I understand about scarfing, but....

[coopercdrkey]

Novice builder, here, so be kind with your rebukes!

I was just wondering, since scarfing plywood to gain the necessary length is so prevalent, is it possible and prudent to consider laminating sheets for thickness?

Specifically, several of the designs I am considering for my next project require a relatively small amount of 3/4", (or 18 mil.), plywood for the transom. Other requirements will result (I think) in leftover 6 mil. and 12 mil. plywood. I have worked with some pretty stout adhesives over the years, and I believe I could glue up enough for the job.

Comments solicited- Criticism humbly accepted- Smart a** remarks no problem! (I'm prone to make them on occasion....)

[Gib Etheridge]

It's done all the time as double layered bottoms and decks and as cold moulding. Suggest you use thickened epoxy spread with a fine notched trowel to avoid any voids. Roll or brush a thin coat on one piece then thicken it and apply with the trowel to the other.

[Tom Lathrop]

Sure, done all the time.

[wizbang 13]

yup, commonly done, even good practice. It uses lots of glue.
But don't use it in lieu of scarphing. do both. ( don't butt the joints)

[Ian McColgin]

It's an especially handy practice for things that go through difficult bends, but whether flat of bent you want to be very sure that you don't leave voids. I go with unthickened epoxy on both faces, then trowel on your thickened epoxy spreading it but having the spread a bit thicker towards the center. Then as you bring the two sheets together apply pressure starting at the center and moving out so you see some squeeze-out at the edges.

On bigger boats, this is often done with cambered decks. With bigger heavier dories (no need in light units like a swampscott since that would be 1/4" or 3/8") you might have the boat cradled somewhat on its side to make hanging the plank easier. Dryfit with very smooth nails at key edges to ensure an accurate fit to the layer already on, slather up and get the guide nails alligned. Then start fastening through the existing layer to the frames to pull the parts together. You may also need to put on sand bags or nail on temporary slats spanning the width of the plank to get the pressure for good squeeze out. With slats, I like 1x2 that have one face a little convex so when you pin the ends it applies more pressure to the center.

Epoxy does not take more than contact pressure, but there's no reason to make a thicker layer than it takes for a bond.

G'luck

[tigerregis]

Another, this is why I love this place.

[Bob Smalser]

Even in a traditional boat, lammed plywood for high-stress parts like this mast step can be stronger and less prone to split, especially with a couple layers of glass and epoxy added to the surface.

[coopercdrkey]

Many thanks! My next project just got (financially) closer!

[Bill Huson]

Speaking of transoms - after I ripped out the original partly rotted transom on my `87 Starcraft runabout I replaced it with 3 layers of 1/2" fir Marine plywood soaked and laminated with WEST resin. Other lams = cockpit sides on my outboard race boats, and of course the transom/engine board on the race boats.

Downside - uses up more square feet of pricey marine ply but in many case saves bucks if you only need a small thicker piece.

[Michael Wick]

In laminating a high-camber plywood deck, I used two layers of 3mm occume epoxied together. To make sure there were no voids, I drilled occasional small holes in the lower layer. It helped my confidence to see the thickened epoxy come through.

[Tom Lathrop]

In laminating a high-camber plywood deck, I used two layers of 3mm occume epoxied together. To make sure there were no voids, I drilled occasional small holes in the lower layer. It helped my confidence to see the thickened epoxy come through.

To do that once to see that your laminating method works properly is fine but:

This admonition to religiously avoid any voids (small pun) in a limanate can go too far. Many perfectly acceptable building practices do include intentional voids in the schedule. I use a glass/plywood/foam/plywood cambered deck and cabintop which obvioulsy includes a lot of air. Gudgeon's bible on wood/epoxy construction uses the same kind of construction in many projects. Balsa core hulls are mostly air as balsa is so porus. Nomex core is probably 95% air.

There are many cases where voids should be avoided (sorry), like laminates subject to compression loads and fastener locations, and long linear voids in interior plies of plywood are always bad. Voids near the surface ply are also suspect. I am not advocating voids at all but spending too much time or effort worrying about them is unnecessary. Don't take this out of context, interior voids are only a non issue if the rest of the structure is designed and built correctly to meets the needs at hand. In our case with plywood boats, that usually means that environmental sealing is all important, as it always is if these structures are to be long lived. Keep the water out and all will be well---epoxy lives!!