How many fiberglass layers for different areas?

No need for testing. It has 25 psi compressive strength. That means when your 50# toddler nephew jumps in the boat and lands on a heel; it will crush and delam. When your 250# buddy gets on board; if his foot or heel covers less than a 5 sqin area; it will crush.

Mostly, it will crush.

and then it will delam and peel apart because the shear rating is about the same..

 

Not true. I have an RC model airplane which uses a pink foam core with no shear web, even though it uses carbon pultrusions for spars. It has no problem with a hard pull up at 50 or 60 mph. The Sky Pup, an ultralight aircraft that people have been building for years, uses blue foam as a shear web in the wing spar. A cantilever wing, no struts. Apparently, there has only been one structural failure, and that one was AFTER a crash.

Dow has Highload foam in 3 types, withstanding 40, 60, or 100 psi. The latter's compressive strength is getting close to that of a light flavor of Airex. I'll admit I haven't looked it up, but I suspect that the shear strength is much more than the regular stuff, too. Highload 60, at least, is not that much more expensive than blue foam. Highload 60 is only a little denser than regular blue foam, I think something like 2.4 lbs per cubic foot, and Highload 100 is significantly denser. 4 lbs? I'm guessing that other brands have foam that can take similar compressive loads.

I think if the skins are sufficiently resistant to abuse, they will be more than adequate for bending loads, and most loads, except maybe around engine mounts, deck fittings, etc.. If it was my project, maybe I'd use Highload foam 1/2" thick, because thicker would probably be overkill. If I wanted buoyancy, I'd probably mostly use air chambers, since air is a whole lot lighter and cheaper than foam. A guy who builds fiberglass boats with foam cores told me he would use about 18 ounces of stitched biaxial glass for decks. I don't know how strong the foam he used was, but it was some other chemistry, not polystyrene. Don't count on this without verifying. For instance, I might not have noticed if he used some other layup for decks; I think I was only helping him with the hulls. It was a long time ago.

Anyway, I think the structural issues are quite solvable, although MAYBE it would be necessary to use something a bit more substantial than Foamular 150, which, if I recall, is only good for 15 psi in compression.

I'm more concerned about what gasoline does to polystyrene. I think it's supposed to just melt the foam and turn it into goo. Also, if the foam gets too hot, it will soften and melt. I had a set of foam cored, carbon/epoxy covered wings. I had them in what I thought were opaque wing bags, which turned out not to be true. When the sun hit them for a few minutes, part of the foam core melted. There's a reason all those VariEze and LongEze homebuilt airplanes, made with blue foam, are painted white. If memory serves, I think the service temperature of Dow Styrofoam is something like 160 degrees F.

The other thing I don't really get is how this boat will be better than someone's old, beat up aluminum jon boat. Maybe it could be lighter, if engineered properly, but fiberglass is heavy, floppy stuff compared to some other materials. I could see using thin ply over a foam core...

 

your remark is offensive and unsupported by fact

dow corning big box xps has a compressive rating of 25 psi

the bottom of my foot pad is about 3 square inches

I weigh 220 pounds. 220/3 is over 70 psi

bring your rc airplane over and I'll test it for you!

the reason your rc aircraft is able to withstand the loading is because it is not experiencing any significant crushing loads; neither do my xps foam amas I use for outriggers

step in a boat, step on an rc aircraft and the compare is then the same

 

I was referring specifically to shear loads, so compressive loads are irrelevant. If something has NO shear strength, it's not a solid material. You said "no shear strength" which is not true. In fact, as my example demonstrated, pink foam has enough shear strength to sustain a useful load, though perhaps not in this particular project. And blue foam has usable shear strength, or all those ultralights built to the Sky Pup design would have plunged from the sky years ago. Not to mention the VariEzes, LongEzes, and any number of other homebuilt designs in fiberglass and blue foam. You also wrote "no...compressive strength." And 15 psi compressive strength is NOT 0, nor is 25. It's not even an order of magnitude less than the expensive materials.

As far as crushing goes, that depends on what the layup over the foam is.

Your response is "offensive and unsupported by fact".

The usual blue foam is indeed specified at 25 psi compressive strength. But Foamular 150, from the OP's example, is only 15 psi. You need Foamular 250 to get 25 psi.

As mentioned, I'd consider using Highload 100 for a project like this. Still XPS foam, but stronger. Specified as 100 psi compressive strength and 50 psi shear strength. It's a little harder to dig up, but I think the density is around 4 lbs per cubic foot. Service temperature is given as 165 degrees F. I've bought one sheet of Highload 60 from the local insulation place, which is close enough for me to carry it home. I suspect they could get Highload 100 as well. According to specs, Airex T90.60 weighs just over 4 lbs per cubic foot, withstands 116 psi compressive load perpendicular to its plane, and has a shear strength of 58 psi. So extruded polystyrene foam is NOT hopeless compared to Airex of similar density.

https://www.buildsite.com/pdf/dupon...ulation-Installation-Instructions-1935145.pdf

 

Well, it is really easy to respond to someone's good intentioned comment with 'not true' and completely evade intent, but at the end of the day, you were simply offensive.

Xps has no shear strength is a relative comment. Relative to a typical marine foam, shear and compressive strength for big box xps is roughly 10% of a good foam. Now, you can parse that and say 'not true' it is 12% all you like.

You took my comments completely out of context.

 

At the end of the day, no marine designer uses polystyrene foam for good reason. You might get away with it, if making a pool toy is the object.

If you really want to make a jon boat that will work, ride well and last buy plans from a reputable designer and build it as designed.
This one is built, proven and will set you back less than a good night out at a bar. And it will not fall apart if you build it to spec.

Jon Boat / Garvey 12 Boat Plans (GF12) - Boat Builder Central

 

From a practical standpoint the time is the greater of the two costs. My experience with John boats involved stumps, logs and some shallow water bottom contact. Uncored glass could be layed up thick enough core is moot. In that scenario the styrene would be a moot point and just added flotation. Seems like for the time invested building a little increase in core cost is minimal.

 

Well I know of guys that built those bateau GF12-16s and run them over oyster bars. So if XPS plus enough glass is good enough, then a plywood core with 12oz biaxial would do as well. I'm building a 17' boat from the same material. It's tough stuff. I made a little dinghy as well and that's bounced off the odd rock.

I'd put it over glass and XPS on both a weight and a cost basis strength for strength.

 

A proven stitch and glue design would answer all these questions and far better facilitate your needs (SOR).
Asking such broad questions indicates an even stronger basis for my advice above.

What kind of a dumb-assed location is that?
We don't even know where you live.

Good luck with your build, post lots of pictures.

 

Zip code says -Columbia, Mississippi