[PSUBS-MAILIST] Fiberglass

Mon May 4 11:33:53 EDT 2015

Hi Scott,

Coincidentally your timing is really good, because I just finished four
rather complex-shaped MBTs for the new sub. Since my sub is a one-off
rather than a series production item, I used the method of glassing foam
plugs that are then dissolved, rather than making molds. To summarize:

- The plugs are a sandwich of many layers of pink home insulation foam from
HomeDepot. When I did Snoopy's saddle tanks I cut out the sections with a
hot wire, but this time I realized my jig saw does the job in about a tenth
the time. Step one is to cut a whole bunch of sections and glue them
together. If I weren't so tight-fisted, the ideal solution here would be to
mill a single block of foam using CNC equipment, but I get by with what I
have on hand if it'll save money - and a milling job would probably take
quite a lot of it.

- The glued sections are only a rough draft of the final form. Next you
have to shape them until the steps between sections are gone and everything
is nice and smooth. I use three tools to go from the draft to the final
product, in this order:
1) A plain old wood saw
2) A Stanley Surform shaver, with flat and or rounded blade fitted
depending on the surface. This thing is absolutely essential and I use it
for 99% of the job. See
http://www.stanleytools.com/default.asp?CATEGORY=SURFORM+TOOLS+AND+BLADES&TYPE=PRODUCT&PARTNUMBER=21-296&SDesc=Surform%26%23174%3B+Plane+Type+%96+Regular+Cut+Blade
3) Sand paper

I'm afraid I have no scientific answer to ensuring symmetry. I just put the
plugs side by side and do a bunch walking in circles, taking measurements,
and using a level. The plugs won't come out identical, but close enough to
be functional. This part is more sculpture than science.

- Next, paint the plugs with several coats of water-based paint. This is to
prevent the resin from dissolving them. Water-based because if not you run
the risk of the paint dissolving the foam.

- Now apply layer after layer of wetted fiberglass cloth. I don't mean all
in one sitting, I mean iteratively for about a month. There are two main
considerations then, cloth and resin.

I can't give you a specific recipe for cloth or the cost for cloth, because
I used a combination of material I already had on hand and new stuff. I
have purchased from http://www.fibreglast.com in the past, but this time
found lower prices on eBay for what turned out to be perfectly good
material. Mostly I used 6.5 oz (quite light) cloth so that it would drape
better, but that depends on the shape of your tanks. For instance, on the
inside face that goes against the hull, you can get away with much heavier
material since its a gentle curve in just one plane, and the same goes for
any flat surfaces. But if you have compound curves or tight curves its way
more challenging to drape cloth without forming air bubbles. I make the
walls that go against the hull thinner, since they aren't going to be
banging against things like the outer or upper faces will. On average I
probably put down ten to fifteen layers of cloth. The pros use vacuum
bagging to prevent bubbles. With my caveman fiberglass skills I just try to
avoid them in the first place by selecting better-draping cloth, and when I
get a bubble I remove it with a flap wheel before putting down the next
layer. In the middle of my layup I put down several layers of Kevlar. This
material is trickier to work with than fiberglass, it can't be sanded and
once cured you can only really go through it with carbide tools. The idea
is to increase survivability by making the MBTs puncture resistant. If I
hit a rock or a dock I still expect the resin might crack, but it should be
quite hard to put a hole in the Kevlar. You might think this hard layer
should go on the outside, but I put it mid-schedule so I could sand
imperfections out of the fiberglass layers above it.

This job (new sub, not Snoopy's tanks) took ten gallons of resin. Everyone
will tell you epoxy is stronger than polyester resin. However, epoxy is $76
per gallon vs. $34 for polyester, and Snoopy's polyester tanks have held up
perfectly well for years. So, for me, this is a case of
cheaper-is-sufficient and I went with polyester.

- Iteratively sand and fix imperfections with a fairing compound. I used
West Marine's #410 fairing filler (
http://www.westmarine.com/buy/west-system---410-microlight-filler--P004_120_004_016).
You can do this with Bondo as well, but #410 is easier to sand.

- Dissolve the plugs. When I made Snoopy's saddle tanks the pink foam
dissolved instantly with gasoline. This time, despite being the same brand
foam it was somehow gasoline resistant, but acetone did the job.

Best,

Alec

On Mon, May 4, 2015 at 9:22 AM, via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Alec,
>
> I was curious if you could go into details about the making of your saddle
> tanks?
>
> *Where did you get the foam and fiberglass and what types did you use
> *How did you shape the foam to ensure symmetry
> *Was there any difficulty or anything you would do differently
>
>
> Thanks,
> Scott Waters
>
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>
>
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