Foam core laminate design/build questions

[wet feet]

The job seems to be progressing in a rather individual way.It certainly wouldn't have been my choice of method,but it may work well enough with some trouble taken.You have almost zero chance of wrapping the edges of the foam without a void behind the glass and that will be the single biggest source of defects if you aren't super careful.How you deal with them and how to minimise the amount of repair work will be challenging.Personally,I would have added sides that extend down a couple of inches and gained some stiffness into the bargain.You might be surprised at just how large a radius can be applied to the outside of two 12mm sheets without breaking through and you can add a corresponding fillet to the inside,which would make it quite easy to wrap glass round the curves.

 

[TeddyDiver]

300gsm Biax goes well around R5mm rounded corners, even tighter. Just use a bit thickened epoxy beneath..

 

[Steve W]

As others have pointed out, waaay too much glass and too thin of a core. One ply of the 17oz on each side of a thicker core would be more than adequate. Core thickness contributes more to the stiffness than the thickness of the skins. For example a 24 ft ULDB sailboat i designed and built back in the 1980's when stitched fabrics were a new thing had/has a single ply of 12oz double bias on each side of a 1/2" H80 thermoformed ( round bilge hull from 4x8 plain sheets) core with acrylic modified epoxy matrix and it is still like new 37 years later. It has never had even the slightest structural issue since new. Of course it has localized extra layers where needed and a very stiff light interior structure. I have built 5 hardtops over the last 50 years by a number of different methods from ply/foam/ply to some balsa cored in temporary female molds and one where the 2x400 watt solar panels are the hardtop. You really need to keep them light. A thicker core also allows for a nice large radius on the edges which the glass will wrap over easily and give the look of a turned down edge. Btw, good choice on using epoxy if working in your basement, the best reason to use epoxy really.

 

[gonzo]

We made many hardtop with foam core. Our solution to the tight radius was to first laminate the top and bottom. After the resin cured, we cut the pattern of the hardtop, then scraped off about 1 inch off the sides. The gap got filled with a mix of resin, silica and microfibers, which can be routed to get the rounded edge. The finish with a layer of 4 oz cloth.

 

[SV Luna]

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.

Would you happen to know an old friend -Oscar Goldman? I appreciate and value the advice. I have a 24" x 48" panel of 12 mm H80 on the way. only $110 delivered.
I should of ordered it right off the bat because shipping is nearly half the cost and my material was only about 10% over build estimate. I used some to experiment with to get a feel for the foam. I love the way it is shapeable with a light touch. I also hate it for that reason.
Between you and the other generous folks on here I think I've learned a lot so far. It's not a reflection on the teacher if I'm off kilter. so here's my revised design along with a fact I've been avoiding.
First the avoiding problem is the back stay (mast support cable to the stern) goes through the hard top. I don't like that for a lot of reasons, mainly safety. The original hard top had a 3" diameter hole for the cable to pass through. It's along the ridge line about a third from the stern edge of the hard top. I'm going to put a 7" piece of 12mm foam centered over the 3" hole tied to a 5" possibly 7" wide longitudinal from forward to aft for a 25mm thickness down the ridge.
Replace the teak with foam core. I might mount the teak on the finished top as a rub rail for the boom.
or if I need a place to hit with a hammer.
Add 2" wide 12mm foam around the perimeter on the bottom for 25mm thickness and shape a down turn outside, with an epoxy fillet on the inside edge.
Add a D shaped gutter strip on top, port and starboard.
My measurements from the original hard top to centerline of attachments are going to be skewed since I'm taking a flat top and arching it a bit. Not a whole lot. but enough that I'm going to do a test fit before attaching the mounting blocks. Love to find some Coosa board or similar laying around.
Well thanks for reading and as always look forward to any comments.
here's a picture of Luna on the hard with the original hardtop before complete demise.

 

[SV Luna]

The job seems to be progressing in a rather individual way.It certainly wouldn't have been my choice of method,but it may work well enough with some trouble taken.You have almost zero chance of wrapping the edges of the foam without a void behind the glass and that will be the single biggest source of defects if you aren't super careful.How you deal with them and how to minimise the amount of repair work will be challenging.Personally,I would have added sides that extend down a couple of inches and gained some stiffness into the bargain.You might be surprised at just how large a radius can be applied to the outside of two 12mm sheets without breaking through and you can add a corresponding fillet to the inside,which would make it quite easy to wrap glass round the curves.

Thank you for the reply. Yes i agree it is not the ideal build method or approach. I'm incorporating additional foam and removing the teak insert. It is difficult finding information like the knowledge on here. Oddly as many times as I googled questions this site never popped up. I tried an AI search recently and there it was. much to my benefit.

 

[SV Luna]

300gsm Biax goes well around R5mm rounded corners, even tighter. Just use a bit thickened epoxy beneath..

Thank you for the reply. after I'm done with the core modifications I'll do a dry run with the glass to see if I'm confident to try .

 

[SV Luna]

As others have pointed out, waaay too much glass and too thin of a core. One ply of the 17oz on each side of a thicker core would be more than adequate. Core thickness contributes more to the stiffness than the thickness of the skins. For example a 24 ft ULDB sailboat i designed and built back in the 1980's when stitched fabrics were a new thing had/has a single ply of 12oz double bias on each side of a 1/2" H80 thermoformed ( round bilge hull from 4x8 plain sheets) core with acrylic modified epoxy matrix and it is still like new 37 years later. It has never had even the slightest structural issue since new. Of course it has localized extra layers where needed and a very stiff light interior structure. I have built 5 hardtops over the last 50 years by a number of different methods from ply/foam/ply to some balsa cored in temporary female molds and one where the 2x400 watt solar panels are the hardtop. You really need to keep them light. A thicker core also allows for a nice large radius on the edges which the glass will wrap over easily and give the look of a turned down edge. Btw, good choice on using epoxy if working in your basement, the best reason to use epoxy really.

Thank you for your reply. I'm going to incorporate additional foam around the perimeter and along the ridge line to add stiffness and a little esthetic as a bonus. I detailed my plan above and open to comments.

 

[SV Luna]

We made many hardtop with foam core. Our solution to the tight radius was to first laminate the top and bottom. After the resin cured, we cut the pattern of the hardtop, then scraped off about 1 inch off the sides. The gap got filled with a mix of resin, silica and microfibers, which can be routed to get the rounded edge. The finish with a layer of 4 oz cloth.

I actually did see this technique on youtube and it was an option I might of considered more.

 

[wet feet]

Thank you for the reply. Yes i agree it is not the ideal build method or approach. I'm incorporating additional foam and removing the teak insert. It is difficult finding information like the knowledge on here. Oddly as many times as I googled questions this site never popped up. I tried an AI search recently and there it was. much to my benefit.

You could substitute a less oily wood for the teak and retain the ability to add fasteners.A sketch detailing what you plan to do around the periphery might be helpful in conveying your exact intentions,but adding thickness there is a good principle.

 

[fallguy]

You can run a router with a bearing to build edges; just avoid dwelling.

 

[starcmr]

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 for espaço invisível grande...
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
View attachment 201813View attachment 201814View attachment 201815
cut away of original-topside original bottom original mounting point
View attachment 201816View attachment 201817
design foam shaped and curved starboard

View attachment 201818
aft

To maintain strength while minimizing weight, consider using 2 layers of 1700 biaxial fiberglass and 1 layer of 6oz or 8oz woven fiberglass on each side. This will provide adequate strength and rigidity without adding unnecessary weight. You can also use Divinycell H60 for a lighter core while still maintaining structural integrity. For attachment points, reinforce with additional layers of cloth or patches of carbon fiber or kevlar. This should help keep the weight down while ensuring durability and strength.

 

[carputech53]

Would you happen to know an old friend -Oscar Goldman? I appreciate and value the advice. I have a 24" x 48" panel of 12 mm H80 on the way. only $110 delivered.
I should of ordered it right off the bat because shipping is nearly half the cost and my material was only about 10% over build estimate. I used some to experiment with to get a feel for the foam. I love the way it is shapeable with a light touch. I also hate it for that reason.
Between you and the other generous folks on here I think I've learned a lot so far. It's not a reflection on the teacher if I'm off kilter. so here's my revised design along with a fact I've been avoiding.
First the avoiding problem is the back stay (mast support cable to the stern) goes through the hard top. I don't like that for a lot of reasons, mainly safety. The original hard top had a 3" diameter hole for the cable to pass through. It's along the ridge line about a third from the stern edge of the hard top. I'm going to put a 7" piece of 12mm foam centered over the 3" hole tied to a 5" possibly 7" wide longitudinal from forward to aft for a 25mm thickness down the ridge.
Replace the teak with foam core. I might mount the teak on the finished top as a rub rail for the boom.
or if I need a place to hit with a hammer.
Add 2" wide 12mm foam around the perimeter on the bottom for 25mm thickness and shape a down turn outside, with an epoxy fillet on the inside edge.
Add a D shaped gutter strip on top, port and starboard.
My measurements from the original hard top to centerline of attachments are going to be skewed since I'm taking a flat top and arching it a bit. Not a whole lot. but enough that I'm going to do a test fit before attaching the mounting blocks. Love to find some Coosa board or similar laying around.
Well thanks for reading and as always look calcular finiquito forward to any comments.
here's a picture of Luna on the hard with the original hardtop before complete demise.

Yes i follow this