Saturday, March 30, 2013

A Better Use for Cheap Foam Insulation: Bulkhead Replacement

Bulkhead replacement....  This was a project that I wanted to do about six months ago, but was reluctant to break into the major structure of the boat.  Since then, I have gained far too much confidence, and recently revisited the thought, deciding it would be a piece of cake.   I turned out to be almost correct, and would now recommend this method to anyone in need of a new bulkhead.  Two years of boat work have gone by now, and I have discovered that I would rather replace a major bulkhead than sand and paint even one square foot of anything.

The original bulkhead was badly rotten in a couple long streaks where leaking water had seeped through its core.  Apparently Chinese wood glue of the 60's was not up the the standards of modern marine plywood either, and bad delamination had occurred across the majority of the top layer on both sides.  Faced with two large sections of rot, and a plywood panel that had gone from 5/8"x 5 ply to 3/8"x 3 ply, we decided it was time for a truly strong and everlasting rot-proof solution: foam-cored fiberglass.




Now, cored fiberglass panels can be purchased in 4'x8' sheets just like plywood...  for serious sums of cash...  and marine plywood is not much cheaper.  This has always been a disappointment to me as I know that a finished product made in epoxy and fiberglass costs about $1 per pound in materials at wholesale prices (my labor is, of course, always free), and there isn't much weight to a flat foam core panel.  Apparently the +1,000% mark up seems to be because so many people view epoxy and fiberglass as a "black art", and that is a shame, because with a little knowledge and practice, it can become the fastest, easiest, and best solution for most things you will do on a boat.

For those that doubt its strength, good fiberglass* has a compression strength of ~25,000 PSI.  So at at 1/8" thick, it only requires about 8" to support the entire pressure of the rig. The only problem is that a 1/8" sheet of fiberglass obviously has some serious buckling issues.  That is where the foam core comes in.  With two separate-but-linked panels of fiberglass (via a foam core spacer), one skin will have portions in compression with a matching portion on the other side in tension as it warps and tries to buckle.  This warping and tension-compression cycle is virtually invisible, but it is how any vertical panel withstands compression forces, plywood included.  Anyway, static physics lesson over, now to the construction.
*"Good fiberglass" means minimizing excess resin and maximizing glass fiber compaction.  The best way to do this is with the vacuum resin infusion method, but since I don't happen to have a vacuum resin infusion machine (...yet...) we opted for the easiest and most DIY-friendly method: hand layup.  It also happens to be the second strongest way to build in fiberglass as a good, hard squeegeeing can pull out most of the excess resin.
Other things to note about the project: epoxy is slightly stronger than polyester resin, but is also a bit more expensive.  That expense is offset by the fact that polyester resin requires obscenely expensive PVC foam for the core, but cheap polystyrene foam (styrofoam^tm) is ideal for epoxy.    The styrene in the polyester resin will dissolve polystyrene foam... (styrene dissolving polystyrene?!  Imagine that!  #DuhChem101)

Bottom line: armed with a half gallon of epoxy and a few yards of biaxial fiberglass cloth remaining from previous projects, we decided to make our own bulkhead panels using the 1/2" blue polystyrene insulation panel from Lowes.  And at $11 a sheet, the price was just right.

We started by cutting out the bulkhead and carefully removing the tile work around the heater.  I made a cardboard template for the bulkhead, and cut it in half since the full size bulkhead would be too large to insert into place.  BTW, the best place to get cardboard for oversize pattern making is not refrigerator boxes, but rather from a place that sells high-quality plywood, (or low-quality doorskins), as there is a 4'x8' sheet of perfectly flat, unmarked cardboard protecting the "A" face veneer of every sheet.  They tend to have piles of this perfect cardboard laying around for the taking.  Also, if you are handy at origami, you can use it to make your own boxes in any size you need.

Of course you transfer this pattern to the foam core, and give yourself a bit of extra edge for final fitting...

... and then transfer that to some fiberglass cloth.   In this case we used two layers of stitched-biaxial-cloth-with-chopped-strand-mat-backing on each side.  The key to a good hand-layup is to use just enough epoxy to wet out the fiberglass then squeegee it rather hard to ensure full saturation and maximize compaction of the glass fibers.  Also, wet the underside of the cloth before laying it on, because getting all of the epoxy to saturate from the topside will inevitably be difficult if not impossible to avoid air gaps in the structure.  Finally, do all layers per side at once, because secondary lay up adhesion (adding another layer of fiberglass over already-cured fiberglass) is not nearly as strong as a single monolithic layup.  Don't worry about the depth of the epoxy, it is not like paint that dries; epoxy cures exothermically, and the additional thickness will actually help in the hardening process.


When fully impregnated with resin and compacted dense and flat, two layers of biax-w/mat comes out to about 1/8" thick, yielding nearly 1/4" of fiberglass on a 1/2" polystyrene foam core. It should be a more than sufficient replacement for the bulkhead.

After each side cured, I cut off the razor sharp edge overhangs with a vibrating multi-tool with plunge cutting blade.

Then we filled the exposed biaxial weave pattern with epoxy thickened with glass-spheres and silica powder to give us a glassy-smooth-yet-sandable finish.  I wouldn't do this if the panel wasn't going to be visible.


It takes a bit of edge sanding to bring the panels into a friction fit.  Then we screwed in a small block to keep the two panels aligned and used 4" squares of biax-w/mat cloth to tab the bulkhead into the boat, and join the upper and lower halves together.

Then we came back the next day to fully 'glass the bulkhead to the deck and hull.  We tabbed the bulkhead the day before so that it wouldn't move during the joining process, which would result in an impenetrable monolithic total disaster instead of a successful bulkhead replacement.  It also allowed us to squeegee the glass tape hard against the bulkhead to ensure a better bond.  For this portion of the project, I pre-wet (pre-wetted?) 6" wide biax-w/mat fiberglass tape in 16-18" sections before handing them to Sarah for installation.


I am not allowed to do fiberglass seam taping because I am terrible at this sort of work.  I can lay a nice smooth layer of fiberglass cloth on a flat board, and I can lay up a fiberglass part in a mold, but when it comes to fiberglass seam taping I am absolute shit; I can't explain why.  I tried one piece and messed it up so badly that I had to peel it off and throw it away.  I expect it is the massively oversize gorilla-like paws on the ends of my arms...  Sarah on the other hand seems to have no problem with her skilled, delicate hands.

Now all we have to do is rebuild all of the fascia in that entire corner...  :-P


  1. Awesome info!

    I'm a novice fiberglasser, but I've spent the last two years doing my own DIY fiberglass boat repair. I definately picked up some usefull tips from your post. Thanks!

    I just published a few posts on my blog about doing simple fiberglass blemish repairs. I also just uploaded a YouTube video of the bulkhead repairs I've been doign on my boat. In my case, I'm using 1/2 inch marine plywood with a single layer of 6oz cloth. I'm not really using the fiberglass for strength, just sheething the plywood to prevent rot.

    1. Thanks Chris! We checked out your blog and videos, and also picked up some useful tips. Great blog!

  2. Great post on your project. Thanks for taking the time to take photos as you were working. I am always too stressed for photos when using epoxy!

  3. Technical knowledge of the foam specification has been given by the Foaming Agent Manufacturer with great liability of the reliance on it. Such a nice blog mostly used in the commercial organization.

  4. This comment has been removed by a blog administrator.

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