Foam vs Plywood

Stan, use 1/4" bead board on the inside, which looks nice painted and oriented fore and aft. With a 1/4" inside skin and a 3/8" external skin, you have more then enough material in play, to make a stiff, strong roof, without without foam. Without foam, some laminated or sawn beams on relatively close centers, much like Tom's arrangement will do fine. If using foam, the beam spacing can be spread out considerably, say doubled. If employing a laminate, you can skip the beams, though the goo factor will rise considerably.

 

From the Hexcel composite design manual;

core shear stress = F/hb

where F = the derived load, dependant on the type of loading (simply supported, cantilever etc)
h = the panel thickness
b = panel width

So if the h is increased, you can see the core shear stress is decreased. So if you make a sandwich panel from a relatively weak foam, like styrofoam, then provided you have enough thickness for the designed load, it will handle the shear just fine.

The compressive is another story, as it does not matter how thick the panel is, the resistance to compressing from an impact or localized load transfers directly through a thin skin laminate and must be handled only by the cores compressive mechanical properties, which is very soft in the case of styrofoam - so it needs to be protected from these localized surface loads or it will ding. Besides aesthetic blemishes, these dings can then lead to buckling of the laminate and total failure of the sandwich, so careful consideration needs to be applied.

Upchurch,
These panels are perfectly fine to walk on... with a very thick panel, of 4inches thickness, the strength and stiffness is tremendous with only a very thin high tensile skin of 1mm thickness. Like i said, we are allowed to span these panels upto 21feet between supports, and obviously people need to walk up there in order to fix the panels. Weve had 4 people all standing in the center and there is only a very slight deflection felt. They are strong enough and certified to withstand +120mph wind loads. And these panels are dead flat, with a camber or compound curvature they would be even better.

Correction, the panels weve used were 150mm thickness, not 100mm...
Heres a pic which highlights what im talking about;


They can be used for flooring aswell, spanning upto 15feet @ 40psf residential live load no less... full specs here--> http://www.ibpanels.com/durability.php

All this with little old styrofoam core...

 

Groper,

I certainly believe your illustration and design of the building panels.

I just don't believe this transfers to a relatively thin cabin top, if it does not have internal stringers like Tom28751 and Rasorinc both discussed.

 

Here in the US we have Dow Hiload foam. it is a very strong version of blue styrofoam. There are 3 types, 40, 60 and 100. The number refers to the compression strength in psi.

One major concern I would have is how the panel strength decreases with temperature. Dow Hiload states max temp 165 F. That does not mean it retains 100% of its properties up to 165 F. That is just the point where they made the call to call it quits. Technically, usually there is a knee in the deflection curve.

165 F is risky for a panel in tropical direct sun. I recommend some testing to prove the panels retain enough strength for the application.

 

The old Airex foam was used for a lots of boats in the 70s and 80s. One of the cautions was to always paint the deck white. There were pictures of foot prints in the deck from a hot tropical day and the sailor walking on it. This showed up even though the deck surface was 3 plys of glass/ epoxy.

 

Upchruch, what part of the physics dont you think applies to a cabin top?

When a relatively weak core material is thick enough, it generally will handle the shear in most design applications.
When the same material is thin, it generally will not be able to handle the shear, and the panels strength will be limited by the core shear failure mode, long before the laminate stress approaches its failure limit. ie. the panels design is not optimized.

In my catamaran design as with most other bridgedeck catamarans, the cabin roof (and walls) main structural load is the global torsion loads from the entire boat twisting. This could be modelled as 2 diagonal arched beams. The simple athwarthships bending is handled by the 2 main cross beams... The roof itself will be mostly covered in solar panels, and will not see much people traffic other than to get a vantage point from time to time and maintenance etc.

By choosing an appropriate thickness for the roof, based on the design loads which can be taken from the class rules, a thick polystyrene cored panel roof will suit my purposes quite well - as it also would in many other designs. If i was expecting more traffic on this roof, i would have to protect the core against the compressive loads of people jumping around etc and i might consider something like a plywood skin or in keeping with my all plastic build, a higher density foam under the laminate - in much the same manner a modern windsurfer board is built - EPS or XPS thick core, with a thinner divinycell outer core layer, followed by the glass laminate.

 

Groper,

Thin sandwich verses thick. As I said. I think your third sentence says the same thing.

I have built thinner sandwich structure with blue styrofoam and watched it bend under load much more than other foams.

If your configuration will accept a thicker, heavier structure I am sure you can make it work.



Have fun.

 

I have read over thoroughly all the ideas presented here and want to thank all of you that replied. I cannot build a 1 piece roof as I have not the room to do that. I now will do what I stated in my post #15 on this matter and use 1.5" stryofoam as insulation and floation. Par, I have not heard the words Bead Board in 25 years but I put a lot of it in houses I built in the 70s and 80s and it looked great--back when you could get it in most any wood from black walnut to Brazilian rose wood. Thank you for suggesting that as it will look great and I'm going to work to find a very interesting material I can stain or oil for the ceiling. Again, thanks all, Stan

 

I used 1" XPS foam core for my Tolman cuddy and cabin roofs, 1.5" XPS foam core for the rear bench seat panels, skinned with 1/8" Meranti. The skins are bonded to the foam core with 3M Fastbond 30 water based contact cement.

The panels have hardley any deflection on them, even when 2 people are on them. After 2 seasons of rough service, there is no sign of degradation or delamination.

 

I built a doghouse top about 40 years ago similar to the way Tom builds his,most of the structural strength is of course in the internal stringers but if you didnt have the foam you would need a thicker top skin, i used resorcinal for the adhesive and i feel it works better than epoxy for this purpose. When you are comparing foams its important to compare apples to apples, the pink and blue stuff is only 1.5 or 2 lb/ft3 so of course you will have better luck with H80 pvc. Incidently CSK built a little 24ft cat back in the 60s with 4lb density styrofoam (strip planked) (which is more like the density we typically use with pvc etc.) and epoxy by the name of "Foamy", they raced it to Acapulco in 1968, a race of 1000 miles 2 weeks after launch. I would love to know what happened to her, would be an interesting study.

Steve.

 

4 pound foam is a bit light, I don't spec anything less then 5 pound for structural applications. There's a lot of ways to skin this cat, depending on use and availability, compared to the budget.

 

Agreed, 4lb is a little lighter than the more typical H80 which is 80kg/m3 (5lb/ft3) but apparently it worked, it was certainly light, the boat weighed in at about 1800lbs which is pretty good for a bridgedeck cat. As i said, it would sure be nice to inspect the boat if it still exists. Another guy who used styrofoam way back is Russel Bowler of the Bruce Farr design office, he built a series of 18 footers that were sponsered by Benson and Hedges which had an aluminum space frame that took all the rig and daggerboard loads and then a very light styrofoam/epoxy /glass shell to keep the water out. He rented space in my shop in the mid 70s to build the second one i think it was. Of course these things didnt need to last. What i have noticed when i have built with the blue or pink stuff is that it has very poor peel strength with epoxy, peel strength is the weakest bond with most materials but it is particularly poor with styrofoam which is why i prefer to bond it to plywood with resorcinal but the bottom line is you need to design your part to minimize/eliminate this situation which we typically do anyway.

Steve.

 

I have a cruising catamaran on the hard nearby that has been out for 10 years with an extruded polystyrene deckhouse roof. No problems.