Lapstrake Plywood Edges

[alkorn]

All the instructions I've read for sheet plywood boat construction insist that I cover the exposed edges of the plywood either with battens or with fiberglass and epoxy. Just coating the edges with non-reinforced epoxy isn't supposed to be good enough.

Then I read descriptions of glued lapstrake plywood construction, and practically the whole bottom of the boat consists of exposed plywood edges. Epoxy coating the edges is recommended, but there's no way to glass over the sharp corners.

Why are exposed plywood edges bad in sheet ply construction and OK in lapstrake ply construction?

From what I've seen of the performance of plywood edge grain, one landing on a rocky beach could leave plank edges on a lapstrake ply boat with lots of little splinters torn out. My experience is with ACX, not marine, but I don't imagine marine edges would be strong enough, either.

I think I'm missing some important info here. Can anyone help out?

[Howard]

Not sure how thick the plywood is you are talking about, but assuming it's 1/4 inch or greater, and the lap strakes are glued and not moving, you could use a roundover bit on the edge, a small fillet on the inside corner and cover the whole bottom and topsides with Xynole polyester cloth. You will have a "rounded" look vs. a knife edge, but beyond 5 feet it will look right. Xynole is stretchy ...conforms to tight curves better than glass, and is the most durable of coverings you can use. Rolling on three coats of epoxy will give you a tough, durable exterior. Not sure it will survive pounding on the rocks, but then most boats of any type won't last long under those conditions.

As for the edge of plywood, it has to be sealed no matter what. The edges are open, end grain....imagine the end of a straw, vs. the side of the straw. Water wicks in easier on the end. Any type of plywood or regular wood has the same problem. If water and air wicks in...rot is possible.

[PAR]

Glued lapstrake or conventional lapstrake can benefit from having the edges sealed with a penetrating epoxy (CPES, etc.) and this is the only function for the treatment. A penetrating epoxy sealant will impart no additional abrasion resistance or strength to the plywood. Regular epoxy will harden the edge, but provide little abrasion resistance. A CPES or similar treatment is a wise and very recommended method to increase the life of the planking.

Radiusing the edges of the planking and 'glassing (some folks do just the garboard) isn't recommended by most lap builders or restorers. The radius necessary can't be had in less then 3/4" stock, which is a pretty hefty boat. Some of the more exotic fabrics can conform to tight radiuses, but it's crap shoot in the end, which most don't wish to risk. It would also complicate repairs a great deal and lap is one of the more difficult to fix in the first place. The edges of the planks should be "broken" before they are hung, so the finish can stick to it.

[Greg Luckett]

PAR,

Is a "penetrating epoxy (CPES, etc.)" like using West 105 resin with the 207 hardner? I was thinking West had another resin which was less viscous but was unable to find anything on it.

I would probably also use a two part epoxy paint for sealing the edges and sides of a glued lapstrake boat. I have found that sort of paint to be excellent for the bottoms of my dinghies. Very abrasion resistant and seals the wood a lot better than any of the 1 part paints.

Greg.

[capt jake]

like using West 105 resin with the 207 hardner

That combination is not a 'penetrating' epoxy. It is comparable in viscosity to the Raka products.

Now, a little off topic; West promotes their 207 as a 'clear' hardener, though I found that Raka's standard product is much clearer. I think the West is over-rated. I used it on the kayak and though it looks great, the raka product gave far better results.

[Greg Luckett]

I have often wondered about the Raka products and look at the website reguarly. It is more difficult for me to obtain Raka and they do not have any online ordering system either. The West products are "off the shelf" about 10 minutes from the shop, for me. I like what Raka displays and someday I will take the time during a week day to call them.

[capt jake]

I have been very happy with Raka's service. A simple call and I have it on my doorstep ina few days. And they are about as far away from me as anyone can be. Price is at least 1/2 of the local West.

Oh, and the 2:1 mix is easier for me. ;)

[Greg Luckett]

The price is about 1/2???? How much does a gallon of the West 105 + Hardner cost you there?

[Howard]

I'm with Capt. Jake on endorsing RAKA. It's what I'm building my Spindrift with. Before I started, I did up a test panel using West with the 206 hardener, System 3 (Silvertip laminating) and RAKA, and in my view got the very best results with the RAKA. The others were bad about leaving bubbles, which become pin holes, left a cloudy appearance, or both. RAKA is generally clear and bubble free. It tends to be less viscous than the others and will thus run worse as mixed. But you can thicken it a bit, and thicker mixes also work fine as adhesive. I've done a few tests on edge glued plywood and the plywood always breaks first. Can't ask for more than that.

Mixing at 2:1 is alway easy from as little as 7 ml (I use small medicine cups) all the way up to a pint or so. For most jobs, I've been using clear plastic cups from Walmart, purchased by the hundred. I carefully calibrated three cups...blank cups fitting inside the calibrated cups....for 30, 60 and 90 ml batches (about equal to 1, 2 and 3 oz) and have never had a bad mix....but I always pay attention when measuring. I'd be just as happy pouring out of the bottle as using pumps, which are not accurate.

And they also stock almost any cloth or tape you would ever want. I give them a call, leave an address and credit card number and within 5 days, it's on my doorstep.

Most recent purchase was $87 per gallon of Resin, vs. $102 for West from West Marine. RAKA would yield 1.5 gallons of mixed epoxy, West 1.2 gallons.

I liked the concept that System 3 developed with specialized mixes for laminating or glueing and another for coating....but couldn't justify having both types around or predict how much of each I'd be needing. But having a really strong glue that is thick and won't ooze out of joints would be attractive. I also played with the System 3 fillet mix. Very good stuff and a long pot life....which is good for that type of work. But again...too many containers for me to keep track of. And the fillet product is not a liquid so it has to be measured by weight. Accurate...but involves math and scales. Too much work for me.

[capt jake]

Regardless of what West says, it is more amber than Raka. Seems I paid close to $120/gallon for West (the 207 is significantly higher than 205 or 206). That is total cost of resin and hardener.

Maybe just me, but the Raka sets harder in a shorter period of time, still allows the same open time to work it though. I usually mix the slow and fast hardeners together, to modify the open/working time.

I have been buying the 3 gallon kit from Raka; $144/3 gal.

[Stump]

I also was a little concerned about not being able to order via the net. I took the plunge and made a phone call...this was just after several hurricanes went though the area. Even though at the time they were without power and had other issues happening locally, I recieved the items I bought promply and in good order.

VERY happy with their service and product. Only issue I had with the epoxy was operator error. :roll: :wink:

[Howard]

this is drifting from the original thread, but since we are on epoxy...has anyone but me noticed that some batches tend to "spread" the two pieces being glued? As in it somehow swells and spreads the two pieces being glued.

First noticed this with a plywood butt joint. Dry, they fitted so tight you couldn't have slipped a cigarette paper between them, but after the joint cured....it had maybe a 1/16 inch or more joint between the two. I've since seen the same thing happen a few times...unless it is very tightly clamped, which I don't do with epoxy.

Also....clamps left tight are later found loose after the piece cures. I can see that if some of the juice soaks in........it all still works, just noting some strange things that happen that I wouldn't expect.

[Levi]

I have built several times with their product and it almost seems like they are a local business because other than having to call them to order product, I seem to get delivery as if they are not 1000 miles away, but local. Calling & talking to someone gives me more confidence in getting exactly what I want. They answer questions on the spot & give you the right amounts for what you tell them you are building (including glass & epoxy with fast or slo hardeners.) I told them I was building the Maine Fisherman Skiff and bingo, the right amount of glass for the job. My friend built the Vacationer and had the same good service. Most epoxies have their merits but Raka is so easy to deal with and the product is amazingly forgiving to first-time users as well as old pros. I have never had a bad mix using their pumps (5 builds so far) and the product mixes easily with cups also. No, I dont work for Raka but they sure work for me....