Dealing with holes through a foam sandwich deck

I'd noticed one or two small leaks where deck fittings came through the coachroof. And I saw that there were patches of mould growing in odd parts of the inside of the coachroof, usually beneath a deck fitting. This suggested dampness in the foam. I took a screw out of a foam sandwich bulkhead and was surprised that a little stream of water flowed out. I drilled a hole through the first layer of glass lower down on the bulkhead, and another little stream came out. I drilled a series of holes, one below the other, each draining a little water out, until I drilled one, and found the foam behind it dry. No more water.

You hear horror stories of foam sandwich boats where the foam is saturated with water, and it loses strength as it is stressed when the hull is driving through waves, leaving you at some point with two thin layers of fibreglass no longer connected to each other, only a little stiffer than wet toilet paper.

When I was thinking of buying the boat, I'd heard of a survey having been undertaken for a prospective purchaser, and I bought a copy of that for £300. It seemed a bargain. The survey was rubbish. There was no mention of several problems that were evident to me after a quick inspection. Potential problems were merely hinted at rather than investigated. Vaguely, it said the design was good, most of the boat was good, but parts were shoddily built and needed attention. Rather than kick up a fuss at the waste of money, I thought perhaps I'd leave things as they were, and if I bought the boat and it fell apart, perhaps I could claim against this lazy surveyor. Maybe that was just me being lazy, ducking out of making a fuss.

I decided I'd have to rely on my own experience. It was after all my 6th boat. And nearby, I had a friend who was actually born on a boat, lived most of his life on boats, and had built a few himself. I persuaded him to spend an afternoon with me tapping everywhere, listening for a dull thud, looking for signs of delamination. We found no signs of that, and no further problems that I wasn't capable of fixing myself. On the basis that the boat was structurally sound, and that there were no problems that I wasn't capable of dealing with myself, I bought it.

With water in the foam in some places, but not others, I wasn't sure what to think. How could water get into a bulkhead at all? Certainly not through the deck. Why was there water in an upper section, but none below. And did water in the structure not mean delamination was likely? What does a dull thud sound like anyway? If I'd heard one with all our tapping around, I'd know I'd found a problem. But what if I had failed to recognise the difference in sound when you tap a solid laminate from one that was delaminating. And even if there was no delamination - how could I go about fixing these leaks, how to get the water out of the foam. Was it perhaps, time to panic?

I did. I decided to call a proper multihull surveyor. Someone with a bit of form. Someone the experts called on. Someone I probably couldn't afford, but who I also couldn't afford not to call in. I called Rob Feloy, who conveniently lives just up the road. I told him I didn't want a full survey. In fact I didn't need him to give me anything in writing, as the advice I wanted wasn't for insurance purposes nor in any way legally binding. I just wanted him to come and look at what I'd found and advise me on the way ahead. No holds barred. If I was wasting my time, he wasn't to say nice things about my boat in order to not hurt my feelings! He understood, and showed up at Old Mill Creek at the appointed time with his magic moisture meter.

First, the mystery of the bulkhead with water in the top half, and none below. Rob explained that the bulkhead was made of contour foam - that is, foam with cuts in it that can be bent to make curves in all directions. Not what you'd choose to make a flat bulkhead, but if there was some contour foam left over from building the hulls, no harm in using it.

contour foam

If the cuts aren't filled entirely, which is often the case, any water that gets through to the foam fills the voids left by these cuts. My bulkhead had a few unsealed screws through it (to hold a piece of timber in place on the other side), and leaks in my hatch guttering allowed water to drip past these screws, eventually finding its way into the voids in the foam. The reason the bulkhead was dry below a certain level was that often big panels of foam sandwich are made from several pieces of foam, which are glued together before lamination. The line of glue between the pieces of foam acts as a barrier, preventing water that has penetrated part of the foam to enter the whole of the laminate. Using his moisture meter, Rob was able to easily distinguish that part of the bulkhead that had become wet. That was neat. He tapped around just as I had, and heard no unpleasantly dull thuds. This was much more reassuring - a report of no dull thuds from a man who'd heard many a dull thud before, and knew exactly what one sounded like.

Later, I drilled a plug out of the bulkhead, and I was fully persuaded that the glass had remained firmly attached to the foam on both sides.

Rob's magic moisture meter revealed moisture in the foam in the coachroof, downhill from several deck fittings. My heart was preparing to sink, but Rob didn't seem to regard this matter as serious. Again, it would be water in the voids of the contour foam. And there were no signs of delamination. Clearly, the water would have to come out - but that's easy, with a small hole drilled through the inner layer of glass at the bottom of an area that the meter designated as damp. Rob didn't think the residual moisture was significant. If the drain hole was left open, and the source of the leak stopped up, it may eventually dry out, but either way, not too important.

I knew how a hole for a bolt through foam sandwich should be created, and it became clear that none of the holes in either the coachroof or the deck had been made correctly. Here's the right way, a way that seals the two layers of fibreglass around a hole, and provides a robust collar that will absorb some of the strain imposed by the compression applied by the tightened nut and bolt:

A hole through glass/foam/glass sandwich	Use an Allen key attached to a drill to remove a few mm of foam
Close the bottom of the hole with tape, and fill with epoxy.	When cured, drill a bolt-sized hole through the epoxy

But how could I seal a hole this way when the foam and glass is already wet? Rob recommended I contact the Matrix Composite Materials Company who could supply me with some epoxy that sticks to wet stuff! Epoxy that sticks to wet stuff! Of course! I got some, and it did! I tested a bit on a piece of wet firbreglass, and it stuck. Magic! (Matrix has many specialist epoxies - and I found good use for one of these with another job, to be described in a future posting).

A detail - when sticking tape over the bottom of the hole before filling with epoxy - sometimes it leaks. The solution is to put a small circle of mastic around the holes first, and press the tape over that. Also, add a little filler to the epoxy to increase its strength, but not too much. It needs to fill all the voids. Aim for runny honey viscosity!

A tip - if you drill through the epoxy, but don't have time to refit the deck fitting, you can fill the hole temporarily with expanding polyurethane wood glue. This also sticks to wet stuff. It expands to fill a hole, and being foam, is easily drilled out when you want to refit the deck fitting.

Another tip - apply mastic only to the deck fitting - not to the backing plate under the deck. That way, if the mastic on top leaks, you'll know about and and can redo the job. Applying mastic to the backing plate might hide the fact that the deck fitting is actually leaking, and who knows where the water might seep to?

Clearly, fixing all these holes would be a long job. Would I need to call on Rob again to check that all the holes I fixed had solved the problems? I decided I'd better have one of those magic moisture meters so that I can verify for myself that my work to remove moisture and seal up holes really was working. It was expensive, but I think it might be a worthwhile investment for anyone with a foam sandwich boat.