Glued Lapstrake Plywood construction

[Dean4140]

Can glued lapstrake marine plywood construction be moored full time in the water? Examples: Gartside 16' and Penobscot 17'.

Would it be better to trailer plywood construction and use cedar planks, etc. if mooring in water year round.

I've heard of some plywood delamination problems. I also have heard that glueing solid wood planks can cause splitting due to expansion from swelling. Any thoughts or experiences out there. Dean

[Andrew Craig-Bennett]

I would certainly go for:

1. Glued ply lapstrake, if the boat is to be kept dry.

2. Conventional (real wood!) lapstrake, if the boat is to be kept afloat.

I have not been happy with the durability of the glued ply lapstrake dinghy I built back in '92.

[Keith Wilson]

I'd say it depends of the durability of the wood species used, and the quality of the surface coating. If the plywood edges are well sealed, I see no reason that a glued-lapstrake boat built of good-quality materials shouldn't last at least as long as traditional construction. In fact, I suspect glued-lapstrake construction could last even longer, because there are fewer little cracks and spaces behind frames for water and debris to accumulate. Since the plywood is more dimensionally stable, the paint should last longer and seal better (Douglas Fir excepted - it checks abominably).

[Andrew Craig-Bennett]

There was a time when I would have agreed with you, Keith!

However, since I have owned a lapstrake glued ply dinghy (kept dry) and a larch on oak lapstrake launch (kept afloat), I find that the ply dinghy is far less able to withstand moderate abuse.

[Keith Wilson]

Well, different experiences I guess. My traditional lapstrake boat is now awaiting new deckbeams and sheerstrakes after being kept afloat for four years (four seven-month sailing seasons, actually). I've never had any rot troubles with ply lapstrake. Plywood boats tend to be more lightly built because of the superior cross-grain strength of plywood, and may be more vulnerable to abuse because of this. You could build a truly bulletproof ply lapstrake boat; just use the same thickness of planking as in traditional construction.

Larch is similar to Tamarack, yes?

[ 04-11-2005, 12:07 PM: Message edited by: Keith Wilson ]

[Dean4140]

Thank you Andrew and Keith,

When you seal the ply, do you do it by saturating the ply with epoxy or epoxy with a 4oz. figerglass cloth? Either way acceptable?

Do you do the epoxy and/or cloth on both the inside and the outside?

Dean

[Keith Wilson]

Well, take what I say for what it's worth; I've only built two small boats in lapstrake ply (plus three lapstrake cradle boats, but they don't really count.) I used fiberglass/epoxy sheathing only on the bottom of the hull for additional strength and abrasion resistance. One of the great virtues of this kind of construction is it requires a minimum of sanding. The plywood is already flat and smooth, and fairing an epoxy/glass layer would be no fun at all.

I wouldn't use epoxy alone as a coating on the face of either plywood or solid wood; if you build up a thick enough layer to do any good it will crack as the wood flexes, and then where's your sealing? Fiber reinforcement in epoxy allows a thick layer without cracking (think rebar in concrete) and adds considerable strength, but would be too heavy on my boats. For larger craft, sure. I use CPES under Kirby paint, or CPES and Cetol for bright-finished parts. I do seal all plywood edges, particularly the plank edges, with unthickened epoxy; on edge grain it soaks in nicely. My .02, anyway.

And one more tip, the single best one I know: use a heat gun and scraper to clean up cured epoxy blobs. Heat softens the epoxy and makes it incredibly easy to scrape off. You can get all the hardened drips off the inside of a 14' hull in and hour or so with this method (and I'm by no means neat with the glue).

[ 04-11-2005, 02:48 PM: Message edited by: Keith Wilson ]

[Andrew Craig-Bennett]

Yes, so far as I know, European larch ( larix decidua , I think) is close to your tamarack.

Commonest planking wood in British boatbuilding, but very many older boats planked with it are described as "pitch pine" planked because few people know different.

[dmede]

There is no need in most cases to use cloth over ply in glued lap ply boats.

You should find that in many applications, especially smaller boats, only the edges of the ply are sealed with epoxy. Marine paint (or even latex house paint) is applied directly to the ply strakes.

For a boat that is to be kept afloat I would might want to coat with epoxy first however. You are sealing, not trying to add strength here, so you don't layer it up untul it becomes thick enough to crack if flexed. One to two well applied coats of unthickened epoxy followed by several coats of paint will be more than enough to seal the ply.

dave

[Keith Wilson]

Right. Tamarack is Larix laricina, grows in swamps mostly.

[Dean4140]

Thanks Dmede,

That certainly makes it easier than trying to do to much. Keep it simple with a step-by-step sequence. Dean

[Gary E]

Can glued lapstrake marine plywood construction be moored full time in the water?
Yes.. many are, a lot of Chris Craft Sea Skiffs are only pulled from the water to paint the bottom.


Would it be better to trailer plywood construction and use cedar planks, etc. if mooring in water year round. I would think that depends on you, If you live close to the boat and use it often keep it in the water. If you live far from the water or cant get there often, use a trailer.

[paladin]

i built a glued lapstrake dink in Vietnam (Nha Trang) in the late 60's....I used a marine plywood that had been used as part of a shipping crate for helicopter blade...and a local wood similar to african mahogany for the keel and basic framing parts. The "chine" was made by leaving a gap and filling it with tape similar to Jim Brown trimaran design for "taped chines"...i used Gerry schindler's T-88 epoxy throughout, no fiberglass covering and coated the boat all over with resin....two coats where it soaked into the wood.....then sprayed inside and out with primer and Imron..I dragged that dink for three years until in the Seychells someone decided that they needed it worse than I.......it was still a good boat and no rotten wood...although I did recoat the bottom more than once dragging it onto a less than sandy beach.....

[Keith Wilson]

T-88 is my favorite glue. System 3 bought out the original company and they sell it now. It's by far the bsst thing for plank laps.

[Trojan Dog]

Over the winter I took a 1963 glued and nailed (clench) ply lapstrake Trojan down to wood and repainted. This is a 28' sedan cruiser and to the best of my knowledge it's been in the water all of her 42 years. Aside from discovering one replaced plank (looks like it might have been collision damage), everything looked really good. There are LOTS of old ply laptstrake boats around here and nearly all of them live in the water year round. As an interesting aside, Trojan used marine mahogany ply with a layer of paper factory glued to the exterior surface. This allowed for easy painting at the factory. When I "wooded" the hull, I was able to take off the paint (heat gun and scraper) and leave the paper fully intact with no problem. Even after all this time, the paper is still perfectly bonded to the wood.

[Dean4140]

Thanks for all the input guys. Ply sounds to be much more durable than I thought. The key is the quality of the ply and the construction of the boat. All the info has been very helpful. Dean