any tips for improving the lifespan of a fiberglass-over-Xps insulation foam boat?

before anyone gets mad at me, yes, i know XPS foam is not very good for boatbuilding. it was my dad's idea and he was very stubborn.

now we have a near completed microskiff hull made of polystyrene foam and im trying to figure out how to make it all actually work out.

attached are some pictures for reference. the boat is 4 foot one way and 12 foot the other and it's about 12 inches deep at the thickest section, 6 inches at it's shallowest.

 

Re how to improve the lifespan of the boat in the photos above?
I would say leave it as it is - ie do not throw any more materials at it - and use the fibreglass materials that you have allocated for finishing this skiff on building another boat that is more suitable for what you have in mind.
I presume that your intended usage is that you want to go fishing on shallow flats with it?
There are many very fine designs out there that are intended for this type of fishing, and they are available for a very reasonable cost.

Re applying fibreglass to the above polystyrene hull - polyester resin will simply eat / dissolve the polystyrene, so unless you sheath it in layers of eg clingfilm (but then you have no bond between the glass and the foam......) you cannot use this resin.
Epoxy does not dissolve polystyrene - but throwing gallons of epoxy resin and glass cloth at your foam block now would be a shame really.

 

I would say the opposite. Add more epoxy glass. That is the price to pay. Make it puncture proof and consider the XPS as floatation material. You have an unsinkable boat.

 

we have 2 goals for this boat:
1) look kinda lika a boat
2) float

I know we would have been better off finding a design online, but the most fun part of any creation, at least for me, is coming up with a terrible idea and then trying to make it work

 

A rough calculation gives approx 210 lbs / inch immersion once the first two layers of foam are immersed.
By the time the next two layers are under water (with just one layer left above the surface), this has risen to approx 255 lbs / inch immersion.
But you do not have much freeboard left then......

It would be an expensive price to pay though, for building an unsinkable raft which might (I say might) only be suitable for (eg) sunbathing on.

 

That was our idea. Add so much glass that the foam could be removed if it ever got waterlogged and the hull still keep its shape.

 

Personally, I'd buy a few sheets of 1/8" ply for door skins, laminate that over the hull, then glass over that. You should get pretty decent puncture resistance for not much weight, I think it would be cheaper than laying on enough glass to get the equivalent performance, and I think you could end up with a really useable machine. If you throw a layer of 12oz biax over 1/8 lauan, I don't know, I'd be pretty confident I could have some fun times on that thing.

Maybe I'm just dumb, but I think it's cool and with minor tweaking that looks like a lot of potential to me. I guess I might also buy a few 2x2s, router off all the edges and inset the 2x2s all the way around, then use a big roundover bit and go back and radius them that way.

Damn it, now I kind of want to build something like this. I don't think you should be hard on yourself at all, I love this kind of thing.

 

thanks a ton! these are exactly the kind of ideas i was looking for!

 

If you do the epoxy+cloth over plywood thing, you still need to paint it and/or keep it out of the elements. I had one fail at about 4 years outside with no paint / no tarp. The cloth will help but the pinholes and movement of the plywood will bite you.

 

If you expect the boat to plane the transom-hull edges need to be sharpened up. Fiberglass will conform to a 1/4- 3/8" radius IIRC.

 

You can only use glass with epoxy really at this point. The problem with xps is it shears easily away from the glass and epoxy.

So, predictably, it will delaminate.

The way to reduce delamination is to add some shear web to the structure. You don't need much and you already have some. This is no guarantee it won't delam easily from impacts, just that one delam area won't spread to the entire boat.

How? Just cut some grooves into the xps to split the delam potential down. You already have a web with the lower piece of plywood, but it isn't filled in with cabosil and epoxy peabut butter yet.

And the cuts don't need to be too fancy or pretty either.

I'd use a skilsaw woth the widest blade you can find; not narrow. Set the blade at 3/8-1/2". Cut the keel lines down the middle and a crossing line every foot or so. Make some cuts on the bow every 6-12" and same on the sides.

Fill all the holes and weblines on the boat with cabosil and epoxy.

plan to glass the thing with two to 3 layers of 6oz woven fabric-more on the bottom..

I prefer to hotcoat a boat like this with thickened resin before laying glass. The surface you have needs to be sanded to 40 grit all around. The 40 grit increases the surface area a bit and then a thickened coating makes sure the glass is in contact everywhere. You can't afford poor quality laminate workmanship here as you already have a poor quality substrate.

Do not sharpen the transom as suggested; that is done AFTER glassing; not now.

If you follow these instructions and you bang the boat hard; the idea is that is can only delaminate up to your webbing and you will have time to fix it before it takes out the entire bottom which happens under power easily.

 

The doorskins idea can work, but you'll want to sheath them in 6oz glass.

 

Damn I love this kind of thing. We need more of this good natured "messing about in boats", and less mean spirited "shouting past each other" on the forum, and more excited beginners would stay.

BE_, thanks for sharing. The cost of glass and epoxy seems well worth it, to be goofing around with your dad. I miss mine.
Otoh, Bajansailor is not wrong. What you will finish with will be worth less than the materials cost, but he is a professional, and professional ethics obliges him to take this view to protect his clients. He is a man of integrity. Me? I'm just some guy on the internet.

 

@BE_ we have not seen you on here for a while - do you have any updates for us please?

Sorry, I was unduly harsh above - but if you did finish the boat off, I hope that you got rid of that console, and have it so that the crew are sitting lowdown while underway, rather than standing.