Foam core construction method

Hi all, first post hear but have learnt a lot reading in the background.

We have a Prout Snowgoose 35 Catamaran which, unlike later Snowgeese, has coffin Shapes aft cabins, i.e. only 4ft high. In the 2 years we’ve owned the boat the kids have grown and we’ve realised these aft cabins aren’t going to work for our long term plans so we are looking at adapting them to match the later models. A few other ‘35’s have done the same so not breaking new ground here and I have attached a couple of pics showing the original and ply mockup of what I am looking to achieve.

The simplest way I can see to bond the new section to old is to cut out the section no longer required and form a mould out of 20mm foam, then grind very long bevels at all contact points before laying the grp (Biax with West System). That is all fine for the outer surface but it’s the inner I have the query on.

The hull is solid grp so no core at present but I will be insulating everywhere. I have attached a sketch showing the 2 options as I see it.

Option 1 would mean having to remove the foam mould and do loads of clean up but would give an excellent bond inside and out. This would then need insulating and some sort of hull liner panels attaching.

Option 2 is my preferred and in this case the 20mm foam mould would remain in place so I would epoxy it to the hull then apply a 2nd, internal skin. This would then also continue past the foam to be bonded to the existing hull, effectively making double skin foam core panel but with the outer skin being structurally strong enough on its own.
The big advantage here speed of construction as the foam would stay in place as insulation and the inner surface would just need fairing and painting, no additional lining. However, I have absolutely zero experience of foam core past adding a few stringers so here, at last is the point of this post. Can any of you see any major reasons not to go with option 2? What have I missed?

All advice and opinions welcome

 

With big flat panels like that the core will make it much stiffer with less glass - and be easier to build. I don't see any problem with your option 2. Is there a way you could tack the foam together and then remove the assembly - maybe with a plywood cradle to hold it aligned - and then fillet and glass the inside? It might be much easier to fit these in place and then do the glassing and fairing of the inside at least off the boat. Overhead stuff is no fun!

Another possibility: Could you assemble the thing from pre-laminated foam cored panels that you lay up on a sheet of something flat in one shot? If you cut and fit the foam and then took it apart and sanded some 2mm or so rebates into the outside edges and then glassed both sides... you could re-assemble the already smooth and flat panels and tape them together and fair them - just to make it easier to keep the large flat areas flat and smooth. Foam core might be floppy and make for more fairing later on... You might also be able to get good results with polyester or vinylester resin so you could gelcoat the completed area and avoid large-scale repainting? Would be cheaper and faster too.

 

Stay in place plug. Modified option 2.

Basically, you need to support any foam structure until glassed and bonded because it has nothing on its own.

If you use say a 12mm foam; you would get some insulation and need none.

I would build a jig from timber and plywood all the way to the existing grp or jist short of it to allow for core and glass to flush to existing.

Mock up the male plug over the timber jig. Mocking up 12mm core is really almost fun. You can use 1.5" screws to temp hold the panels. radius edges, tape relief meeting sides

Then laminate the flat panels on the inside side on a table. Lamination depends on the spans. Then put the panels back up and screw in place with thickened resin to glue in place. Use ship tape on the jig to keep from sticking. Keep in mind the jig must be removed when you put it together. After it cures, glass the seams inside; then glass the exterior. Use 3/8"-3/4" radius. Relief the existing hull and anywhere outside you want tape seams or overlaps.

Remove jig and Glass inside seams after outside is all done.

Finish. Doubt you need any insulation.

 

In the context of your ply mockup, the mockup is probably a bit too wide now...and high and would not support the glass foam perhaps.

And it would be a pita to remove.

Another way to do this would be to use marine ply and laminate the core to it and glass the core on the outside and only glass marine ply seams on the inside. But spanning say 6 feet is a bit much with just ply and core..I would probably build a thicker roof so it won't sag. Perhaps marine ply and then two layers of 12mm core bonded to each other and glass on top say 24 oz.

 

Thanks very much for your replies, sounds like I need to tweak a few methods but basically on the right track. The ply mockup was really just a quick and dirty thing to show my wife what it may look like so don’t take too much from that.

I hadn’t really considered that the foam itself might sag across those spans. The widest point across the roof is about 600mm, is that too much?

Interesting that you both say about laying up flat panels off of the boat then bonding together. That would certainly make life much easier in terms of glassing upside down. My thought was using large sheets of glass inside going from roof right down to overlapping the existing hull by a good 300mm inside but am probably over engineering things as usual so would a 6 inch wide tab be sufficient?

One concern is the shroud chainplates will be attached to the outmost side so I want to make sure the new top won’t be pulled off in a blow but it is a Cat so adding too much extra weight is not ideal.

I’m not sure I fully followed with the ply and double foam roof idea. Do you mean it all stays in place to give a 36mm thick roof (12mm ply, 2 x 12mm foam) plus glass inside and out? The existing coach roof is approx 6mm thick glass so I was thinking the same as this on the outer face plus a maybe 2mm thick layer on the inside of the core. Too much?

 

There is no reason to do 6mm of glasswork by hand! The original builders had a mould and production facility. If you try to build a 6mm thick glass shell; you'll not like the result, nor the effort needed.

You said the widest area is 600mm, but there is a length, too. The real concern is someone walking on the area or stepping on it.

There are basically two ways to do it.

You can build a stay in place plug or you can build a removable one. Up to you.

Stays in place use 6mm marine ply for the mould. You can glass it on the bottom side or not. Seams must. Laminate foam to it with thickened epoxy using a 1/16" trowel. If it has a curve anywhere, use pieces or scrim foam. If you are worried about people breaking it or want more insulation; add a second piece to the top. The top pieces need to stack on the sidewalls for strength. Then glass the exterior and the interiors seams. You can relief the side and top seams and the seams meeting existing. You can use two layers of 400 g stitched biax and a 200g woven overtop of those if you like, but the 400 is enough. Seams are easiest with 600g/225, but you want thinner seams, so 600 will work, but tricky to work with for amateur.

otherwise, you can build it all in foam, you glass the foam on the inside with say 2x 400g stitched biax and then build it all over a removable jig. Glass exterior. Then you go inside the boat and glass the seams real well with say 150mm and 100mm 600/225 tapes.

The nice thing about the plywood method is there is almost no work to do inside. One or two layers of 200 gram staggered tapes on filleted seams is all you need. It'll look noce enough sanded and faired with a single pass of compound to paint. It costs a bit more, but finishes easier inside and is faster with no glasswork needed on the foams.

For the outside work, make sure to use peelply and masking tape run stops or the resins will run all over the boat.

A 150mm overlap is plenty. If you two layer the exterior, the first overlap is 150, the next one is 100.

Other glass options exist. I am just going with what I know and have done. The picture is a built up plywood foam roof. It is awfully big to do lots of inside work. All we did was glue the plywood to beams below and glass the plywood to the sides, then glass taped the foams to the finished glasswork below. The 6mm ply will crack when walked on before foam and glass is on. The building up helps disperse point loads.

It should be noted that I had a temporary beam in place to support loads until the glasswork was completed. You may wish to also put a temp beam across the space to avoid a sag until cured up. The boards you see here are to avoid point loads cracking the 6mm ply; not needed after glassing foam.

 

I did not want the hassle and did not have enough glass for the seams, so we had to use tapes and seecondary bond the roof glass to the tapes. You can do yours in a single go or the same way. Except you'll get humpy on the existing. The foams had a relief cut in to reduce fairing...both sides.. seams are 2x 600/225.. it would have been nice to peelply this, but just so big to peelply well, hard enough to wetout

 

Reliefs are hard to do for a one off job like yours. You can buy an electric planer or find someone with one. Set the planer depth for 3mm and guide for 2.5". You will want them where foam meets foam not in plane.

You can also get away with a single layer of 600g stitched biax and this would reduce the humpiness, but it isn't really enough for the seam alone...

 

I'll first caveat this by saying that my experience with grp work is very limited, so perhaps others with more experience would care to comment, but a couple of thoughts.

1. Assuming a fair surface to the outside is more important than to the inside, would it not be possible to construct an accurate external (female) mould insitu, somewhat similar to your ply mock up, but out of mdf to give a fair finish, then transfer the mould to a table, the other way up, radius fillets in the corners, and build up your laminate and foamcore sandwich on the inside? You can add a lap joint to the mould, which would slide inside the existing and then allows the finished piece to be glued in place.
2. Is it possible your 'roof' might get sat or stood on? is strong enough to take it?
3. If you do end up glassing insitu inside, ensure you have plenty of ventilation, and only work a small area at a time - especially upside down. My only experience of trying to do this was horrendous - both with fumes from the resin, and trying to get an over ambitiously large piece of wetted cloth in place upside down.
   
   

 

Thanks very much for taking the time to write such detailed replies, it’s clear I need to give this a lot more consideration that just cutting off the top, bonding foam in place and glassing 6mm over the top.

One other question, the later models of my boat use 1/2 inch balsa core for not only the aft cabin roof and sides but also the entire coach roof. This span is at least 6ft x 8 ft completely unsupported on the coach roof (albeit slightly curved) and is plenty strong enough to walk on. Would that be a more suitable core option?

Many thanks

 

What would be more suitable is some beams. The ones pictured here span 8' length and are 3x0.5" thick and have a camber. They are for a roof about 12' by 8' with a few walls below. Whether it is walked on is a big factor in designing the structure.

The alternative building up with plywood as the bottom and glass on top with foam in between to 24mm is quite good, but it may sag a bit at 3' without any beams. Another fellow on the forum built a much larger roof with marine ply on both sides of the foam. I can't recall the thickness.

To your question, a heavier core material will not help. The rigidity of the core and structure is perhaps one of the best kept secrets in boat building. But the stiffness is defined a bit by the cube rule. The cube rule basically says that the stiffness of a sandwich is related to the thickness. If you have a thickness of 12mm, something 24mm will be not twice as stiff, but 2^3 stiff or 8 times as stiff. So of you want to avoid beams; you need to build the middle of the sandwich fairly thick...like 24mm or so. Easy to do. Or use some beams and build it like I did. But remember, mine is walkable.

Quite honestly, I would build it with supported from beneath 6mm marine ply top and sides and laminate two layers of 12mm foam core on top and one layer on sides, making sure the tops sit on the sides. You may need to use hotglue to hold the side panels in place on some blocks; hotglue can be broken easily later if you only make a dot of it. Relief the seams 3mm for 2.5" each way before putting up. Then radius the exterior foams once glued up and two glass tapes 4" ~600/225. Then fair those tape seams or prefill them with cabosil and glass the top with either 1x 600g stitched biax or two 400g same.

To glue; use epoxy and fumed silica at 1:2 by volume or add a bit of cabosil of that os too runny.

If the top seems to drop on you; instead of beams running the 8' span; you can just put a 3/4" hardwood timber on the 6' span in the middle. I doubt it will move.

In the picture, you can see temprary jigs to hold shape of walls and roof and even timbers to hold the beams in proper camber. You'll need to support the 8' dimension square during building and keep the roof from sagging until glassed up. Or, you can also add some beams. The beams here are 11.6" oc. But like I said, this was designed for walking and 100# point loading. All you need is a light shell with enough thickness to support itself.