lauan plywood voids

Hi all.
I'm just starting my first build, a 5m kayak and I am a rank amateur. I am making it out of lauan plywood....I have no choice at this point as where I live it's all I can find.

My question is "What can I do to improve the strength of ply with voids?"

It's a hard chine so i can see some voids around 3mm in diameter running through some of the strakes I've cut out. I was thinking i could try to get some epoxy inside with a syringe or something or perhaps just glue another piece of lauan over the top on the inside of the hull of course.

Any advice is greatly appreciated. Cheers.

 

Epoxy will do fine. Don't worry too much, the boat should last long enough to have fun with. A mistake many novice builders make is trying to get a masterpiece on the first attempt.

 

I'd be maybe a little concerned that the ply wasn't waterproof glued what with the voids and all. Probably is, but is it always the case the glueline is a dark colour in that case ?

 

Thanks for taking the time to reply Gonzo and Mr E,
I'll inject some epoxy then. The ply most certainly isn't waterproof glued but I'm going to the glass the whole thing inside and out. I have heard there is marine ply in Saigon but I'm central Vietnam and it's not available here.
It's more a structural concern, not wanting to build a death trap....guess I'll just glass the crap out of it. I agree about not making a masterpiece, already thinking of the next 2 to build. Getting my head around freeship and bulkheads is a another story, lots of staring at a computer screen with a beer in my hand. No proper designs yet but have drank a lot o beer.

 

I'd just follow the plans, not worrying about the voids. The glass inside and out is what really gives the boat integrity. No need to over-glass it to compensate.

 

From a structural stand point you should be concerned about the number of veneers used in the panel and their relative thickness to each other. Many times, in this type of plywood, the outer faces are paper thin, offer zero strength longitudinally. Coupled with non-WBP adhesives and a light 'glass skin, just a set of problem waiting to occur. "Glassing the crap out of it" will just make the kayak heavy, but it'll still likely delaminate with use. Small craft, especially human powered light weights like yours, will flex, twist and strain ever veneer glue line. If the veneers are thin, the glue not WBP, nothing short of 4 - 6 mm of 'glass skin is going to help, which will make a kayak a pretty heavy thing. Build it, have fun, don't go crazy trying to make a silk purse out of a sow's ear and you'll be fine, knowing it's limitations in durability.

Michael, a light 'glass sheathing doesn't really add much to the "integrity" of the boat, in regard to strength and stiffness. What the sheathing does is help seal and waterproof the seams and panels.

 

Hmmm...really?

The advantages of sandwich cores on strength is pretty well-known, no?

Empirical data - on my strip-built kayak, prior to glassing, the hull perpendicular to the strips was weak and flimsy and would not hold much weight without deforming or breaking. Cedar strips are weak. After glassing it's a structurally sound craft. Same thickness (about 1/4") as his luan panels.

 

You have to compare apples to apples. A plywood, taped seam build handles loads and stresses differently, than a sandwich or strip planked build. There are also several different types of sandwich and strip build types, but a taped seam plywood build doesn't really have any other variants except the few that combine a couple of different build types, which in itself is different.

Simply put, if you skin a 1/4" plywood kayak with 2 ounce (68 GSM) cloth, you'll get about the same amount of "strength and stiffness" as a 4 or 6 ounce (135 - 200 GSM) sheathing. The real benefit of a sheathing on a kayak or canoe is abrasion resistance. You'll need a fair bit more skin weight, to make significant differences in strength and especially stiffness, on a taped seam plywood kayak. Of course, this comes with a weight.

 

He said he was going to "the glass the whole thing inside and out", so it will be a sandwich construction as opposed to just taped seams.

OK, now I'll have to go out in the garage and make up a couple of test "coupons" of luan plain, luan sandwich, and okuome and plop some weights on there and see what happens.

 

Well, ply that isn't waterproof glued is going to delaminate, is it not, even if in sight of water ! Poly resin is not waterproof, and water will infiltrate the veneers.

 

What he said was he was going to "glass the crap out of it" which is self defeating on small plywood craft.

Just because something has a light sheathing, doesn't mean it's a cored or sandwich type build. It just means it has some goo and cloth on it. A sandwich build requires two materials (core and sheathing skins), neither of which is sufficient in dimension to serve as the complete hull shell. This isn't the case with a taped seam build, in spite of a sheathing or not.

 

Seems to be a bit of debate going on, which is healthy. The beach is only 100m away so I'm not to worried it it's a bit heavy, It's a learning process for me so I'll go ahead and make it... glass inside and out and a bit extra where I feel it needs it. I just want to get paddling, I wont go far out and I'll try to give it a hiding and see how it all pans out. Will let you know down the road what happens.

 

I had assumed that he was going to use Epoxy - not Poly

Polyester is a bad solution for the OP.

If the OP is stuck with Polyester resin, then the plywood can be best regarded as a sacrifical mould for a 100% glass craft.

 

I built my first Buccaneer with a ply called "Samba" mahogany.
I think the name Samba was a proprietary name and it was just Luan ply.
It had a 3mm core with 1.5mm skins. Staple pressure had to be exact so that the staple head did not crush the inner core. Voids were negligible, the glue was waterproof and it worked just fine for me.
This was just before the saturating epoxies were available and so the hulls were coated on the inside with copper napthenate,(Green Cuprinol), and on the outside with epoxy tar. This was not up to the present day methods, but was an interim stage leading to the products like the WEST system, Bote-Cote epoxies, and others.
It worked well at that time and made a very light structure, which produced a game changing trimaran.

 

Sam, there are two issues here.
1. Waterproofness of the glue used. If the luan ply gets moisture intrusion AND your luan ply used non-waterproof glue then they will delaminate. You would have to soak/boil test your luan ply to know the property of your plywood. There is no moisture resistance standards for luan ply made for door skin or underlayment.
2. Even if glue used for your luan ply is waterproof, there isn't specification for delamination from repeated twisting and flexing. I have seen unevenness of adhesive strength between exterior sheath (APA CDX and ABX grade) plywood. Lay the plywood flat on uneven surface (dirt or over 2x4 scraps) and step on them few dozen times, then some of the plywood will delaminate partially. No amount of glass and epoxy will prevent this kind of delamination.

If you have no access to any marine plywood then I would test small pieces of sample plywood (4"x8") and soak in water for few days and also do boil test for an hour. Take the soaked/boiled sample plywood and twist and flex them for delamination. If the boiled plywood delaminates but not the soaked plywood, then I will assume the plywood is water resistant but not waterproof. If soaked plywood delaminates then it probably is not even water resistant. I would be cautious about spending money on non-water-resistant plywood boat. The fiberglass fabric and epoxy are expensive too.

If plywood is at least water-resistant then I would flex the plywood to see if there is any weak adhesion between plies. You can put 1x4 or 2x4 wood spaced 2 feet apart and gently walk on them to flex plies. Do this in x, y, and 45 degree directions. If they survive flex regiments then I would use them for boat building.

I would build exactly as designed but I would add additional 4-6" width glass tape layer on keel and chines for added abrasion protection to resist moisture getting into plywood core.
Just my 2 cents.