Glued Ply Lapstrake

[MarkWoyWoy]

Hello All,

I've recently finished planking a glued ply clinker(lapstrake) launch(12 feet).

The overlap on the laps ranges from 12mm(1/2") to 20mm(@3/4'). I tried to be uniform but to get things fair they sort of went this way.

I think this is enough but this is my first boat so I'm not sure. As insurance I was going to install quite a few frames, around 12 or so.

However I thought about running some 6oz glass tape along the laps so I made up some test laps. It was pretty difficult to get it to lay over the laps.

However I found that by tacking the cloth down with a light dusting of spray adhesive and using a wide roller on the flat and a window screen spline roller on the corners I could get nice sharp laps.

Applying resin rounded them slightly but they still looked nice and sharp from a foot or two away.

My question is does anybody have any comments on this. It seems an easier way of getting a bit more strength and rigidity than more frames.

Thanks

Mark S.

[G. Schollmeier]

In this case I think the glass will only add work for you. It will also subtract from the looks. Unless the plans call for them, skip the frames also. Glued lap makes a very stiff hull. For a 12 footer you should not have to add extras. Unless you went cheap on the ply or glue.
Gary
Almost forgot, pictures please. :D

[ 03-26-2002, 01:09 AM: Message edited by: G. Schollmeier ]

[wolfietuk]

I would think that the spray adhesive would prevent the resin from bonding with the wood. This will show up in about a year when it starts to pop off. I think G is right. You should be plenty strong without it. The difference in the laps shouldnt make a difference in that small a boat. call the designer if you think there might be a problem.

Rick

[Greg H]

I would say skip the fgls tape, and 12 frames sounds like a lot. I'm working on a 16' glued lapstrake and it has no frames, just floors and thwarts to stiffin it. If you're worried about the strength of the lap joinst, you could run fillets (?) of thckened epoxy along the edges of the laps.
---G

[Billy Bones]

Interesting issue.

I've hunted all over for information on failures of glued lap ply construction. I haven't found any, regardless of quality of construction and materials.

If your materials were good quality and if your design is a proven one, you'll have no problem.

You might have problems, though, if you add stuff to the design like frames. And the glass tape on the laps will be superfluous and will look ugly, all the while being a pain to do.

The junction of each lap acts as a stringer along a chine. There is one thing you can do to make your chine stronger and more rigid. You can cut a small strip of hardwood the thickness of your plywood and a quarter-inch thick or so, and glue it to the bottom of each plank, increasing the size of your laps. You can then fill in behind the strip with thickened epoxy. This will increase your beam strength greatly at each lap/chine (much more than glass), and won't detract from the looks.

Trust the glue, or build differently.

[ken mcclure]

The only thing I've seen recommended over glued plywood laps is dynel. Apparently it's flexible enough to make most of the curves. It would still, however, "round over" the lap edges and make the boat look more like a plastic lapstrake imitation.

This was recommended (in my readings) more for waterproofing and abrasion resistance than for strength.

[NormMessinger]

Ah, grasshopper, now ya ask. A half inch overlap is plenty. No need for the glass. Half the fibers in the tape run along the lap and do nothing. The contact cement used to hold down the glass will reduce the effectiveness of the bond, etc etc etc. as has been advised above.

However, don't let all this negativeness get you down. You'll know next time and in the mean time proceed as you are, finish the boat and enjoy. I have no experience to base my prediction on but my bet is the superfluous glass will stay put and you'll have a nice useful boat.

If you are interested here is my Humble Bee album: http://www.imagestation.com/album/?id=4292123583 . It is build with 4 mm okoume, 1/2" laps. It was quite flimzy until the gunwale, floors and thwarts went on.

Best wishes.

--Norm

[Dale H]

As the others said. About the only way to use regular glass is to put it on the plank's flat surface and put fillets of thickened epoxy on the laps to seal/protect. All you'll get is some abrasion resistance...and weight.

I'd suggest a small colour matched epoxy fillet on the laps and a coat or two of epoxy and finish away as usual. Did this on our Wee Rob canoes and have not had a stitch of trouble though they see rough service, to say the least.

Dale

[Andrew]

I'd leave it as is unless you want to put some glass on the bottom for abrasion resistance. 1/2" laps are adequate.

[Don Maurer]

For my 13' 6" Tammie Norrie, Iain Oughtred recommends 5/8" laps on 6mm ply. Mine actually came out between 1/2" and 3/4". As others have said, once the floors and gunwales go in, the boat is plenty strong. The boats are designed so that stress is spread over a large area. As long as the stress isn't concentrated, a joint shouldn't fail. Even then, it will most likely be the plywood that fails and not the glue joint.

[kevinwal]

I'm building an 8ft Acorn (or whatever Iain Oughtred is calling it these days) and the plans specify a 5/8" lap. I'm well into fitting out the interior, and while Iain calls for using stiffening battens to be attached across the beam at the gunwales to help it hold it's shape while I work (mostly to prevent twisting) the boat seems plenty stiff to me, and I every so often remove them to marvel at how well it holds its shape without them. My only concern about it holding together has more to do with my workmanship rather than any perceived insufficiencies in the design of the boat. Even with less than superior joinery work, I'm assured by many that epoxy clinker ply boats are very forgiving and will last and last and last. I am of course counting on this, and my advice to you is to do the same.

By the way, this group thrives on pictures, and photos of your work would be most welcome. You won't regret posting them either, because even if your work is obviously atrocious, you will receive many, many satisfying oohs and ahs over your superb craftsmanship. :|

Regards,

Kevin

[MarkWoyWoy]

Thank You,

I will skip the glass tape. More work than necessary. Collective wisdom isn't it good.

As far as pictures go I lent my camera out. I will try to get it back and post some pictures over the weekend.

Mark S.

[DougWilde]

All these tales of laps varying in width only confirms in my mind the wisdom of Tom Hill's method of construction. Unless you really aren't paying attention, all your laps will be the same width the entire length of the boat. On his 11'6" Charlotte caones, built of what, 3-4mm ply the laps are 3/8".

And I've tested construction to destruction. The second time Merrisa was on the water and friend and I were paddling the Illinois River (NW Arkansas). To make a long story short, the current grabbed his canoe, turned him sideways and pushed the canoe into the roots of an overturned tree. CRACK!! as one of the roots stove in the hull above the waterline. Two strakes were stove, the plywood failed, not the glued lap. Slather on the epoxy, clamp and brace properly, some fresh paint and you cannot find the damage.

No need for reinforcement.

Doug Wilde

[Mike Field]

And to add to what Doug just said, I understand that adding frames to a glued clinker hull can actually make the completed structure weaker (by virtue of the rivets) than if one had no frames at all, but relied solely on the glue in the laps for strength.

[K Wilson]

The collective wisdom is dead right on this one. Assuming you did a reasonably OK job of fitting things together and got enough glue in the laps, 1/2" overlap is fine. The great thing about glued-lap construction is that stresses are distributed over a relatively large area compared to mechanical fasteners - think welding as opposed to rivets. If you have some legitimate reason to be concerned about the strength of the lap joints, gluing in frames wouldn't hurt, I guess. If you really don't trust your glue, I suppose you could back it up with mechanical fasteners, rivets or the like; again, it wouldn't hurt but it would be a lot of work for little gain.

You could, I suppose, glass the inside of the hull for tensile strength only, since it obviously wouldn't contribute anything to abrasion resistance except from sand on your shoes. It would be a lot easier than glassing the outside, but I wouldn't do it unless one of the lap seams had already popped open, and even then I'd think twice. With glued lapstrake, you have to either trust the glue or go nuts; that's all that holds the boat together.

One additional suggestion - I've always put a good thick layer of unthickened epoxy on the end grain of the plank edges (dribble it on while the boat's upside down). It seals the end grain and possibly strengthens the joint a bit.