New Guy, just saying hello.

[Condor1970]

New on the forum, mainly as a result of wanting to build a small 14ft Jon boat over the winter, so thought I'd register on here, and do some research.

Just to let y'all know of my idea for a winter project. I'd like to build basically a 14ft Jon, but with a flare. I'd like to build it more conventional style, with a rib and cedar strip on the outer hull, 15-20deg sides, 14-15deg transom, a plywood interior, filled with liquid poly flotation foam between the ribs.

The idea for the strip design, is I'd like to build the bottom of the outer hull with two channels, similar to what one sees with a typical bass boat.

Other than that, Hi everybody, and I'll be doing some searching and reading a lot before making other posts. Thanks.


Oh, I guess I do have one question. For a hull design like this, using a 10hp outboard, what thickness of cedar stripping would you recommend with 1 or 2 layers of 7oz glass/resin?

 

[rasorinc]

Welcome to the Forum. A Jon boat beaches many times so I would use 3/8" ply on the bottom or at least near the bow.

 

[DCockey]

Why not plywood instead of wood strips? The channels could be formed using stitch & glue?

My guess is a wood boat filled with foam-in-place floatation will tend to rot.

 

[messabout]

Not only inviting rot but flotation in the bottom invites the capsized boat to remain bottom up. Consider air tanks as part of the structure, that eliminates some of the frames. Foam and wood are not happy bedfellows.

 

[Condor1970]

So, basically don't use foam. Ok, I guess that makes sense, if water gets in there, it will tend to stay in there. I was thinking of cedar strips on the bottom and plywood on the sides. The bottom will be concaved with channeling, and as the bow has a curve up, there's not much of an easy way to make a smooth transition with plywood. It's like a reverse curve.

So, if I was to cedar strip the bottom, what thickness of strips would you guys think is about right?

I was thinking 1/4" with 2 layers of glass outside, and maybe even one layer inside.

Does that seem about right?

 

[thudpucker]

Post a photo or three of your Jon. Someday I'll build one for Flat Water fishing down here.
Just wood and PL goo for sealing.
I'd thought of an Oak bottom sawn to 3/8". No plywood.

 

[Condor1970]

Oh, I will post pics. Matter of fact, I may post pics of my drawing here soon. Not done yet though.

Also, I've been thinking of a way to do it with plywood. It may just work.

What I still want to know, is what the guru's on here suggest for strip thickness for the bottom.

 

[messabout]

What will be the width of the bottom? How many frames will you use? What do you mean when you say "concave" bottom? How much curvature is involved in the concave? These are questions that matter when choosing the thickness of the bottom planking. If you strip build, you will surely want to glass the inside surfaces. Two reasons. 1; keep the strips permanently dry. 2; The inner glass will become a tension member that will help stiffen the loaded areas.

There are many small boats built with 1/4 inch ply bottom skins, which is somewhere near the lower acceptable limit of thickness for a powered boat, especially one that is expected to plane. Your 10 HP is obviously for a planeing boat.

Canoes and kayaks typically use quarter inch strips which is entirely adequate. But paddle boats do not get the pounding that a power boat is likely to endure. Stiffness of the strip increases roughly with the third power of thickness. So a 5/16 thick strip will be almost twice as stiff as a 1/4 inch strip. An ordinary 9.9 outboard will probably drive a 14 foot boat 25 MPH or a little more, if modestly loaded. The bottom is going to get hammered by chop and wakes. I,d be inclined to use 3/8 strips and fewer frames on my boat.

The thicker the strips the more important the bead and cove joints unless you want to do a whole lot of careful hand planing of the fitted edges. But bead and cove has limits on the acuteness of athwartship curvature that you can use. It's one damned thing after another when building boats. We, the nautically afflicted, do it anyway.

 

[Condor1970]

I figured out how to do it with 1/4 plywood, so I think I may go that route.

It will have 2 concave longitudinal channels on the bottom, instead of a V-hull, using 3 aluminum bottom strips for protection, one being in the center. Sort of like a bass boat. The bow and sides will be flat plywood, with a curved up bow, like a fan boat uses on its bow. The channeling is used mainly to help with speed/stability a little, and also help prevent excessive bottom scratching on the lake and rivers I primarily intend to use it on.

I plan on using 1x3 and 1x4 boards (clear pine or hemlock) for the ribs, placed 12" apart. The bottom being 4ft wide, with roughly a 20" side/total draft. The bottom will get 6-7oz glass, and I'm thinking also the inside as well to fully seal it.

I talked with a fellow about using two-part poly closed cell foam. He said to epoxy the wood on the inside, and go ahead and foam it for rigidity and buoyancy, because it will strengthen everything on the bottom. This will help a lot, because with the ribs being concave, the center arcs will be only 1" thick, so the foam will really help firm it all up.

 

[SamSam]

Conventional 14'jonboats don't have a v hull, they have a flat bottom. Why do you want to have 2 concave longitudinal channels on the bottom?

Also, don't forget why foam in the bottom has drawbacks.

 

[thudpucker]

Maybe he's thinking of a Tunnel hull, made of two small hulls?

 

[Condor1970]

No, I was thinking of doing basically a flat bottom, but with 2 slightly concaved channels along the axis to help it plane and maintain directional stability when turning, etc. Sort of a flat bottom lake boat hybrid design. Also, the concave design will allow me to have 3 railed skids along the bottom to help soak up any mild rubbing from the lake or river bottom without punching holes in the hull. I've seen it done before, and it seems to work really well.

As far as foam goes, I was thinking of using some 2" Owens Corning foam board insulation between the ribs for the majority of fill and stiffing/insulation, and the liquid pour will just fill in the rest.

 

[PAR]

Ribs, pour in foam, rigid foam, "concave" bottom panels. Why all this convolution? You don't need ribs, nor concave strips to make the boat plane off and the foam should be on the hull sides, not the bottom, for floatation needs if she's swamped, assuming you couldn't just work in a few buoyancy chambers fore and aft, like most designs.

Maybe it would be best if you just bought a set of plans, you know, where the engineering and hydrodynamic stuff has been worked out, by someone that understands the realities and structural aspects of the arrangements. If you insist on "designing" a boat, you'd be best advised to get a much firmer grip on the principles, dynamics, engineering and terminology involved, don't you think? Now, maybe you're thinking "how hard could it be for 14' sort of jon boat thing" right? Yep, it's a pretty simple project, if you have a clue on the various dynamics at play, but facing a swim to shore, farther out then you're capable of, is a reality you may not have yet considered, all because you didn't take expected bottom loading into consideration, when you setup your scantlings (or something).

Jon boat plans (12' - 18') range from free or nearly so, to not very much at all, so what's the deal? Do you really want the transom to pop off, because you didn't understand cantilever dynamics, with a 10 HP outboard trying a hole shot.

 

[Condor1970]

I understand everyone's concern, which I guess is a good thing. I've built tons of wood projects over the years including airplanes, from RC to small experimental, and the majority of my house. So, wood construction, and engineering are something I'm quite familiar with, even though my "boat" terminology is admittedly poor, at best. I guess what I should point out, is this design I have in mind is basically an exact copy of another 14' boat I already saw for sale on Craigs', and really fell in love with it. I even went out, looked at it and measured it, to see if it was about the size I wanted.

Unfortunately, the guy sold it to someone else. It's actually an older fiberglass hull design from the 80's, and is no longer made. I wish I had an actual picture of it, but don't. What I did do, was sketch it out on paper when I saw the add before it went away, so I could build a version of it to almost exact dimensions out of wood, with essentially a foam core in the hull.

The way I see it, doing a rib design with 1/4" plywood to get the water channels is the easiest way to do what this boat had, and filling in between the ribs with foam board or liquid, is the best way to firm up the whole thing, since the ribs are fairly thin (about 2" in the centers), so the ply, fiberglass, and foam will all add the structural integrity it needs.

 

[PAR]

'Glass boat construction and engineering approaches are wholly different than plywood. There's very few "ideas" you can take from one method to the other. Again, you're making assumptions about material properties that you don't fully understand. You don't need ribs to stiffen plywood and you don't need "water channels", whatever these might actually be. It's likely you've seen some 'glass panel stiffeners or possibly some strakes and are assuming they do something they don't.