small boat design, Plat. (any help appreciated)

Discussion in 'Boat Design' started by bigjimmy, May 24, 2009.

  1. bigjimmy
    Joined: May 2009
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    Location: NZ

    bigjimmy New Member

    HI Lads,

    the brief,

    i need to build a 'Plat'. which is a fish carrying boat that can be towed through the water while swimming. the idea is that it is towed by rope from the bow.
    typical dimensions are 1100mm long, 550mm wide and about 300mm deep.

    most designs are coffin shaped, tow poorly, are affected by wind, and flip in a swell.

    what i am wondering is if i was to take a typical skiff style open row boat. down size it to the above dimensions, the close the top in. add a hatch in the top to aid access. do you think this will work?

    it needs to be stable when empty (not flip) and hold a max weight of about 30kg.

    i don't understand boat design principles, so what happens to weight, water dispersion for stability as you downsize an existing design?

    anyone have any lateral thoughts as to how to make this work? or alternatives?
  2. peterAustralia
    Joined: Mar 2006
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    Location: Melbourne Australia

    peterAustralia Senior Member

    well this is a weird one for sure

    I get the idea, you are going spearfishing, you want to put the fish on a little craft that gets towed, as opposed to tethering the fish to your person, so sharks do not see you as breakfast.

    Let me think, maybe have a pointy end to go through small waves. For stability, maybe flare would work. Your design seems a bit boxy at 2:1 length /width ratio

    maybe something like this but scaled down say 1.6m long and 50cm wide

    take this photo, open it in ms paint and stretch the horizontal axis to about 60 percent or so, to make it fatter. I just did it on my computer and looks good. If you need them I have offsets.

    and here is the boxy looking version, just a topview...........hey I tried.,,...the quality of the replies is probably related to the amount of money spent in getting them... zero means your not getting rocket scientists
  3. thudpucker
    Joined: Jul 2007
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    Location: Al.

    thudpucker Senior Member

    With a fish net restrainer for the cargo so when it flips, you dont lose all your fish.
    LOL, but you might be wary about trying to right the bugger and get caught in the feeding frenzy...
  4. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Let us deal in generalities. The stability of the boat, or the ability to stay upright is a function of the width and is influenced by sea state. If you place a small boat in breaking waves things will be dicey at best. I would not want to make a pointed boat for fear of being accidentally (or inevitably) speared by it.

    Your dimensions will yield a boat that might carry about 18 kg. A short boat like that will be more difficult to tow than a longer and narrower one. To design your own boat you can use simple math to determine the displacement or carrying capacity of the boat. One liter of displacement will support roughly one kg. of load. There is a slight difference in fresh water versus salt water. You can ignore that difference for this project. So you need a boat that has an interior volume that will hold 30 liters of water. Actually it needs to hold much more than that because you do not want the boat to be awash at the deck. Peters boat, the one that resembles a dory will be quite stable in fairly rough waters only if it is laden relatively heavily. A boat of that type has exceptional stability when the load is kept near or on the bottom of the interior. I would think that a boat to be towed by human power will need some rocker (fore and aft curvature)_ in the bottom. The reason being that the boat will more readily change directions when the ends are not deeply immersed. If the boat is box like, as in a barge, pronounced rocker will make it a bit easier to tow.

    I have no experience at spear fishing so I can not imagine every detail that is important to this pursuit. My first thought would be to buy, borrow, or steal a kayak. The kayak will be easy enough to tow, probably as easy as the fish holder thingy. The kayak will support a lot of fish and, furthermore, give you the ability to get the hell out of the water when the sharks or crockodiles arrive. The kayak would also give you a far wider range of exploration than you could hope for while just swimming. There are many kayaks whose length is a manageable 3 meters, they are plastic, and cheap.
  5. bigjimmy
    Joined: May 2009
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    Location: NZ

    bigjimmy New Member

    thank you for your generous replies.

    "messabout" i take what your saying about the kayak as a good idea. but it isn't always practical.

    scenario, you arrive at your destination by boat. dive over the side and swim off in search of your catch. sometimes you swim for an hour away from the boat along a coastline. if your doing well you might end up with half a dozen fish. the 'Plat' (fish carrying boat) then aids you in transporting your catch on the long swim back to the boat by reducing drag, keeping them away from sharks etc......

    so, fore and aft rocker. how much are we talking to make this useful? i understand windsurf board design. would you recommend a similar rocker combined with a shallow v?

  6. bigjimmy
    Joined: May 2009
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    Location: NZ

    bigjimmy New Member

    to show you an example of what people are using. here is a coffin/ barge type design that has been quite effective here.

    if this had fore/aft rocker do you think that would help?
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