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Older Wood Cherubs provide a special challenge. We are talking
about a structure that is operating right on the limits of what
is practical with the materials. Right from the early days in
the 1950s up to the present it has always been difficult to build
a really robust and reliable wood boat that is also down to minimum
weight. Unlike most classes where minimum weight was a sine qua
non, it was always accepted that on many boats a few pounds of
extra weight was just one of those things, and there was no point
in attempting to reduce weight and risk compromising the structure.
As techniques and materials improved over the years changing rules
and techniques also saw the loads on the boats increasing, so
it never really got much easier. Perhaps the worst time for reliability
in the history of the class was the early 1970s, when poorly built
boats, often using glass tape and polyester resin gained the class
a terrible reputation for structural failure.
Fortunately epoxy has changed much of this. Its still a challenge
to build a down to weight and reliable wood Cherub, but it is
now easily in reach of the skilled amateur. More to the point,
the techniques that have been evolved using epoxies, especially
combined with appropriate fillers and reinforcement fabrics, mean
that its possible to maintain and restore a wooden boat to a higher
standard than was possible before. There are also Cherubs built
using plywood as a core system for glass skins, but that's another
game entirely.
A typical wooden Cherub will be constructed without a false floor, and with a transverse frame at the back of the centreboard case as well as the front bulkhead and transom. The cockpit floor will have a central hog and one or two stringers, with one supporting the side tank sides, and perhaps another one in the middle of the floor. The topsides and decks will be of 3mm or possibly 4mm plywood, and the floor will often be at least partially made from two layers of plywood, perhaps totalling 6mm, or may occasionally be "cold moulded" in two layers of overlapping strips of 3mm ply. The stringers will be quite thin - normally less than a square inch in cross section. There will be a foredeck - or there should be - and most probably a low bowtank which will go from chine to chine.
There are really three main areas to consider as troublespots, and most of them come down to the effects of hard spots where flexible ply is glued onto stiff stringers or frames, and the flexing gradually breaks down the glue joints. There is some evidence that Aerolite - the classic wood boat building adhesive - becomes brittle after 20 years or so, which exacerbates the problem. Epoxy and fillers really help here. Pick out the failed adhesive if possible and replace with epoxy. Then put a generous fillet along the edge. This should be a mixture of lightweight filler with microfibres to combine strength and low weight and some flexibility. About 3 parts lightweight filler to one part microfibres is appropriate. Avoid silica, which tends to reduce flexibility. Typically you will want to use microballoons as a significant part of the filler as they approximate a hardwood colour and will be less obvious. Unfortunately there is, so far as I'm aware, no filler combination that will resemble spruce stringers...
The most difficult to access is the area in front of the mast each side of the front bulkhead and forward. This area is both difficult to construct well and takes the main wave impacts in upwind sailing. The worst of the 70s boats would literally break apart here. What would happen is that the panels would flex under load, break away from the glue lines on the stringers, and the whole integrity of the structure would be lost. Sometimes for ease of construction the bottom channels reverted to single skin here which made problems all the more likely.
If you have to work in this area you are going to have to remove both foredeck and bow tank top. The aim is to reduce the hardspot effect around the stringers and bulkheads and to stiffen the panels. The amount of work you have to do will depend on what problems you find. Best practice is to put in generous lightweight fillets of filled epoxy wherever frames and stringers touch the skin in order to reduce the effect of the hard spots, and then consider reinforcing and stiffening the skin itself. Very often the best boats were made with internal glass reinforcement running from the chine to the hog on each side. This should be done if you have concerns with panel stiffness below the chine. Your reinforcement should run from on the stringer, across the skin and onto the hog. A lightweight carbon cloth is of course the ultimate choice here, but a high quality glass cloth - best is one of the bi-axial cloths - will be very satisfactory. It is best to arrange for the fibres to be laid at 45 degrees to the centreline. It would be possible to lay a criss cross of unidirectional carbon reinforcement with a glass cloth above it, but this will be unnecessary unless the underlying wood is very weak.
The second troublesome area is glue joins to the stringers and hog, especially along the centreline. Judicious use of fillets and glass tapes is appropriate here. Again you need to evaluate how much of a problem exists. If considerable glue failure is evident then the best choice is to run a double bias 45/45 tape along the join. These are not available in very light grades, but provide the best properties. Where there is less of a problem then lighter tapes can be used. The finer grade glass tapes will scarcely be visible at a distance. Moving to the outside of the hull there will often be problems at the join between the hull panels at the centreline. As boats became flatter the centreline join was often eliminated in the after part of the boat to reduce this. Again this is a case where glass reinforcement pays. Run a layer of glass across the seam from bow to stern. This one is very much a job for 45/45 alignment of the fibres, because of the loads you are protecting against. For some reason problems at the chine join are much less common.
Finally the most prevalent problem of all is the sidetank to floor join. This is a complex 3 way join, with loads in all sorts of directions, and so reinforced that there is virtually no give at all, so any panel flexing breaks the glue joins. They usually ended up bodged up with glass tape and polyester, applied to vaguely damp wood, and once it got to that stage were rarely ever satisfactory again. Start again is almost the advice! The most likely glue failure will be between the side tank and the stringer. If the decks are off anyway then for the cost of the plywood against the quality of result then I'd be tempted to suggest that you scrap the tank sides, clean up the stringers etc and put in new tank sides with properly filleted and epoxy glued joins. If you do that you will need just a fillet on the outside with maybe just the finest of glass tape, and the job will be so much neater as well as stronger. If you can't get access to the tank stringer join then clean out all the rubbish and dry things out properly, then put in a new taped and filleted join. This will have to be strong enough to take the entire load, because you haven't got any way of getting glue into the join between the tank side and the stringer.
Materials wise you will be using SP106 for most jobs. Its a
good general purpose resin which works well with wood and laminates.
The only thing you have to be aware of is that it is UV sensitive,
so need protecting with a UV resistant varnish.