Custom 12' in need of some help

Discussion in 'Boat Design' started by mystic957, Oct 22, 2013.

  1. mystic957
    Joined: Oct 2013
    Posts: 1
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    Location: United States

    mystic957 New Member

    So I decided to design and build my own boat, 12' long about 5' beam, straight sides and flat bottom.

    Was the plan. Never mind the big box in the center that is for an experiment i want to run.

    Where we are so far, I turned this [​IMG]

    in to this

    Ok, fair start.

    But I have some questions on laying the lower hull. Almost every photo or drawing i see shows angled planks for the bottom, where one side connects to the other the ends are just butted against each other.


    I took issue with that as it creates a weak point down the entire boat. I decided to try taking the tongue and groove to the end of the board in kind of a parkay design. This has shown its self to be quite strong and yet its still a seam and a joint.


    So my first question would be why do i need or want to use angled boards vs just running them straight across?

    Second as part of this project im trying to use nontraditional and cheep materials. How would we feel about using some kind of cheep silicone (window caulk for example) as... well... caulking. In the tongue and groove it should be well protected even without paint. Any thoughts? Oh freshwater use only.
  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    It appears that you have a start on your boat. It is going to be unforgivably heavy. Tongue and groove is a bad idea. There is no need to use a herringbone planking pattern on a flat bottom boat. You would use that pattern if it was a vee bottommed boat that have varying deadrise angles.

    Your basic design leaves some elementary principles untended. You have evidently made the bottom a straight line from bow to transom. If it is to be a powerboat it will be disconcertingly twitchy at any speed above walking speed and maybe even at slow speed.. If it is to be a sail boat or row boat it will not turn well. If a sail or row boat, it will drag a big wake behind it because you have not given the boat any aft rocker therefore the transom will drag badly.

    When you put the boat in the water, it will leak unless you caulk the seams skillfully. I suppose that you could use weather seal caulking to seal the seams as you mention. After the boat has been exposed to water for a day or two, the wood will swell, so you will need to leave some space between the bottom planks to account for the swelling. Whatever you use for caulking needs to have some resilience or the planks will cup and the bottom will look like a washboard.. The wood will swell and there is no way to avoid that reality unless you use a wood epoxy saturation technique, which will raise your budget by a good bit. Cross planking for small boats went out with buggy whips. You can certainly build a boat that way but there are far more practical methods now.

    You will be way ahead of the game if you use plywood for the bottom and avoid all the pitfalls of spacing and caulking. You will be even further ahead if you get some professionally drawn plans and instructions, and build according to those plans.

    I do not mean to belittle your boat or your ideas. I am just telling it like it is. I will presume that you did want comments from experienced correspondents.
  3. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I'll second what Messabout has mentioned. File plank bottoms where fairly common a 100 years ago, but have been all but abandoned now. Many reasons, but simply because we have simpler and more water tight methods now.

    If you're not going to sheath this boat, those tongues will likely cause you problems in the near future as they swell in their grooves. The only advantage to a file planked bottom is the very economical use of materials. Speaking of materials, that looks to be flat sawn, #2 pine, with a significant number of defects each. This is about the worst material choice you could make on hull planking.

    The exposed edge grain of the bottom planks will be problematic, as will the dozens of seam making a fine path to the inside of the boat.

    Messabout covered the boat shape choices you've made, but she we be repetitively stable, if a bit of a slug, regardless of propulsion type.

    If those bottom planks aren't fastened down yet (they don't appear so), pull them off and place them athwartship (actually if run fore and aft, you'll have less seams and more strength). Also build up a rabbited chine log, so the bottom plank ends, have a place to land where the end grain is protected, a good location where caulk can live and the multiple direct paths (seams) to the inside of the boat can be easily sealed (caulk and no, bathroom caulk will not do).

    A noble effort, but a set of plan may have been a better route, if some level of performance is desired (easy rowing, sailing, modest power requirements, etc.).

  4. Dave Gentry
    Joined: May 2010
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    Location: Charlottesville, Virginia

    Dave Gentry Junior Member

    Just an FYI, cross planked bottoms are alive and well on the Chesapeake Bay, both with little flat bottom skiffs and dead-rise hulls (which are evident on every one of the two dozen or more active wooden oyster boats based out of the local cove here).
    That being said, the smaller boats being built in this manner all seem to be coming out of the various museum boat building shops in the area.

    Anyway, this effort is worthy as an exercise to see what one can produce with little knowledge about the subject. However, this is also a classic case of "reinventing the wheel." Traditional flat bottomed boat building was figured out and more or less perfected some hundreds of years ago. No doubt your boat will eventually float, but as a useful watercraft, there will be issues as noted. Before proceeding further, a little reading could help you produce a more workable boat.

    I recommend "Boatbuilding" by Chapelle, or most any of the books by John Gardner. Harold Payson and Jim Michalak are authors who also specialize in simple to build boats, albeit of more modern construction. Amazon and/or interlibrary loan are your friends.

    Good luck!

    Dave Gentry
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