Help designing a knockabout 3.4m cat dinghy with FREESHIP

Discussion in 'Boat Design' started by catdinghy, Sep 11, 2013.

  1. catdinghy
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    catdinghy Junior Member

    Hi, first post here so be gentle;)
    Wanting to build a new tender for my boat and I do have boat-building experience.
    Before I have had a flat bottomed ply punt (8x4) which worked well over short distances with a 2hp outboard and found it very stable, but to small and pounds.

    I had a 12 ft mono tinny for a try and was unimpressed with lack of stability and have decided that at a catamaran dinghy would be closer to what I need.

    I have looked at the ripple range http://www.spiriteddesigns.com.au/index.php?pageId=71517 and while nice, they do have some shortcomings for me, I never treat my dinghy's well, they get used and abused so after cheap build and easy repair

    So, I am looking at doing a quick and easy single chine 6mm gabboon ply one.
    around 600mm sides,400mm beam, flatish run aft turning into a v at the front and 1600mm wide overall.
    Probably strap a 15 to 20 hp on the back is the plan.

    Using FREESHIP for the first time to save time developing patterns or so I thought and the problem I have is I keep getting these supposedly undevelopable sections shown in red.

    I doesn't matter how much I shuffle things around I seem to still have some red and this is the least red shape.

    Is it anything to worry about? The curvature to me doesnt look that bad, but I'd hate to loft and cut and find I cant bend it around or have I totally misinterpreted what the red means?

    Any help given gratefully recieved
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I doubt cats are the way to go in such a small vessel. With an effective waterline beam of 800mm, it will be very sensitive to fore-and-aft weight shifts, and one-up the nose will point to the sky with just you on the tiller and the outboard back there. I'd go for a beamy alloy v-dinghy with full bows, or a vee-bottom punt.
     
  3. catdinghy
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    catdinghy Junior Member

    Actually the 400mm was incorrect, should have been 500 at the WL
    Webster twinfisher do an incredibly stable 4.3 metre cat with similar dimensions on the waterline and that's alloy.
    The craig schionning cat linked to before is narrower again on the waterline
    There is a video here of a narrow overall beam 3.6 that seems pretty stable
    http://www.youtube.com/watch?v=2FS1eYxuM6g
    Livingstone in the states do a range of fiberglass cat dinghies 10 ft up and they also seem fine
    http://www.youtube.com/watch?v=cMmIssvpGbw
    beamy alloy v-dinghy with full bows, yeah thats a monumental fail, I have one that I just sold, felt very tippy to me and I tested it for a year and never really felt safe.
    That was my second choice

    But back to the picture above?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, 4.3 Vs 3.4m is quite a difference, in fact volumetrically it is twice as big, if scaled up uniformly. And that boat has forward steering, I assume you are tiller control. Buy yourself an old Clark Abalone 4.2 if it will fit, weighs about 150 kg hull only and an extremely seaworthy conveyance.
     
  5. catdinghy
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    catdinghy Junior Member

    If it would fit dont you think I may do something like that?
    Its a tender for a cat, 3.4 is as big as it can get.
    And there is no way in hell I am shelling out $3000+ for a bit of tin when $300 worth of ply will do in this instance.
    The whole idea is if I am in the middle of nowhere I can pull out some patterns and build in a day or 2 if someone steals it.

    None of the boats in the video has fwd steering and the twinfisher comes in tiller steer as a 4m version
    [​IMG]
     
  6. HJS
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    HJS Member

    My way for a 4,0 m cat for a customer

    js
     

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  7. Mr Efficiency
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    Mr Efficiency Senior Member

    I think you really need to specify what kind of duty you require of this tender, 3.4 metres is quite small, far too small for open waters, I don't think a cat would be anything more than a gimmick, a beamy vee-nose punt would be the best choice. Incidentally that 4.3 Twinfisher weighs about 500kg, and is not in the same category as a lightweight tender.
     
  8. lewisboats
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    lewisboats Obsessed Member

    The red isn't bad at all. The red line is the chine... you will get that with every model that has seams like that. The bow chin or forefoot isn't too bad and should be attainable. Freeship originally was designed for ships... which are made from sheet steel. It's algorithms don't take into account the tortureability of sheet plywood. I have done that kind of shape without any problems.

    [​IMG]

    [​IMG]

    [​IMG]
     
  9. catdinghy
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    catdinghy Junior Member

    I thought I made it pretty clear it is a tender for my larger cat
    Many would disagree
    Care to explain why?
    Never said it was but there is still no reason why a cat cant be built from lighter material.
     
  10. catdinghy
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    catdinghy Junior Member

    Thanks Steve, just the sort of feedback I was after, I know what the red lines were it was the red shading I had concerns about.
    Thanks again

    Looking at your dinghy I am really just building a half beam version x 2, with higher sides, cool.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    A cat is structurally more difficult, requires more material, is difficult to move around in unless you have a level floor (raising the COG unwisely in so doing, in such a tiny thing). An all-round dud idea in my opinion, then you have to worry about the single-engine cat bug-bear of prop ventilation. I love power cats, but under 5 metres it starts to fall apart, and I go back to the inherent sensitivity to fore-and-aft weight placement, particularly troublesome when a couple of people are much heavier than the boat.
     
  12. philSweet
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    philSweet Senior Member

    I built a 9.5 footer out out three sheets and some glass and scraps. Weighed 84 pounds with paint. I paid a little extra to keep the weight down to the pick-it-up-and-walk-off-with-it range. But I had US$800 into the materials. The good news is it has survived tender duty to a live aboard and a cruiser and I can handle a 60# anchor on 100' of 3/8 chain with it - just barely. And it tows ever so nicely. Set the motor forward on a recessed transom to gain balance. You may still need a short extension, but one that is just long enough to let you stand and steer should be the goal.
     
  13. catdinghy
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    catdinghy Junior Member

    Cat dinghy or normal dinghy?
    If a cat dinghy have you any pics?
     
  14. catdinghy
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    catdinghy Junior Member

    OK, as a V bottom punt was my second choice after a night on freeship I cant for the life of me figure out yet how to do a v nose bow.

    Any tips from users?

    ^^^ Belay that, new day and its making sense.

    [​IMG][​IMG]
     

  15. philSweet
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    philSweet Senior Member

    I was in a hurry this morning and didn't answer you question about developability.
    You need to define each developable panel as a separate layer in Freeship and then run the plate development on the whole thing. It will give you some stats on the accuracy of it's approximation. For a dink, no, I don't think there is anything to worry about for a stitch and glue build. But you can plan your joint so that one edge is a left a bit wild, sat 1/4 inch, and the other butts to it. This can be helpful during the build and assuming the joints are bigger than about 25 degrees from perpendicular. If you want to learn the mechanical way of doing this which will also give you the amount of bow in any frames, the Rabl paper has been posted here many times. I'd recommend sandwiching a 1" sheet of foam with a 4mm ply top onto the bottom insides leaving a scupper all the way around and a bailing sump at the rear. Dynel makes a nice but thirsty covering for the inside bottoms.

    I found a cartoon from a few years back. A 10'8" version, but this one hasn't been built.
     

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