Question: Small flat-bottom boat

Discussion in 'Boat Design' started by VV Cephei, Apr 20, 2009.

  1. VV Cephei
    Joined: Apr 2009
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    Location: Ontario

    VV Cephei New Member

    Hello all,

    I was wondering if I could get a bit of feedback, or a few questions I have answered about building a small ( 4' x 11') flat bottom boat. I will be using regular plywood, but I do plan to coat the boat in marine epoxy.

    I plan to just but the ends together, and use fiberglass cloth on both sides of the seams. The boat will be pretty much a box shape |____/ .

    It will be used in calm water that is very shallow with a 4hp outboard.

    Now for a few ques:

    What thickness of plywood would you recommend? I know the bottom will need to be thickest. I plan to put narrow 'runners' on the bottom of the boat for extra stability and protection from rocks etc.

    If I make no seats in the boat, will just the epoxy coating and the 'but joints' with cloth be strong enough? It would be pretty much an empty plywood box just with a coating of epoxy. I know it dries very hard, but I want to know if the way I'm picturing doing it will be ok.

    I'd like to keep it empty without permanent seats if possible. I may have a few more questions, but I'll start with those and see how things go. Thanks for any help or advice you can give.
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    That's a very easy boat to build. You could make one from your own design and it would perform to some expectations, but you could also get a set of plans (I have low cost plans for just the boat you describe) that will optimize the shapes, keep it simple to build and answer your questions about how thick things need to be, plus where to precisely put them. An added benefit would be better performance from the little outboard and improved maneuverability as a result of the shapes employed in the design. In other words, boxes with sloped fronts don't do very well, compared to other shapes.

    From a structural stand point, not having a midship thwart and a bow piece (breast hook, foredeck or thwart) the sides of the boat will likely flop around a good bit.

    Butt joints will need some sort of reinforcement. The Payton butt joint works very well and is one I employ in my taped seam designs. You could use a butt block on the inside of the joint or a scarf joint.

    To answer your question 1/4" plywood will be fine, if it's good plywood. 3/8" on the bottom if you feel it's necessary, though the rub strips will absorb most of the punishment, 1/4" will still work and keep it light.

    Keeping the structure light is the key to a good small boat.

    It would be possible to have a small thwart at each end of the boat with reinforced sheer clamp and maybe a topside stringer, to provide the open layout you desire. You'd have narrow side decks of sorts, but the middle of the boat would be wide open.

    Contact me by email (click on my name) if interested. It could be continuing a run on Canadian customers of late.
     
  3. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    A 4 foot wide boat as described would have quite a wide bottom, and it's strength (to resist transverse flexing) would be achieved by having a reasonable amount of rocker or making the seats full boxes for flotation and a set of transverse beams.
    In your case, wanting to have a clear open area, you can't just modify an existing plan without help from someone knowledgable.
    Removing seats or transverse frames along the bottom will affect the bottom's strength. Scuppered transverse frames with floorboards (even if removable) keeps the floor area dry and works well with gussets at the chines, which is similar if not identical to dory construction.
    A good flat-bottom boat is one of the most useful boats you can own. It will pay, I think, to have at least a rowing station with oarlocks and seat for when the motor won't go, but the seat can be shipped to one side when not in use, so that still leaves the boat free of cross members.
    PAR will, I'm sure, provide you with a good set of plans or modify to suit, but by all means maske sure the bottom is rigid enough and that the connection to the sides is well braced. Otherwise, the boat will practically build itself.
     
  4. VV Cephei
    Joined: Apr 2009
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    Location: Ontario

    VV Cephei New Member

    Hey guys,

    Thank you for the replies. I've spent a bit of time reading the forum and looking at other 'jon boat' plans on the net, and I've modified my initial design.

    In the future, I may purchase plans and modify them to suit my own design, but for this first project, I think I'm going to just go with this basic design and consider purchasing plans in the future for something a bit more complex, even though I have questions about this 'simple' design!

    One of the goals of this little project is to just keep it as simple and cheap as possible for now.

    I was wondering if someone could answer a few ques that I have:

    In the attached picture, there is the rough outline and dimensions, as well as the proposed 'skeleton' of the boat made from 2x2s. I would make the skeleton out of the 2x2s, and then cover the boat with 1/4" plywood, then I'll cloth and epoxy the entire thing.

    I have a center 'transverse' beam in lieu of making seats so I can have an open concept. Does this plan look stable enough now, or are the few 'extra' transverse beams that I've now included overkill since it will be epoxied?

    I would be putting cloth and epoxy on the outside and inside of all seams, but should the entire boat surface be coated as well, or just the seams? The entire underside would be epoxied of course, but does the entire inside need it as well, or just the seams?

    Do I now have 'too many' transverse beams?

    Again, the boat will be used for shallow, calm water, and will have rub strips along the underside for added stability and for protection from rocks etc.

    Thanks a lot for any input/advice any of you are able to give me.
     

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  5. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    Too complicated...too many frames. Use a 1x6 for a buttblock in the after section. Don't try to have a sharp angle forward...Jons have a curve for a few reasons, one being that plywood bends. {shameless self promotion coming} check out http://angelfire.com/ego/lewisboatworks for free plans (LilJon) for just what you are proposing. 4 hp is just on the border of enough to get you planning if the boat is light enough but I bet not so internal framing is not so necessary. There may be a bit of flex to the panels but that won't be damaging to the structure. If you double up on the wales and use some external skids you won't have many problems. Why don't you want any seats?...hard to stand and motor at the same time. If you want to add frames then use some 1x with 1/4 ply gussets in the corners...lighter than your 2x proposition.

    Steve
     
  6. ancient kayaker
    Joined: Aug 2006
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    At the very least put a bit of curvature in the bottom as it will help stiffen it and prevent oil-canning. Curvature can make the ply act as if it was twice as thick, without the weight and cost, and it will go faster and smoother. You can google PDRacer for an example of a boat with curved bottom and flat sides.

    A bit of a curve will do the same thing for the sides: they do not have to come all the way in to a point at the bow. If you make the front plate (pram transom) slope a bit the boat won't come to an abrupt stop when you run into a wave form a bigger boat (see PDRacer again). At this point you will find you have something looking more like a boat.

    It is actually easier to do it this way than the way you have sketched: ply bends easily and it will save you have to make joints that are strong enough and waterproof. The butt joints can be made with the planks flat. Most ply boats have a stiffening member along the top (sheer) of the side planks which is called a gunnel.
     
  7. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    You're not alone in avoiding curves at all costs! First boat designer/builders present absolutely flat-panelled drawings here all the time.
    Curves are not hard to do at all, especially on a jon boat. You may not curve the sides but absolutely try and curve the bottom into the bow.
    You need no more than a 3/4" x 2" chine log where the boat is straight (it's been suggested to design in some rocker there, and why not? The rocker will as mentioned add stiffness across the boat, eliminating the need for so many frames. Just spring a batten along the sides to give about 2-3" of rocker and copy one side to the other. If the sides are parallel, lie the chine logs down flat and they will, if coaxed, curve right up to the bow as well as curving easily along the bottom.
    Usually, chine logs are vertical, but in your boat they won't take the curve that way.
    Use seats as frames, and hide flotation in them Three cavities, both bow and stern shelves and the center thwart (seat).
     
  8. ben2go
    Joined: Jul 2008
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    Location: Upstate, South Carolina,USA

    ben2go Boat Builder Wanna Be

    I agree.Check out Lewis' website,link is in the post.
     
  9. VV Cephei
    Joined: Apr 2009
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    Location: Ontario

    VV Cephei New Member

    Hey, It's been awhile, and I thought I'd update this thread with what I ended up doing.

    First, thank you to everyone who gave their advice and input in this thread, it was much appreciated, and much needed. Well, I realized that my square design that I had initially come up with wasn't going to cut it, so I took the advice to make some curves in it.

    The length of the boat is 10'.

    1/4" plywood was used, with a 3/4" transom.

    It is 46" wide at the stern, 48" in the middle, and narrowing to 38" at the bow.

    It is 18" high at the stern, and it's 20" high in the middle before tapering off towards the bow.

    There is a layer of 6oz epoxy 'mat' underneath and coming up 6" or so on the sides. I used a lot of epoxy on the bottom, more than I should have, but it was my inexperience at 'fiberglassing' that caused me to use so much.

    I decided not to put seats in it, because I want the space for carrying things I have in it, but I included a 2x2 across the top and bottom near the middle of the boat.

    It will be only for slow speeds (2hp outboard) and used in fairly shallow, calm water.

    The pictures I have here are before I put the epoxy and cloth on.

    I have more pictures I could show, but it seems that I'm at my size limit here with the 3 pics I've already posted in this thread.

    boat1.jpg
    boat2.jpg

    Anyways, I wanted to come back to say thanks, and also to show you guys what I ended up doing.

    Any comments, questions, or criticisms are welcome!
     
  10. ben2go
    Joined: Jul 2008
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    Location: Upstate, South Carolina,USA

    ben2go Boat Builder Wanna Be

    Congrats!You have built your self a Jon boat.Add a couple skegs to keep her tracking straight.1" x 2" roughly 10 inches from the sides.
     
  11. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    Looks much better than the proposed drawing...congrats. To post pics your best bet is to find a free site that hosts pics then link to them. were I you I would put a couple of knees in the corners of the aft transom to tie the transom to the sides and give it a bit more torsional strength. You may find that it will be fairly easy to twist it without any bulkheads to give it rigidity so a couple of knees from side to bottom might be in order to help with this. The quick and easy way to find do this is to get corner molding at the box store...it is prefabbed and decorated too. Two or three per side should work

    Steve
     

  12. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Well done. It will serve.
     
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