Glass both sides?

You don’t gain much putting glass in between. You gain lots by glassing the sides to the ply. So, even a single piece tabbed 2” onto the ply would be super strong.

We got real fancy doing beam deflection calculations. I just don’t want to do it. I will tell you my layup.

3x 1/2” thick by it was 2-2.5”; they were 8’ long redwood with a bit of an arch; the arch helps shed water and offers additional strength we did not calculate into our ratings

above that 6mm okume dryfit, then screwed and epoxy putty bonded

above that 12mm corecell, various widths and some scrim on the edges; edges were filled with putty; then inside grooves, then 16-17 oz hexcel woven atop; the only glass

I did not bond the beams to the plywood. The beams are about 11” oc; so things are strong as they are…

I did have the plywood crack in one spot before I got the glass over the top and that I glassed over from under. I did tie the first and last pieces in with glass as well as the ends of each beam tied to the sidewalls. Then I put a small piece of glass on each side of each beam.

 

Just a side note for fun.. those redwood sticks came from the Hamms Brewery in St Paul. They were repurposed for my boat. In the situ picture; there are two of the uncut staves from the 15’ high beer barrel they were from I put in place to help keep them in place until things were all connected.

The middle timber was also temporary and removed after the glasswork was all done atop.

 

Fallguy, thanks for sharing!
I saw a video where honeycomb was used to make a cabin top with honeycomb beams let in to the underside.
Looks really light, but isn’t the honeycomb reinforcing running the wrong direction? Does this make its strength entirely dependent on the fiberglass skin?
EDIT
Scratch that, no local source for honeycomb core, shipping costs are beyond belief, so it’s going to be either plywood or foam.
Best locally available foam is 6.5#, is that dense enough for roof or deck application?

 

Found a scrap of plascore next to some 6 pound foam next to a half inch section of coosa. I'd prefer foam over honeycomb for a beam that's open ended. It's biggest weakness is radius an edge like your drawing. My preference is coosa if it needs to hold a screw, coated foam if you don't.

 

Forget plascore. It has its place, but resin thirsty and flexural rigidity is not high alone. I used it for bdeck and bdeck cabin sole. Not the roof because both sides need glass. And like com said; open sides are another pita.

My cabintop is beams, 6mm okume, 4# foam, 17 oz hexcel. Beams are tabbed to sidewalls and okume. I plan to add insulation of 1.5”.

 

Update:

 

My post June 17, 2024, I said I did not bond the beams to the plywood. I misspoke. I dryfit the plywood with screws, then I epoxy putty bonded them. I have no idea why I said I did not bond them, I meant to say I did not tab them. I’m a bit annoyed rereading an error like that. The plywood needs to be bonded or tabbed to the supports, I just did not see reason for tabbing. Once the glass is on the top; it would take a sledge to remove it.

the work looks great; how much glass did you use? You may want to sand and neat cooat that with epoxy to fix any pinholes

 

The 1/2”plywood was glued and screwed, not tabbed, only glassed on the outside with 1 layer of 1708 and a top layer of Matt.
Not sure how much yardage, but it took about 15 gallons of resin, trying hard to get consistent wet out without any excess.
I put a coat of resin with surfacing agent over the Matt for a sanding coat, big mistake!
It was super hard to sand, should have used gel coat.
It is quite solid to walk on, I’m very pleased with that.
Inside is still raw wood, I’ve got to decide soon what to paint it with.

 

That sounds like an awfully large amount of resin, even in US gallons.

 

I really like Sherwin Williams precat epoxy for the inside. It is durable, non-marring hospital paint you can scrub. Add a drop of blue and it becomes ultrawhite with any sun coming in.

the 1708 uses about 25 oz resin per yard so 15 gallons seems a bit

 

“The 1708 uses about 25 oz resin per yd”
I think that is an idealistic number for lamination only, possibly vacuum bagged?
I glassed and tabbed a full bulkhead below as well as a big anchor hatch/curb in the bow, tabbed in the inside where the top meets the inner liner and made a lot of filler from my 15 gallons.
The total area glassed was Somewhere around 27 sq yd x 25 oz = 675 oz or 5.27 gallons not accounting for the goop, precoat, matt, and sanding coat, and miscellaneous patching on the deck.

 

I wouldn’t worry about being resin heavy. Work looks great.

 

Thanks, it came out pretty well for the amount of sweat I dripped on the work!

 

That looks like some tough paint, think I’ll get some.
What did you use for primer?

 

That precat is my interior paint. You can bump into the wall like a rum soaked sailor and not mar the wall.

For exterior, I recommend interprotect 2000e; three coats within the time window, sand, and then another; go up in the grit rating and repeat until no glass is showing raising grit as needed. Then I used Epifanes two part poly, but it blisters under immersion of a drop of rain, so not recommending it. I also recommend you stay away from awlgrip 545 unless spraying; it dries too fast for any wet edge work, but they sell it as brushable. I don’t have a great paint topcoat recommendation. If you use 2 part polyurethane, wear a respirator and work doors open whenever possible. I’d use interlux perfection if I painted again, but have not used it..