Most sailboat manufacturers like bolt-on keels for a number of reasons, mainly economic. But for the custom builder there can be better ways of doing it.
Among the most vulnerable components of a modern sailing keelboat are the appendages below the water – meaning the rudder and keel. Whereas the loss of a rudder can be an embarrassment – hopefully more a nuisance than a threat to life – the loss of a keel is a catastrophe. Back in May 2015, the Beneteau 40.7 Cheeky Rafiki lost her keel while being battered by 5m waves in the North Atlantic. Sadly all four on board perished. Six crew members were also lost when a 42 footer lost her keel off the coast of South Africa. And these are not isolated incidents. Over recent years several other yachts have met the same fate and other lives were lost. Make no mistake. If you lose your keel you are in serious danger.
Having been a professional yacht surveyor for several years until my retirement in 2003 there were a number of occasions when I had come across serious damage to fin keels – either to the keel and its fastenings or to the supporting structures within the hull. In nearly all cases the keels were of the bolt-on type.
Very often the problems had arisen due to grounding, as shown below. Having run aground, it's all too easy for the owners to forget about such incidents – indeed embarrassment may encourage them to gloss over the events. Then later, with the boat hauled out and blocked off ashore for the winter, owners may have a quick mooch around to see if there's often little to be seen. The boat's weight could well have closed any gaps, with little evidence of any serious damage being visible.
Bolt-on keels have long been a favourite of boat manufacturers. Keel are generally simple objects cast in iron or lead and relatively inexpensive to manufacture. There are specialist foundries who do such work. One of the attractions of such keels is that they can be fitted late in the build schedule. This means hulls can remain conveniently near ground level for the bulk of their assembly period.They are also easier to transport, delaying their fitting until they arrive at their new destination.
So when I came to design Shindig I needed to find a way of devising a keel that,without resorting to traditional attachment techniques involving keel-bolts, could be fitted relatively late in our schedule. After a good deal of head scratching, the drawing below shows how we went about it on Shindig.
The concept was simple enough. Basically the plan was to have a stub that had a wide horizontal flange inside the hull and a foil-shaped protrusion extending about 40cm downwards below it. A bit like a mortise and tenon joint with the stub being the tenon. The photo below shows what the stub upside down having epoxy adhesive applied to it ready for turning over and dropping into place. A few screws of no structural significance were added to restrain movement while the adhesive cured.
Later, the keel shell itself would be bonded to it; the accurate matching of the two components being guaranteed by using the stub as part of the mould. This ensured that they would fit together perfectly. Incidentally, vinylester resins were used throughout for the laminating because they cure more slowly and evenly then polyesters, thereby limiting the distorting exothermic heat effects that can arise with the latter.
Naturally, ballast keels are not exactly feather-like objects. Of necessity they must be heavy. In our case the weight would come in the form of a solid lead casting when the time came – the pattern for which we supplied to the rather appropriately named Irons Brothers foundry based in Cornwall. This was lowered into the keel shell before it was bonded to the stub (see below). The casting was deliberately quite a sloppy fit so there was no risk of it jamming. Any gaps around it would later be filled by injecting a slurry made up of lead shot and, again, vinylester resin. This was done after the keel had been attached.
Shindig's keel has a flared bulb at its lower end which required a separate lead casting at its base. Since this was a simple, relatively shallow casting we thought we could cast it ourselves as you can see below. The lead was melted in a gas-heated crucible which had once been the tank from an air compressor. A simple sand mould was used to obtain the right shape. Its flat upper surface was left bare but the curvaceous underside was sheathed to the same thickness as the fin shell. Toys for boys did I hear someone say?
After some careful attention to abrading the mating surfaces, attaching the keel was simply a matter of lifting the hull with a crane, applying a high-modulus epoxy adhesive to both stub and shell surfaces and gently lowering the boat so the two components slid together. See below.
Once we had checked that the alignment was spot-on, a pair of 10mm diameter countersunk stainless steel were added on each side, passing through into the the cavity inside the stub. These added very little structurally but prevented any movement while the epoxy cured.
A relatively minor task was to bond the bottom of the cast bulb to the keel. As you can see, a thick laminate had been added to the underside to take any abrasive wear that might later occur. The keel and the bulb were bonded together with a moisture cured polyurethane sealant and, later, the 'seam' would be heavily glassed over to give continuity to the laminate.
And how has the keel fared? Well, I must admit we've touched bottom a couple or so times but Shindig has traveled many thousands of miles since she was launched in 2001 and we've discovered that not all underwater obstacles are charted! Yet I'm pleased to report that there's absolutely no trace of movement or any other forms of structural distress to the keel and its surrounding areas.
Yes, it was a fairly time-consuming task, but what price can you put on peace of mind?