design for three part nesting pram?

Discussion in 'Boat Design' started by Tony Eaton, Jan 1, 2007.

  1. Tony Eaton
    Joined: Jan 2007
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    Tony Eaton Junior Member

    I plan to build a 10' stitch and glue boat out of 1/4 plywood. At about 100 pounds weight and 4x4x2.8 nested dimensions, this boat will nest into 3 pieces so that I can easily cartop the boat, even on a small sedan. Outside of a design from the 1950's, I could not find a 3 piece nesting dinghy design so I decided to come up with my own. It will have a 4' max beam centered in the middle of the boat, with a a 3.5' wide stern, a 2.5 wide bow and a flat bottom. I plan to put about 1' of rocker towards the bow and 9" of rocker towards the stern. The center section will hold the aft and then the stern sections when nested.

    My planned use will be on a fairly large lake with a trolling motor. Most of the time I will stick to protected waters but I want to be be able to withstand some 2 or 3' chops or wake in a pinch. There will be well over 1.5' of freeboard at the max beam. The front section will be filled with foam, with over 4 cubic feet of floatation. 1 inch plywood for the transoms and for the bulkheads. The sections will be attached with 5/16+ carriage bolts, 4 for each joined bulkhead, and well as a seat that will lock the bulkheads in place.

    I am curious what everyone's opinion is.

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  2. Eric Sponberg
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    Eric Sponberg Senior Member

  3. messabout
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    messabout Senior Member

    Tony;
    You have an excess of rocker aft. if the boat is immersed so that the WL is an inch or so below the transom bottom the displacement might be in the region of 700--800 pounds. With the boat weight, the skipper, a trolling motor and battery you'll displace less than 400 pounds (unless you are a really big fellow) at 400 pounds the waterline will be very short and the boat will probably bob around like a cork. These are eyball estimates but I think that they're somewhere near.

    There are two things you can do to improve the design. One: flatten the middle section of the bottom some. If you want 9" of rocker aft, save it for the aft section. Two; go for a sure thing....send Eric 20 bucks and get a nesting pram that has the benefit of his expertise and some prior builds.
     
  4. safewalrus
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    safewalrus Ancient Marriner

    Tony

    Welcome to the forum, Yeah! my first thought was much the same as 'messabout' to much rocker flatten it off a bit mate! should make it easier to build too (got to get something out of this!)

    At that size why three bits? two would be sufficent surely? :confused: and the less bits the less to break up if anything goes wrong. The last thing you want is the different parts of the boat to go walkabout whilst your in in 'er eh!:p
     
  5. messabout
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    messabout Senior Member

    Tony:
    My eyball displacement guesses were light. The boat as drawn will displace a tad over 400 pounds at 4.75' of draft. It will have a prismatic of about 0.63. At 8.75" of draft it will displace more than 1000 pounds. I'd say that is overkill. You can make the boat more boaty and probably better handling by reducing forward rocker to 10 inches and aft rocker to 7 inches, pinch the bow in to about 12" bottom beam and the transom end to about 24" bottom beam. In that configuration it will have 350 foot pounds of righting moment when heeled to 15 degrees. That is when the total weight is 400 pounds. Thats adequate RM for a fishermans boat. The narrower transom will still allow a normal sized person to get near the motor without serious danger of swamping. You can build this boat to weigh 80 pounds or less if you choose the right materials. The best advice is still to buy the plans from Eric and follow them faithfully. Of course that will rob you of the bragging rights of having designed your own boat. You get around the bragging rights thing by getting an established design and then lie about the design work.
     
  6. safewalrus
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    safewalrus Ancient Marriner

    Tony

    Tell you something else mate, the thicknessess of some of the wood is a little bit over engineered don't you think? with the transoms between the seperate hulls built of one inch ply, that gives a thickness of two inches at each join - phew! For a boat of that size the only place you'd need that sort of thickness on the transom is just under the engine clamps - for a start it makes the bloody thing a little heavy, your going to want to pick this up on your own aren't you? arr a contender for the world's strongest man eh?
     
  7. Tony Eaton
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    Tony Eaton Junior Member

    I appreciate everyone's advice. Sometimes it might be a bit dangerous sitting down on the computer with a little inspiration, a CAD program, and a few beers on New Year's day. Since I have a few smaller boats I want something
    big enough to take out my whole family in sheltered water (4 people at 450 pounds weight) and 100 pounds of motor / battery and still cartop.

    Eric, I like your design. I am going to give it some thought. I am guessing it would be a great 1 person or 2 person boat, and in sheltered water under oar power perhaps the whole 2 kids / 2 adults family. I am sure a trolling motor would push it along quite well. I do want to get a sail boat one day, this would be a good start.

    Messabout, thanks for the advice on the rocker. I tried FreeShip and saw that this boat would be high in the water although the practical meaning of that was a little lost on me. I want to provide some floatation in the mid hull section around the trolling battery too. A more tapered hull as you suggest will allow that and still allow the sections to nest.

    Safewalrus, you sound right regarding the thickness of the bulkheads. When I realize more than a sheet of 1 inch plywood will go into the bulkheads, that is a lot of weight! Perhaps .5 inches is more in order and I can glass a bit to firm it up. Why three pieces? Since I am not so strong I do not want any one section to weigh more than 40 pounds and I would like the entire boat to fit nicely on a cartop for highway driving. Too, I already bought 8 sheets of 4x4 plywood for a steal and it is sitting in my garage at the moment. If in practice this turns out to be folly I'll glue and glass one or both boat ends together, buy a trailer, and that will be that.
     
  8. messabout
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    messabout Senior Member

    Tony:
    It's me again. I do not mean to annoy you with too much critique. However, you are going to build a boat, it will involve expense and a bunch of work, not to mention potential conflicts about the time and passion lavished on the project instead of ones lovely spouse and children. With all that, we'd like to have your project turn out well enough to meet your desires and expectations.

    Among my own several faults, I am a Kayaker. I have marvelous 35 pound ones. I have nice Yakima roof racks that works as advertised. The roof rack thing is a pain in the ***. I can get my kayaks on and off my small trailer easier and faster than I can the roof racks. There are a few cases where the racks are best, like when the "put in" location is in the deep woods and no suitable beach area is available or parking the car with a trailer will be a problem. Otherwise the trailer beats the racks for practicality. In your case, having to assemble and disassemble the boat will add to the hassle, and diminish your enthusiasm for useing the boat.

    If at times you will have 2 adults and two children in the boat, the boat needs to be bigger. If the boat is crowded it is less comfortable and thus less fun to use, and most of all, there is the safety factor.

    I built a 15'-6" flat bottomed skiff complete with a small sprit boom sail rig. The boat's all up weight is 132 pounds. It is not at all flimsy. It rows decently, it goes very nicely with an old 24 pound thrust Min-Kota. It sails smartly. I can put the boat in the water and be 100 yards from shore in less than 5 minutes. I mention all this to suggest that a bigger boat that is to be trailered is both practical, fun, and will not take any more time to build than the three section pram. Four sheets of plywood is all the material that was required. The flattie skiff, with its light weight, is a piece of cake to get on and off the trailer single handed. The trailer does not even have a winch.

    As for the excess rocker.....Look at the boat in the water, in side view. Both ends are up too far. The boat will behave in a manner similar to a rocking chair. Small movements of the occupants,fore or aft, will cause a large change in the fore/aft attitude of the boat, much as a rocking chair or hobby horse might behave. Rocker, in a displacement boat, is desireable for decent performance but too much of it will cause discomfort.
     

  9. safewalrus
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    safewalrus Ancient Marriner

    Messabout

    You got it in one, Tony listen to the guy, it's good advice!!
     
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