Adhesive for laminating two sheets of plywood

[Briank2]

I've started building a plywood flat bottom skiff. Going to laminate two sheets of 12mm plywood for the bottom. If I can avoid the expense of marine glues and epoxy resin, I'd like to. I do plan to use epoxy when fiberglassing the outer hull and I'm using Bostik Marine for the rest of the joints.

Theoretically, it should stay dry between the two sheets...I will use lots of screws too, so can I get away with Elmer's Contractor Grade Probond, or should/can I use laminating polyester resin? Or something else???

 

[BlueBell]

I would advise epoxy with no screws.
But a 24mm bottom on a skiff?
How big is this "skiff"?

 

[Briank2]

I would advise epoxy with no screws.
But a 24mm bottom on a skiff?
How big is this "skiff"?

16’ with a 6’ 6” beam. I could go with one layer of 18mm but hear that flex’s a lot on this boat.

 

[BlueBell]

16’ with a 6’ 6” beam. I could go with one layer of 18mm but hear that flex’s a lot on this boat.

What does the plan call for?

 

[Briank2]

One layer of 18 or two 12s.

 

[BlueBell]

Well, there you go.
That is a very thick, heavy bottom.
But, weight in a good place.
Follow the plan... if it's a good plan.

 

[Briank2]

I should have mentioned, it’s Walter Baron’s LYSS 16.

 

[Milehog]

I'd be more wary of the "lots of screws" than the weatherproof glue.
Each screw is an entry point for water. Do you really need them?
Will the screws go from outside in and be covered with fiberglass?

 

[Briank2]

Yes. Thinking I may go with just the 18mm though, cheaper, no glue. No screws. Maybe I’ll add a keelson to stiffen it up.

 

[bajansailor]

Some more info about Walter Baron's boats here -
Power — Old Wharf Dory http://www.oldwharf.com/power

And the plans for sale -
LYS Plans — Old Wharf Dory http://www.oldwharf.com/store/lys-plans

Maybe I’ll add a keelson to stiffen it up.

That would be more effective than making the bottom of thicker plywood.
Do the plans not call for anything like this, or an outside keel?
Are there any longitudinal stringers and transverse frames?
I can see a transverse frame in this photo, in way of the console -

 

[Briank2]

There are. It calls for 2x8 butt blocks across inside of the boat to attached the bottom sheets together, with 2x8 frames/ribs attached to those 2x8s. Plus some addition.

I’m thinking adding the keelson and use some 1/2 ply butt blocks on either side of the keelson. And using 2x4s across the bottom on edge, and 2x8 for the frames which also support the deck. This way it will stiffen the bottom (reviews say it flexes a lot) and prevent water from being trapped behind the 2x8s.

 

[BlueBell]

If you are concerned about reports of hull flexing then epoxy (thickened a bit) two 12mm pieces but no screws, just pressure.
If building upside down, could you laminate the second piece after the first is installed?
Follow the designers advice.

 

[Briank2]

Yes, that's how it goes

 

[fallguy]

Well, an inch thick bottom is workboat grade. As long as 18mm is optioned, use it and never look back. 24mm is really a lot

And the best way to rot plywood is punching holes in it. All holes in plywood need to be overbored and filled with thickened epoxy.

Bottom flex doesn't matter. You can shore it up with transverse members. I have a canoe 20+ years old 3/16" bottom that reflects the waves in the water. A bit draggy; sure, but doesn't matter to me.

Laminating ply panels requires precoating both bonding surfaces with epoxy. Then apply epoxy with 1/16" to both or 1/8" vee trowel to one side. I prefer both. You willbe shocked and amazed at the volume of epoxy and fumed silica required.

 

[Andrew Kirk]

Waterproof PVA like Titebond 3 might do the trick. Although it claims not to be suitable for continuous immersion, anecdotally it works. It is often used for Poor Man's Fibreglass as a fully waterproof non structural membrane.