DIY Small wooden pontoon Questions

Discussion in 'Boat Design' started by minno, Aug 27, 2014.

  1. minno
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    minno Junior Member

    Hi All :)

    I'm about to build a small rowing cat and while I've got some ideas I'm not sure if they're good Ideas so I thought I'd ask.

    the pontoons need to support ~ 350 pounds each which includes about a 60% safety factor.

    I'm planning on making them rectangular 6" wide by 12" high using 1x6 for the top and bottom and light plywood (6mm max) for the sides.

    the sterns will be flat, I'll add something later to fix the hydrodynamics, possibly a second cat :)

    I'm planning on making the bows either wedge shaped, or asymmetrical with the outside flat and all the curve on the inside, in either case the top and bottom will be the same length over all and the same width at any given point and without any rocker, here's an "artists conception" picture showcasing my (lack of) Paint skills :D

    There'll be two double bulkheads in each pontoon to which I'll mount the deck, they'll be double as I'll want the option of raising and lowering my deck by using a longer or shorter mounting piece that will fit between the bulkheads.

    Is it better to try to seal wooden pontoons to keep water out? or to engineer a way to get water out when it does get in? either a drain plug or a hatch.

    can anyone point me to some good information on building hatches out of wood?

    I'm sure I'll have more questions but my fingers are getting tired :)

    Minno
     

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  2. rwatson
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    rwatson Senior Member

    You may like to check out

    http://rogermann.org/puddlecat/designs/puddlecat-89/

    "Is it better to try to seal wooden pontoons to keep water out?"

    Hard to say for all designs. Generally access to the inside of the hull are there for inspection wherever possible, but some designs make this just too problematic.

    "There'll be two double bulkheads in each pontoon to which I'll mount the deck, they'll be double as I'll want the option of raising and lowering my deck by using a longer or shorter mounting piece that will fit between the bulkheads."

    I have no idea what you mean here.
     
  3. upchurchmr
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    upchurchmr Senior Member

    So you must need to float 280#. I assume that is passenger + boat with oars.

    350 x 2 = 700
    0.4 x 700= 280#

    Here is a boat I built for my wife. 125# + about 55# = 180#

    11 feet long x 6 inches wide (max) per hull, 5' total width. The hull in the water is semicircular. There is 4" of freeboard. These hulls are completely sealed, if I ever get water in them I'll cut hatches. No bulkheads. Strip planked like a kayak, glass/ epoxy on inside and out.

    You can see here that the hulls are double ended, no transom.

    [​IMG]

    One for myself required 240# + 100# (a badly built set of hulls).
    The only thing different was that the width of each hull is 12".
    This doesn't work very well at a total width of 5', I had to widen the crossbeams to 7' to make it row better.

    Your proposed hulls will have a lots more drag.

    If you need 280# capacity, it would be better to go to 6" wide hulls with 16' length. Lots of alternate choices in between
     
  4. messabout
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    messabout Senior Member

    Weight is the enemy, repeat, weight is the enemy. Do not use ordinary spec lumber for the top and bottom. Your drawing indicatse a sharp corner near the bow. Use curves, not angular plan forms. If the boat is to be rowed, the after end of the bottoms must rise toward the surface (rockered), not straight and flat as in a power boat. The alternative is to allow the aft end to remain straight but the sides must be curved in to a point. You will now have double ended pontoons. It will track very well with the latter option but it will be difficult to turn.
     
  5. upchurchmr
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    upchurchmr Senior Member

    Tracking depends upon how much of the bow and stern are in the water. If you curve the bottom up until the foot of the stem is at the waterline, you will have a hard time going straight.
    1.5" under the water will give easy tracking.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Minno, what's really being said is there are a lot of variables to consider and a lot of information to absorb and understand, to address these convoluted concessions in discontinuous compromise, that we call yacht design.
     
  7. minno
    Joined: Aug 2014
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    minno Junior Member

    Hi All :)

    I appreciate that boat design is a lot more complex than most folks realize, so I'm designing in ways to make my boat longer, wider/narrower, and higher/lower as I wise up to the need to do so.

    I'll be using the boat for spear fishing and crabbing as well as rowing so everything pretty much becomes secondary to stability as I'll be walking around the deck, pulling traps, getting in and out of my wetsuit and climbing in and out of the water. I plan on going with the asymmetrical pontoon design to keep as much flotation as far away from the centerline as possible for a bit more stability.

    the flat transoms are intended as mounting points, initially I'll have hull extensions similar to those on the pudlecat rwatson posted, eventually I may build a second similar cat designed to use separately just for rowing or mount stern to stern so I can turn this boat into a 24' cat rigged to sail.

    @rwatson - I'm not sure how far off the water I'll need to make my deck so the pontoon bulkheads will be double with a socket between them I can slide stakes into (much like on a pickup box) and then mount the deck onto the stakes, longer or shorter stakes will be used to raise and lower the deck, once I get it where I'm happy with it I'll epoxy the stakes into place, until then I'll use bolts.

    @ UpChurchMr, If my cat turns out half as nice as yours have I'll be more than pleased with it.
    280# covers me and my basic gear plus the boat and leaves a 70# reserve for extra gear and things I forgot to allow for, like oars, doh, some boat designer I am :)
    I could certainly go with wider pontoons, but longer and deeper aren't options at this point due to the folding deck design I'd like to use which also dictates the flat bottom.

    @mesabout: I'm not very good with MS paint, the curve at the bow will be rounded off, I'm much better with hand tools than with computers :)

    I feel that the top and bottom of the pontoons really need to be stronger than 4mm plywood as I'll be walking on the top frequently, and the bottom WILL hit rocks sooner or later, stronger requires either fiberglass or more wood, I've chosen more wood over glass which would save me a bit of weight at the cost of MUCH more work, time, and expense, I suppose 18mm of wood is excessive though, 3/8" (9mm) plywood seems like a better compromise at just under half the weight of 1x6.

    @ PAR : I'm guessing that yacht designers tend toward baldness from tearing their hair out :)

    Thanks for your help and advice everybody, I'll keep this thread going as a journal as I build

    stay safe

    minno
     
  8. rwatson
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    rwatson Senior Member

    "I feel that the top and bottom of the pontoons really need to be stronger than 4mm plywood"

    If you use epoxy and fibreglass, that will be plenty strong enough.

    Dont forget, the light the structure, the less impact when you bang against rocks etc.

    9mm plywood is waaaaay over the top. It will weigh a ton. Dont go more than 6mm on the bottom. Curving the plywood a bit will add a huge amount of strength too.

    "I've chosen more wood over glass"

    I dont understand that.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Minno, you're making several assumptions that will lead to a poor performing craft. Stability will come along naturally if the rest of the package addresses a reasonable SOR. Too much focus on stability (for example) will just lead to a pig you can't row well. Yes, we as well as most engineering professionals have a higher baldness per capita ratio.
     
  10. minno
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    minno Junior Member

    Hi All :)

    I did say in my OP that I wasn't sure if they were good Ideas ;)

    so whats a SOR when it's not at home?

    by making the tops and bottoms out of 4mm and fiberglassing I'd save about 7 1/2 pounds minus the weight of the fiberglass and epoxy, so at most 6 pounds, at a rather substantial cost in materials, time, and labor.

    with 6mm I'd save a bit under 6 1/2 pounds and a bit of money at the cost of a bit of piece of mind.

    the lack of rocker just isn't fixable as it's dictated by my deck design wich is dictated by the need to hand wheel the boat just over a mile round trip every time I use it.

    doh, wrong button, sorry, I'll finish this up later

    minno
     
  11. messabout
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    messabout Senior Member

    Minno, how does the deck design prohibit the use of rocker in the bottom?
     
  12. upchurchmr
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    upchurchmr Senior Member

    A Tornado catamaran was originally built out of 4mm Okoume ply with glass / epoxy inside and outside.
    The 12" wide deck required some deck beams for you to walk on the deck.
    This is a very high powered boat that worked very well, with small bulkheads under each cross beam.

    4mm ought to be fine - with glass.

    Asymmetric hulls will do nothing for increased stability, but will increase the drag.

    Do you have a sketch of the boat?
     
  13. minno
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    minno Junior Member

    the pontoons will swing under and lock together at the bottoms, pretty sturdy with flat bottoms, but with rockered or round bottoms I'd have to either build in some sort of cradle or add supports to the deck to keep it stiff when folded, either option means more weight, I hear that's bad :)

    as I understand it the only problem with no rocker is that it will be difficult to turn.

    This page seems to say that no rocker is better for what I want to use this boat for, large open flat water.

    minno
     
  14. upchurchmr
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    upchurchmr Senior Member

    You could easily make the hulls flat in the center, and rocker'd on the ends.
    That will allow you to turn better and still do whatever you are planning.

    Can't say I understand the swing under process. Why?

    Sketch?

    There really is not much of a point in commenting anymore when I don't know what we are discussing.

    Good luck
     

  15. rwatson
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    rwatson Senior Member

    Do you see a correlation there ?
    For things that need to be manhandled a lot, every kilo is significant.



    SOR means "Statement of requirements"
    If you write down everything you need the boat to do, it saves people getting confused when half way into the conversation they hear "need to hand wheel the boat just over a mile round trip"


    The Tornado was a great example of lightweight ply made to do extraordinary work. The ones I repaired weren't glassed inside.
     
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