Foam core laminate design/build questions

So happy to of found this website and forum, so much skill and knowledge that I need. I have several questions concerning my hard top bimini construction for our monohulled sailboat. The previous owner had made one (see photo) that basically lasted a couple of years due to water infiltration. It was flat on top with only a couple layers glass which obviously didn't cut it.
I'm planning on using the same 4 attachment points as they were capable of supporting the 80 lbs of saturated material...
My design plan is a cad drawing (attached) with feet and metric dimensions. I'm using half inch H80 divinlcell as the core with the longest dimension having an additional 1/2" thick piece of foam, 6" wide, running the length. The foam is tapered 2" wide toward the aft. I'm running another piece of foam 2" wide from fore to aft down the centerline. It will only be about 1" wide for strength since it is tapered on each side.
I have the top curved to port and starboard with 3" fall in 42" run.
I cut a series of kerfs in a 1"x1" piece of teak, to glass in to the fore end, for attaching a light awning
to the dodger. I will fill the kerf cuts with thickened epoxy to add strength. I have Now comes the /8" thick foam that I'm thinking of using on the bottom at 90 degrees to the top for a rain drip as well as possible rigidity.
Now the hard part. Try as I might, I cannot figure out a properly balanced laminating schedule without seemingly going overboard. Originally I was going to go with--
foam core- 1708 biaxial, 1700 biaxial, 1200 biaxial, 6oz woven then 3 oz woven with peel ply.
Same lay up on the opposite side. That's about about 20 lbs plus the weight of epoxy at 20lbs for a hand lay up. Add the core and it's close to 50lbs. Not the end of the world but I obviously would like
it to be as light as possible while maintaining superior strength. Some experts say it's a waste to use the 1708 with epoxy since it's more for the ester type resins to bond and adds resin weight.
I just recently came across an article explaining a truly balanced laminate has fibers running at 8 points of direction with the same weight fabric. So here I am, where I probably should of been months ago, asking for advice.
Thanks in advance
Jeff

cut away of original-topside original bottom original mounting point

design foam shaped and curved starboard


aft

 

The amount layers doesn't improve the water tightness but the quality of layup and finish. Most likely the old top leaked thorough the holes for fasteners and the ply wasn't marine quality. Your top is quite large so a bit thicker core would be better, or now you've half way done some additional foam stringers, then a single layer of 600gsm (whatever that is in merican) biax on both sides. Then smaller patches round the fastener holes plus some hardwood core or high density epoxy filler instead foam core to cope compressive loads..

 

Unless you are planning on carrying a lot of weight, you laminate schedule seems too much. If you already have 1708, then it makes more sense to use polyester or vinylester resin. Epoxy does not need the mat layer. If you already have epoxy, then stitch biax would be better. The fibers are flat so they use less resin and also get a smoother finish.

 

I would recommend adding another 12mm core or 6mm st least. Then, you use some coosa 20 pound or more density for attachment points.

I found the drawing now in black above, so you are about 2M wide. You’ve put the stringers for n aft, but really you need transverse members, perhaps more than the teak.

The flat section of 6”-2” stringer is not going to help much; sorry.

And for the laminate, that is really going to depend on whether you add core.

Consider the weight closely.

A 12mm core at 3.5m by 1.75m of H80 is .012•3.5•1.75 or 0.0735 cum or 5.88kg a layer of db1700 is 10.4125kg and double that for hand lamination and double again for the other side..

Check my math, but you don’t want to trade core for glass in the stiffness game.

 

Thank you for that advice. yes it is always difficult to convey a current situation the first go around. The existing attachments on the destroyed top are, I think, solid fiberglass discs/plates that were bonded to the plywood then glassed in. It looks like instead of finishing the top glass there was a very thin veneer of some kind of wood bonded to the glass then painted. It's a shame and I don't want shortcuts to ruin the effort involved. Your recommendation of reinforcement is greatly appreciated. I'm posting a clearer image of the proposed design I've been working on. yea I know cart before the horse a bit but I have to have a tangible object to work with in order to have questions. I will have more questions to post here and I definitely appreciate input

 

Thanks for the reply. The only stresses the top should see are from gravity and boat movement. The supports are sturdy but have some flex so it's not super ridged. I chose epoxy because I'm working in my basement and the fumes from esters aren't compatible. lol. I do have both 1708 and 1700 biax.

 

Thank You very much for your reply. Here's a better visual of the cad drawing. I apologize for the poor quality of the first post.
I usually work with large land surfaces and my drafting skills are lacking lol. So the overall widest dimensions are 2 meters by 1 meter with the attachment points being
slightly less. When you refer to attachment points are you suggesting like an internal frame from attachment point to point or just for the blocks to screw into?
I'm bummed the 6" wide section of 1" foam doesn't provide strength enough to keep the shape. I initially wanted it for aesthetics then thought it might help with the stiffening. I had the beam thickness in mind but I can see the weakness just behind it now.
The dimensions should be much easier to read now. US feet and metric dimensions. I have a 1 x 1.3 meter sheet of 6mm H80 I can incorporate or should I go with adding 12mm.
Really appreciate the time.

 

I agree with gonzo about the amount of glass proposed,it seems excessive unless you have plans to stand a heavy crew member up there when reefing.I also have some reservations about glassing an oily wood ,namely teak into the laminate and believe there are less challenging wood types for the job.Absent from the discussion so far is any kind of detail about how the laminating process will be taking place.Will you be building some kind of frame to support the foam sheets while laminating first one side and then anther support to hold the partly laminated piece while applying the glass to the other side?More details would be useful if we are to help with the whole process.The additional centre strip of foam plus a modest vertical flange around the edges ought to provide sufficient stiffness if the load is only going to be rain or maybe solar panels.I'm assuming that the flat facets on top are that way to make mounting solar panels a bit easier as most deck or wheelhouse surfaces are cambered.

 

I’m with wet feet on the teak. Just not ideal. Screw holding from any HD foam cores is usually excellent with zero chance for rot.

The only reason for so much glass is standing loads. Even then, too much.

Perhaps you can speak to the total use expected for the top. If you don’t plan walking loads, I’d use Hexcel 17oz woven one piece each side of 24mm thick.

Also, is rainwater a concern? I used gutters on top of my hardtop; they work as stiffeners and also shed water away from people entering the area and no headroom impact.

You could still use your aesthetics plan if you build it up to 24mm thick.. if you are short of foam to get the entire thing to 24mm, I’d still find a way to incorporate all your foam. Glass it up one layer of 17oz hexcel or db and see if it is stiff enough. For walking loads, I’d want a bit more glass. Although I am walking carefully on the hexcel 17 on M80 core.

I walk on core M80 with 2 layers of db1200 without concern.

I do apologize. As I am not a composites engineer, I cannot offer you a deflection calculation for your top.

The top I built was about 42x60” and I used 3 layers of 6mm M80. But I am on a metal frame..

 

I used some weights and laminated three thicknesses of foam over the metal frame and it held the shape well enough to glass.

The rain gutters/diverters were glassed on with 6-10oz fabric after bonding them with epoxy putty and screws to hold, then repaired the screw holes prior to glass..etc

 

Thank you for your reply. I'll answer your questions. The top isn't going to have any real weight other than snow in the winter which has been minimal for the past 5 years.
I understand your reservations about the teak strip. This piece was cut from a board I have had on a rack for about 10 years and it is super stable. I'm planning on coating it with epoxy as I fill in the kerfs with thickened epoxy prior to glassing. For the first lay up on top I built a contoured frame ripping 2"x4"'s at the slope angel. I screwed these to a support frame beneath it so I'm off the platform. Once the top is hand laid and cured enough I will use the cut-off's as supports to flip the top and glass the bottom. Right now the frame is too high to flip so I'm going to reposition it to a height that I can turn it over. The pictures I posted are a little behind the progress and I'm including the most recent ones here. I still need to round all edges. The 6" wide strip running port to starboard is rough shaped in the photo and will be hand shaped to complete. The top has a camber of 3" sloping to the sides. The longitudinal foam is a 51 mm wide arch 6"mm high with a total thickness of 12mm. For the vertical flange I was originally thinking of putting it on the underside by using the 6mm foam notched and epoxied into the base foam with approximately 25mm sticking out to act as a drip leg for the rain and added stiffness like the original had. Having trouble visualizing the actual layup process. I did a test on a small piece of foam and getting the 1700 to wrap around from top to bottom didn't seem possible even with the edges rounded over. I was thinking of putting the 1700 to the edges top and bottom then using 3x- 6oz to wrap the edges but thinking there will be a bulge at the joints. Really appreciate your time spent helping me.


f
frame and supports,,,,,,,,,,,,,,,front with teak,,,,,,,,,,,,,front with teak ,,,,,,,,,,,,,,,,,,,, roughed in latitudinal..... longitudinal

 

Thank you for the great reply and photos. Definitely won't be walking loads on top. For some reason I was under the impression that the core wouldn't hold a screw. ok now I know for my next project. I've only done fiberglass repairs that were like 1m x 1m max and smaller. First step into anything actually constructed. Your pictures help a lot to see your lay up and I like the dual purpose gutter . The original had a glassed drip leg that ran on the underside and I was thinking of repeating that by notching the core to fit a piece of the 6mm foam in place with an epoxy fillet. Have the leg extend about a 25mm or so. Problem is I don't think now that the strip will bend to the camber running to each side. If I get some more of the foam and add it to the underside it shouldn't make a difference should it? Just increasing the core thickness and moving the neutral axis ? I spent some time reviewing some of the composite engineering calculations Then I called a Civil engineer friend and he just laughed and said good luck.. It is definitely a specialized science. I have the 1700 so maybe I will glass both sides with the improvements suggested in this thread and see how rigid it is? Once again, I appreciate the time spent helping me.

 

The teak requires acetone washing after you sand it with 40 grit and before puttywork you would also neatcoat. But not ideal.

The main difference between your project and mine is I have a probably too heavy aluminum frame to bolt to,,

For any work with additions, simple woven glass is best. It is really difficult to glass the drip edges or diverters. The reason is you have an outside radius and then the inside fillets. And when you fin roll an inside corner; you roll in, and outside corner; you roll out. So, when you have both it is a battle to not entrain air. The way I attempt to reduce it is to hot fillet. A hot fillet is sticky and tends to hold the glass a bit. I honestly don’t recall, but it looks like either a single piece or two piece effort by the pic. Anyhow, glass the big flat areas first with heavy glass, then test, then final woven over the small stuff is less fairing. Just sand the threads of db off before any additions.

I also built a cabintop that was 8’ wide. This was done with laminated timbers, 6mm ply, 12mm foam, and 17oz hexcel, but the reason I share is I used 6mm okume under it to avoid any laminate on the bottom. It is less exposed to the environment than yours, well, not at all.

If you insist on the teak, overbore all holes 2-3x the screw major and epoxy puttt fill; then drill the screw minors and use caulking in the epoxy, not timber.

 

Don’t use db on the edges. Just use on the flats and be ready to prefill with putty and then use wovens on the edges. It won’t take a hammer hit, but why would anyone hit with a hammer?