines of vac bagging over a male mold or using prefabricated composite panels. I have read that plywood is particuary efficient in small boats, so it would be silly to go to the expense and effort of building a carbon boat and end up with something not much lighter than ply.

    The reason I ask is because I am planning on building a pilothouse cruiser, and I might be required to load the dinghy on the cabin roof. I am concerned about the implications of additional weight aloft and also the ease of loading a 50kg hull.

    Any information is appreciated. Thanks
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    BOATMIK Deeply flawed human being

    Back in the days when kevlar/foam was a new thing a bunch of people in Queensland were building some ocean going multihulls.

    One of the builders decided to put together a 16ft rowing dory as a tender out of foam/kevlar.

    The sixteen foot dory came out under 20 lbs.

    It seemed a good excuse for everyone to stop their work, set up a BBQ down on the bay and have a boatlaunching for the dory - the first boat built using the new technology.

    Dropped it in the water in front of the crowd and tried to get in. It went over so fast that the movement was a blur.

    So the afternoon became one of "wild bronco riding" trying to get into the boat in knee depth water.

    The builder being a bit disappointed at the end of the day went to the hardware store on the way home and bought a bag of premix concrete and added another 50lbs in the cockpit then glassed over it.

    Result - a boat that you could at least get into, but was still a little flighty.

    You can build a Bolger Nymph - 7ft 9" - row sail motor, carry three adults at a pinch out of two sheets of plywood and it will come in round 45lbs built of gaboon ply.

    If you move to a power tender as you show it will be much heavier and bulkier and won't row and you'll have to work out some way of dropping a heavier outboard onto it - and so on.

    It seems to me the boat is defined by the size of the outboard - if the outboard that is needed to drive that planing hull increases the boat weight to 90kg - you have to scale up to that level.

    IT would seem much smarter to accept a lower speed boat (maybe longer than the nymph which is pretty minimal) with the appropriate displacement hullform and have a 2 or 4hp outboard rather than the 10 to 15 to 25 of the boat that you show.

    It is never easy to work out what will work best for you - but think it through a bit and you'll work out a good way to proceed.

    Best wishes.
    Michael Storer
  3. rwatson
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    rwatson Senior Member

    Micheal - I find it hard to understand how one could blame weight as the cause of the instability of the example dory.
    If it had been a catamaran or flat bottomed - it wouldnt have mattered.
    This was obviously a very sharp V shaped hull if ballast was required.
    The design of the hull was the problem. (I know it wouldnt be one of your designs Micheal :) )
    Its like you can never be too rich - a boat can never be too light (yeah yeah - and strong enough, I know)

    And Deepsix - prefabricated composite panels sound the absolute lightest to me ( a la Derek Kelsall techniques) - they would at least be a lot less work than vacuum bagging on such a small boat.
  4. sal's Dad
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    sal's Dad Atkin/Bolger fan

    I'm no expert in getting dinghies up on a pilothouse roof (but why should that stop me from offering an opinion?;) )

    Seems to me the hull weight (within reason) is not going to be as big a problem as the outboard, gear, and general unweildiness. To get it way up high, in most weather conditions, you'll want some kind of mechanical assist, so saving 10 or 20 kg won't make that big a difference.

    Why not build your tender quick and dirty, with very light ply (6mm, taped seams, no sheathing, house paint)? If that works out, and seems the perfect design, THEN evaluate how much weight matters, and whether an ultra-light exotic version is justified. In the meantime, you can be out fooling around on the water in a couple weeks.

    Sal's Dad
  5. Pericles
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    Pericles Senior Member

  6. tom28571
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    tom28571 Senior Member

    Mertens gives the bare hull weight as 170 lbs which is a bit over 77 kg. This makes much more sense than the 48 kg he also gives, so I think he means 78 kg. 170 lbs is still light for a 13' planing dinghy. The main problem is that planing puts a lot more stress on a boat than lower speed. Getting a motor capable of planing this boat onto the transom while in the water would be a hassle. Lifting the whole thing with the motor mounted would be much worse. It demands a lifting hoist of some kind and your cruiser would have to be fairly big and stout to accommodate all this anyway.

    To build a 13' planing powerboat to 48 kg would more than likely require exotic construction. Carbon fiber sandwich.

    Such boats can be built of wood without exotics that will plane with a small and light motor but they would need to be long and slim, like Robb White's Sportboat, for instance. Not what you might need for a load carrying tender in a rough harbor.
  7. deepsix
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    deepsix Senior Member

    Thank you everybody for the comments and advice. I will have a look at a smaller displacement tender, it makes more sense now. You may have saved me alot of disappointment.
  8. tom28571
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    tom28571 Senior Member

    What make and what size is your cruiser?

  9. deepsix
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    deepsix Senior Member

    At the moment I dont own a cruiser, but I am seriously considering building later this year(assuming I dont change jobs). At the moment I favour your BJ25.5, im very impressed, it looks like a smart piece of design.
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