How many fiberglass layers for different areas?

Hi, new member here and my first question is how many layers of fiberglass should be layed down on different areas of boat? I am build a small boat from scratch using foam insulation board so I will be depending on the strength from the fiberglass. It does not have to be super strong as I will be using it for small shallow sand/gravel bottom rivers. The boat will be in the design of a small 1448 Jon Boat and will be propelled with 15.8 hp surface drive. If someone could break it down for me I would greatly appreciate it. I dont want to order to much or too little. Also, what would be your reccomendation for overall general weight of chop strand mat fiberglass to use, trying to keep it simple and cheap. Thanks!!

Hull?
Transom?
Decking?

 

Welcome to the Forum Sean.
Can you supply a bit more information about your design please?
What is a '1448 Jon Boat'?
What is the length and breadth of the hull?
Do you have any sketches that you can post on here, to show what it looks like on paper?
Will you have a transverse seat or two in the boat? These can be useful structure to help stiffen up the hull sides and the bottom. And watertight box seats will give you some reserve buoyancy if you get swamped.

Re your 15.8 hp surface drive, will it be something similar to the engine in the boat in this link?
Inboard Air cooled Surface Drive https://www.boatdesign.net/threads/inboard-air-cooled-surface-drive.67153/

Re this 'foam insulation board' what type of foam is it? And what thickness?
If it is polystyrene, then the polyester resin will just dissolve it, unless you plan to use epoxy resin.
And if you use epoxy resin, you definitely do not want to use chop strand mat.

 

14 foot Jon Boat 48'' wide with 16" high transom. Yes I will only be using epoxy resin, during my reseach I learned about the issues with polyester resins. Will be using 2" & 1" insulation EXP foam board. Hull will be 4" thick and sidewall 2" thick. Attached pictures are examples and not my work.

 

14 foot Jon Boat 48'' wide with 16" high transom. Yes I will only be using epoxy resin, during my reseach I learned about the issues with polyester resins. Will be using 2" & 1" insulation EXP foam board. Hull will be 4" thick and sidewall 2" thick. Attached pictures are examples and not my work. The surface drive is the type in picture, not through hull drive....this one just mounts to the transom and it air cooled.

 

I presume that you mean either EPS or XPS foam?
Rigid Foam Insulation Types | EPS, XPS, ISO | Insulfoam https://www.insulfoam.com/rigid-foam-insulation-types/

Re the 4" thick hull bottom, will you be bonding two layers of 2" thick foam together to achieve this, and then laminating on each side with epoxy?
Is the 4" thickness to try to get built in reserve buoyancy?
I would be a bit dubious about the shear strength of 4" of polystyrene in your hull bottom, and also the 2" in your hull sides.
I hope that you are not going to use chop strand mat with your epoxy.
As it will soak up the epoxy almost like a sponge, while not providing much strength at all.

Re the mudskipper photo, I presume that you will be using an engine arrangement like this?

 

Yes XPS
Yes Stacking & Bonding 2 to 3 layers for buoyancy, cut and shape as needed
I could either stack more.....6'' hull & 4" sides for additional strength if needed. Trying to keep wood out of the build do to weight.
I was unsure of what types of fiber glass to use. Im assuming you would suggest cloth. What type and weight?
Yes, same set up as the mud skipper photo, but still reseaching best mud motor for the buck$.
Do you have and recomendations on how many layers for each area? I will be painting the hull with gator glide once completed if that matters any.

 

Going overboard on thickness doesn’t necessarily translate to “stronger”. The foam you have chosen is weak, and will delaminate from skins readily under stress.
I’d choose thin plywood as a core instead of the XPS. Great bonding, lightweight, and compatible with all resins.
Although the method you’re talking about is very popular on YouTube nowadays, it is not “best practice”, and undoubtedly will be quite heavy, as you will basically be building a heavily insulated fiberglass boat.

 

Duly Noted

Do you think one thin layer of plywood with resin & glass will be lighter than styrofoam & glass? I guess you saying I would have to double up on the fiberglass on styrofoam to make it as strong and essentially gain weight and possibly make it heavier in the end. Is that correct?

 

As kapnD has said, the polystyrene foam is inherently 'weak' compared to other more suitable foam cores.
This means that it would be much easier for it to delaminate - eg if you jump into the boat, or hit something while underway, or even drop your anchor on to the sole.
If the foam shears or delaminates, then you basically have two thin skins of fibreglass flopping around, and the effective 'strength' of these is minimal.
The fibreglass skins act like the flanges on an 'I' Beam, and the core acts like the web that keeps them apart (ok, this is a simplification).
If you fracture the web, the flanges are not much use any more.

Good quality plywood and epoxy is well proven to be strong and reliable. If you cannot source good plywood, you could consider making up fibreglass sheets to use instead. And then you could use polyester instead of epoxy.

 

The pink foam has no shear strength, no compressive strength and will crush delaminate from you walking in the boat or hitting the bottom or tying it to the car.

 

I don't know the comparative prices of polyester and epoxy wherever 39429 happens to be.I do know how much work it takes to get a decent finish on the outside of a laminate,which makes me question the viability of the project if a useable boat can be bought and put to immediate use.Looking at the fairly simple shape of the boat I do wonder if the suggestion in post #9 might be the way to go.If you could create a female mould from melamine faced board,a simple layup with as much polyurethane foam for buoyancy as you wished for would be quite simple and the outer surface would be reasonably good with no further work.

 

Well I'll break it down.... from a mid level pragmatic perspective.

Forums tend to trend towards the most anarach of a cross section, boy scout method if you will. Often the end result is "best"or the 50 year solution. While the answer is probably right and accurate, answers like expectations need to be.

Are you building a John boat to

A. Last forever
B. Be cheaper and tougher than alloy
C. Try a crazy idea out without a ton of out of pocket.

If the answer is a or b the foam is a bad idea if it's c, and your estimations are realistic.... it's worth a shot.

I've seem some crazy ideas work for a little while... will an xps boat bounce off stumps longer then a commercial alloy hull... also probably not. But if you have some feature or idea you want to try go for it. Just don't have expectations of an heirloom level finished product.

Your not gonna get a lot of strength from the foam, likewise there isn't going to be a lot of research on what's suitable. I made a model to tow test an extreme hull shape out of xps, coated in out of date epoxy and 10 oz cloth.... the out old epoxy was give to me as was the sheets of foam and I had a partial roll of 10 oz that had been wet and set near a furnace to dry. Asked a few local guys and they said " that foam is weaker than you think so put more on than you think" . I had been thinking 2 layers so did 4. We towed the beans out of it and gave it to some village kids to play in the harbor with about 4 years ago. My methods were unscientific and anecdotal, but it worked as needed.

Were I wanting a legacy boat, it would be Coosa bottom and transom with divinycell sides and stringers with 2x sacrificial layers on bottom for good measure.... but the budget just went up 6x or more. It's all trade offs, decide what your willing to experiment with and your risk aversion and go from there.

 

I was only wanting to build a cheap/light hull to hop from one sand bar to the next at my camp on the Homochitto River in SW Mississippi. There is no local boat ramp in the area so it would stay at my camp. The water there is very shallow 1-3' thats why I wanted it to be in a jon boat style with surface drive air cooled engine. I wanted it to be a little more larger/stable than a kayak. The river is very clean, mainly sand and gravel bottom with a few fallen trees here and there "no rocks". It would never see any rough seas and max speed would probably be in the range of 10-15 mph.......but mostly slow poking along. I wanted it to be light so that my UTV could pull it through the sand bars without getting bogged down. I also wanted it light weight as possibe because i may have to drag it to the waters edge by hand because sometimes the UTV can not make it all the way waters edge all depending on muddy the bank is that day. I'm not looking for it to last a lifetime nor do I care if it gets scuffed and dinged. With foam/fiberglass I was thinking if it did ever spring a leak it would not sink like wood or aluminum. So basically "C" try an idea without spending a lot. They main thing is the weight of the project, keep it light as possible. Thanks you for your comments, maybe I should of explained in more detail in the beggining that this project is just basic flotation device to get from point A to B at my camp. I have a 20' Chaparral for all my other needs.

 

plywood, epoxy are the most durable way unless you buy a used aluminum

you can't drag an xps foam boat over anything, like I said, it'll delam

And if you try to clamp an outboard on it; you'll get a lesson the first crank as the foam crushes. So, a clamping board? Okay, well, good luck hitting a boat wake.

people who think you can make an xps jon boat for river stuff don't understand

 

If it were me, I'd take a 18 inch x 12 inch section of your xps and coat it with a few layers of epoxy and cloth. Suspend the test segment between some 2x4s on the floor and step in the middle. This will give you a feel for how much/little rigidity it gives you. Play with thickness and see what you get.

Then do the math on the surface area of the hull in cloth and epoxy and decide if it's right for you.

Sometimes the money saved on core is lost in epoxy and cloth for a foam that provides minimal structure.

About 5 years ago we were insulating an aft fish hold. I had 1500 worth of foam sprayed in mine and covered it with about 800$ in poly and combi with a rolled on gel coat layer. My neighbor didn't like the idea of waiting on foam guy so he bought 4 inch sheets of pink foam board and went to work. He had maybe 400$ in foam, but had adhesive to the wall, epoxy, a much more expensive cloth and paint to coat. He ended up with a product that had functionally similar properties than mine but ended up spending more in the long run. Both still function, just different paths.