Building lightweight sandwich panels for cabinetry & interior construction

G'day.

In the near future I'm going to be restoring a 33' IRC cruiser racer, its going to need a pretty comprehensive rebuild inside.

Now I've done this with plywood in the past, it works well, is available and easy. Its also pretty heavy. This time I want to build with an eye on the overall weight.

These panels are purely for building out the interior, think cabinetry, bunks, lockers, the galley, heads, and doors etc.
Structural stuff like bulkheads and most of the partitions, floors and stringers are already in place, so these panels aren't expected to meaningfully contribute towards the structure of the vessel, the really just have to stand up to normal in use abuse.

The idea was to build large panels as flat stock and then cut them down to size as required, preferably using polyester resin for price reason. (I'm happy to use better resins if required, but if polyester is adequate it makes sense to use it.)

Has anyone got any tips, or suggestions, for maybe the best cores to use or laminate?

At the moment I was thinking maybe something like this honeycomb:
Thermhex PP Honeycomb 2500x1200x15mm | CFS Fibreglass

Or perhaps a foam core like these:
ArmaForm PET Foam Core 100kg/m³ - 15mm
Closed Cell PVC Foam Core Material; 3mm, 5mm, 10mm, 25mm - Easy Composites

There are lighter foams, but a lot of them seem more expensive.
I was thinking 600gsm biax each side (1 layer) + maybe a thin layer of tissue would be adequate, with maybe a bit extra for stuff like bunk tops.

Finishing would be either a 0.6/7mm wooden veneer or paint, depending on location. Exposed ends capped with hardwood.

To those that have been there and done it, does the above sound reasonable?

 

Ive built entire interiors out of divinycell and coosa in the pursuit of making a crewman proof boat cabin.

Currently (well after fishing ends) spending the remainder of the summer outfitting a fiberglass shell into a camper for the wife. Its given me the opportunity to chase weight savings as my boat interior build was designed to hold up to commercial use and hopefully see my career to its end so much to heavy.

Wall panels are 3/8 thick divinycell with 4 oz on the backside with a thin veil type csm front size is 3/4 oz csm and 10 oz cloth. Panels appear to be rugged and conform fairly well to the contour shape. Wall panels were shot with several coats of gel coat shot through a 3 mm nozzle and only a small amount of spray additive. Gives a small texture that cleans easy, ceiling panels we shot 2 mil mixed heavy with brush additive and cut to 320, had planned to go all the way out but the wife decides she found a cloth that looked good and the spray adhesive adhered just fine at 320 so we didnt do more. Im not done with the sette or the kitchen counter but its going to be a combination of 1/2 inch pvc foam and coosa 26.

My first interior was almost all blue water 26 and it was as heavy as ply with the heavy skin. On a friend's boat we did a bathroom and an overhead set of cubbies from a combination of divinycell and coosa. We used a single inner and outer layer of 1708 on the panels and made an outline of the door jamb and door edges as well out of coosa with the divinycell making up the other areas. It ended up so much lighter than my similar sized bathroom walls, but still had 26 lbd foam on the door edge, jamb and where all the metal hardware was fastened. Not a structural guy, but we were remote and he had a roll of 1708 so thats what we used. Id probably do a similar layup and method on a personal boat as the coosa interior was heavy and expensive.


Used to layup panels on a table, but fairing taped seams is a pain. Started to make better jigs and supports and doing as much glass at a time as I can. Have also started to get extremely particular with my peel ply for panel layups, folds or crinkles get printed through so the smother the better. Saves a ton fairing after the fact.

My current laminate table for making panels is a set of laminate faced mdf on top of a full 4x8 sheet of ply thats atop a full 4x8 pallet that some core came shipped on. It does alright, although my last table was 4x10 3/4 inch uhmw smooth tivar mounted on a similar table.... dang that thing made some massive smoth panels.

Away from my main computer and projects at the moment, did a cursory look on my phone and didnt see any useful ones. Ill see if my friend has any of his last spring work.

 

I would always suggest getting a sample of each and trying to produce a section of the laminate you intend to use,in order to get a practical example in your hands for evaluation.Of the materials under consideration for the core,I would think 100Kg/cu.m is a bit dense and 75 kg/cu.m ought to suffice.You would need a good surface to do the laminating on and ideally a vacuum system to produce the best bond between surface laminate and core.The size of the panel you work on is obviously the limiting factor that governs the maximum panel size that you are able to produce.Polyester ought to be fine,but do consider the details of bonding to the hull surfaces and to adjacent pieces of furniture in a neat manner.As comfisherman has posted,it requires a lot of care if a nice finish is the desired outcome.You might also need to plan where hardware needs to be attached to the panels as it could be necessary to include a hard insert to deal with local loads.If you need panels larger than a standard plywood sheet,it is sometimes possible to track down a furniture manufacturer who has a jumbo sheet of melamine faced chipboard with cosmetic damage on one side and who would be happier to sell it cheaply that to dump it-they are typically 2800 X2070.I have used one in the past for bagging a good number of panels together.

As an alternative,have you looked at the lightweight ply intended for camper van interiors?here are a couple of links Lightweight Ply & Furniture Lightweight Ply Furniture Board For Leisure Vehicles https://ukleisureparts.co.uk/product-category/furniture/board/ Shop 15mm Lightweight Ply (Collection Only) On-line - High Peak Conversions https://highpeakconversions.co.uk/category/furniture-construction/15mm-lightweight-ply/ . It could be quite a time saver,as it often comes pre-finished.

 

Thank you both, some stuff to consider.

Coosaboard is out for me, its not superlight, and its basically unavailable in the UK or only at insane prices. (£400+ a sheet I've seen)
The plan if making my own would be to vacuum bag it, its something I've been meaning to learn for a while, and pieces of flat stock seem like a good starting point.

Wetfeet, I have looked into the lightweight stuff, I'll have to ring a few of the suppliers up as they seem a bit coy about putting actual weights onto the websites.
Good tip for the larger panels, but I don't think I can even fit an standard sheet through the companionway of the boat, so I won't need anything bigger.

 

Wet feet pointed out a good point on the time.

Im not a craftsman, but have spent the better half of the last 20 winters kicking about boatyards working on projects for myself and others. Id rank myself on the solid hobbiest level, far from pro but do alright. Maybe better than average with aluminum and below with wood work.

The time it takes me to make all composite one off cabinets, walls, cubbies, settes, generator boxes etc... is by far and away the most time consuming method. Quality ply being so much faster to build.


I'll give a quick example.

Wanted to do a fairly large storage space up against the hull above a galley on a friend's boat. He'd liked my composite furniture so we decided to make one for his.

Steps after we'd cleared acess to the open hull.

Laser lined up the basic shape
Made strip jigs and mockup for door faces.
Cut and fabricate coosa to shape and fasten together... here is where you'd be done with laminate ply.
Then we built a jig to hold the foam core, did the hand lamination being tedious to make it as fair as possible.
That night we built another jig to hold the setup to glass the backside. Less care needed on the inside but its still a tedious layup.
Next morning its mark and cut doors for the cabinets.
Then an extremely long tape session to prep for the door flange.
Layup cloth and core strategically for the cabinet sill, as well as hinge area strut and cam latch hardware.
Pull tape and trim all the sills. (Keep in mind this is slow work compared to jig saws on ply)
Then cut outside of cabinet to shape from lines marked from jigs.
Down inside the boat, jig was good so only took about 20 minutes to do final fit.
Grind periphery of hull for fit.
Tack in place with hot glue
Use tabbing putty on periphery and glass the inner layers (on this one ts 92 inches long 16 tall and 20 deep so its a long putty and tab layup)
Second layup on the outside, lots of extra care as its the observable side.
Pull all peel ply and do sanding
Two rounds of fairing got it good.
Spray gel coat on cabinet and doors, in our case we just did 2x coats and it finished well enough for the owner we didnt do after the fact cut and polish as he liked the faint matte orange peel (think we shot it with duratec and a 2mil so fairly smooth).

Guessing its about tripple the time of a wood cabinet from start to finish. Some carry over like jigs and hardware install but the rest is significantly higher effort.


Not to dissuade, just want to set expectations. To me its still worth the effort on boats I intend to own a long time, but its worth going into it eyes wide open.

On the core Ive done some fit out that's much lighter. Think some is 3/8 5 lbd the rest .5 inch so 9 and 12 mil are close approximation. Haven't had them in circulation long enough to give a solid feedback on longevity. I will say on head doors and bigger segments id still suggest finding some form of higher density core for edges where hardware will be. Doesn't have to be coosa, there are more than a few higher density options around.

Gonna speak on core material,

My neck of the woods coosa is just shy of double the various h80 pvc foam that are common up here, the plastic honeycomb is about 30% less than the pvc. It used to be about half but post covid its only about 30%.

When I started wanting to learn more about glass a lot of the projects were some kind of plascor type honeycomb. Mostly commercial boat deck and fish hold divider sections. A little over 5 years ago when it came time to build my boat we bought a few sheets of coosa, h 80 and honeycomb. After working with it for interior work its the only core I won't buy again. Edge finishing and panel bonding is more labor for an equivalent finished edge to me isn't worth the panel savings.

All advice is best contextualized, boat building/repair is second to fishing for me. On a commercial build like the one we did 4 years ago or the one we're planning in the near future are such that core is only a single digit % of the boat cost. So a 50% increase in core cost makes a minimal difference compared to say my hydraulic or refrigeration system. While that clouds my view, it's also given me the opportunity to buy all manner of building materials as a tax write off.

Last year we brought a big truck up from the lower 48. In addition to my known core we went to 3x different large composite warehouses selling all the major foam formulations from most the major manufacturers. It was probably close to 2k usd of various foam densities and chemical compositions. I've built all kinds of bits and bots with it over the winter.... and am probably more agnostic to what's best now than before trying it all. My reccomendation is find whats available local witb some basic tech support and go for it.

 

I'm sure most of us understand that in a production environment,the windows are one of the last things to attach to the mouldings and that allows for quite large pieces to pass through.If working on a refit out in the open,it may not be an option.For moulded components,it might be simpler to create disposable moulds from melamine faced chipboard and perhaps include plants on the faces that need access hatches.If the item is made with some overrun,that area can be used for sealing the vacuum bag and there is no need to strive for a perfectly sealed assembly.the finish can be very good when removed from the mould but for a novice to bagging things,this might be a step too far for a first attempt.One possible consideration might be whether at some future time there might be a need to find a new owner for the boat and potential purchasers might not like to see anything too avant garde.

 

Plywood is definitely cheaper & easier.
I did the last one with it, I don't think it turned out too badly.



Not quite finished, and before the cushions were recovered and the hull ceiling finished, but you get the idea.

However life is a learning experience and I can do a much better job this time around.

@wet feet I get your concerns about resale value, but the price I'd be paying is so low that realistically I could throw it away and not lose much.

And really, I'm not doing anything radical, when I'm finished the interior will be for all intents and purposes a normal interior, I'm just looking to save some weight, and have a bit of fun learning some new skills.

The windows / portlights are pretty small, I won't be fitting much through these.



@comfisherman , all good info, thanks.
Though I won't be attempting too many complicated shapes, maybe a radiused piece in way of the galley, and a few small other places.
With the right hardwood caps and trims its fairly easy to make flat panels look quite classy.

Most of the existing interior is made right now out of single skin fibreglass with gelcoat, its unfortunately been sadly neglected, has lots of damage, and I want to make changes to the layout anyway. so its not exactly a classy looking place as it stands!
I wouldn't be gel coating anything inside, been there done that, give me a proper 2 component polyurethane paint any day of the week.

 

I’d use 12mm pvc foam core 4 pound or 60-80kg on availability/price.

I’d use db1200 biax glass or 10 oz woven, or 6oz woven if you are not sitting on it ever.

As for making large panels and sawing them out; you can, but you can also fair reliefs into actual size panels; dependi f on finish level you want.

 

Cheers fallguy.

I'm not sure exactly what you mean by 'fair reliefs into actual size panels'?

 

You can make a relief and then any tabbing on the exterior can be done in the relief. This requires making panels custom to fit, but only the exterior panels.. you can also just fair stock glassed material; it comes down to the finish you want

 

Oh gotcha. So that the tabbing becomes flush with the rest of the panel.
Makes sense, thanks.