Bulkheads are a vital component in the structural integrity of any boat, but are prone to damage. Rupert Holmes outlines typical problems and how to solve them.
Whatever the main construction material of the hull and deck, marine plywood is almost ubiquitously used for the structural bulkheads in boats. It’s relatively cheap, has a good strength-to-weight ratio and providing the timber is kept dry it has great longevity. However, if bulkheads don’t stay dry the timber will rot, or the glue joining the laminates will fail.
In particular, it’s vital to ensure deck fittings are properly bedded down on a marine-grade adhesive sealant, to investigate any deck leaks as soon as they become apparent, and keep the bilge, and any lockers dry.
Unfortunately, previous owners of my 1975 Quarter Tonner Minestrone had been remiss in this respect, and when I started to refit her some of the bulkheads and structural ring frames were really quite damp. Although a cursory visual inspection didn’t reveal any obvious problems, a quick check with a moisture meter showed sky-high water content, and it didn’t take long to discover places in which a screwdriver could be poked through the timber.
In addition, there were numerous cut-outs for instruments, speakers and other fittings, which seriously weakened the starboard side of the main bulkhead at the aft end of the coachroof. This had seven holes, many of them close together, and an area at the top of the outer edge was waterlogged thanks to a badly leaking stanchion base.
Drying the timber
The first priority when starting this kind of work is to dry the timber out as thoroughly as possible. There are many potential water traps that
will stay wet even if the rest of the timber is dry, so the first task is to expose as much of the bare wood as possible. On Minestrone this involved stripping the paint and some of the fibreglass tabbing that bonds the bulkheads to the hull. This may sound drastic, but it would otherwise be next to impossible to dry out the relatively thin layer of timber sandwiched between the two skins of fibreglass, or even to examine it properly to check its condition. In any case, polyester resin doesn’t stick well to damp timber, so the structural integrity of the fibreglass was already suspect.
Using a hot-air gun to strip both paint and the fibreglass that needed to be removed also helped to start drying the timber. When stripping
the glass, the hot air was used to soften the material, so that it could be removed with a paint scraper, taking care not to heat any glass that we wanted to keep in place. At this stage we removed any timber clearly delaminated or rotten, generally by cutting it away with a jigsaw, helping the remainder to dry thoroughly.
As we were unable to guarantee dry weather at the time we were carrying out the work, we put the boat in a shed, and used a dehumidifier to speed up drying the interior. Taking further moisture meter readings every week or two we were able to track the progress of the drying of the timber. Once it had reached an acceptable level — less than 15 per cent content — we were able to ascertain the extent of the damage and how much timber would need to be removed.
Finding the extent of the damage
Tapping gently with a light hammer identified the sound timber (a clear ring) from damaged material (a dull thud). Some of the unsound wood could be removed with a sharp chisel, without even needing to tap it with a mallet, but for the most part we used a jigsaw — set at the most acute angle possible to cut the material away, checking carefully that only sound material remained.
Having cut back to solid timber
and allowed it to dry, the next step was to make templates for the new wood. Stiff card proved ideal for this, together with masking tape used to extend the card in places that too much had accidentally been cut away. It was then a relatively simple matter to cut out new ply panels. These were bonded in place with epoxy thickened with microfibres and filleted at the top to spread the join at the deckhead over the widest area possible.
To hold them in place while the adhesive cured each joint was sandwiched between battens clamped each side of the bulkhead. Scrap pieces of thick plastic sheet between the batten and bulkhead enabled them to be easily separated once the epoxy had set.
Rebonding bulkheads to the hull
The final stage was to re-glass the bulkheads to the sides of the hull. Although the bulkhead faces were now clean and dry, the paint on the hull needed to be removed to ensure the new glass would stick. Then the surface was roughened to provide a mechanical key and wiped down with acetone to soften the top surface immediately before laying up.
I used three layers of chopped-strand mat, each one covering a slightly wider area than its predecessor. This helped blend the new glasswork into the line of the hull, avoiding the concentrations of stress that can be created at a hard edge. Instead of cutting the mat with scissors I tore it apart at the edges — although this can appear untidy before starting work, it helps to feather the edge of the repair.
Having cut the glass mat to size, the procedure I adopted was to mix the resin, wipe the area with acetone, and then apply a coat of resin to the boat using a brush. The first layer of glass was laid on this, and more resin added, stippling it in place with the brush. I then passed a consolidating roller over the area to ensure the mat was fully wetted out, and that any excess resin was squeezed out.
The next layer was then immediately laid in position and the process repeated. Note that the most common mistake made by amateur laminators is to insufficiently wet out the mat. But don’t overdo the resin – although strong, such structures are more brittle than necessary because the resin lacks enough fibres to give it support – hence the importance of using a laminating roller to ensure the fibreglass mat is fully impregnated with resin but that any excess is squeezed out.
Not all Minestrone’s problems were down to water ingress — the forward-most frame appeared to have been badly bodged when Minestrone was built. It’s not uncommon for such frames to be made of several pieces of ply, providing they are joined with scarfs that taper over a distance of at least eight times the thickness of the wood. However, on the starboard side there was a badly fitting butt join with virtually no strength. The port side looked better, but stripping the paint showed this was a result only of carefully applied filler — I was able to separate the sections of ply using only a screwdriver.
This, of course, helped to explain why the foredeck felt fairly flexible underfoot, which I’d initially put down to Minestrone’s lightweight construction. I made the new pieces larger than the old. In the case of the forward frame, which was less than two-inches deep in the central section, I replaced it with a much more substantial piece, creating a semi-circular partial bulkhead.
I compensated for the extra weight of this by removing the large door to the forward locker — this area is too far forward to be useful stowage on a boat with fine forward sections like Minestrone. To help prevent future problems with the deck, I was also keen to maximise ventilation, but this door only hindered the circulation of air.
The main bulkhead
The starboard side of this had numerous openings that had been cut by previous owners, including four redundant four-inch circular cut-outs for 1970s-era instruments. Below deck the structure was further compromised by cut-outs, for stereo, switch panel and speakers, some of which protruded into the tabbing that secured the bulkhead to the hull. In addition the top outboard portion was next to a water-damaged area of deck and was soggy and delaminating.
This top section was the first to be dealt with — this was treated in the same manner as the forward sections. I did it before replacing the deck above, which made access easier. Next I made a doubling section to go on the hidden face of the bulkhead below deck and restore strength.
The cut-outs were then filled in — those below deck were cut square and glued to the backing piece, while those for the instruments had the edges feathered, to maximise the bonding area. Both sides could then be filled and faired to a neat finish, and painted with the rest of the coachroof and cockpit area. Minestrone’s new instruments are wireless Tactick units that are removed from the boat when not sailing, and don’t’ need a cut-out.
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