Time to take stock
In 1998 as a 26-year-old designer I was delivering my first design commission across the Irish Sea to Cork Week in fresh conditions at night, worried sick (literally) that I might not have done my engineering calculations correctly. Back in those days I didn’t have access to Finite Element Analy- sis (FEA) software that I could compare against my hand calculations for the keel and the aluminium frame structure that supported it, and the World Sailing plan approval process wouldn’t be in place for another 12 years. Apparently the calculations were fine,
or at least the boat and keel are still around and together, and we’ve never lost
48 SEAHORSE
any other keel of our design either, but it does illustrate quite how ‘unconstrained’ an industry we were in. Nowadays our keel engineering is always
carried out under the direction of an experi- enced engineer, usually myself. To ensure realistic analysis we always carry out a com- bined FEA of the keel and the structure to which it is attached using an integrated FEA tool (Siemens NX Nastran), the FEA loads and constraints are checked, hand calcula- tions are undertaken to ISO regulations and then the drawing package is reviewed by Det Norske Veritas Germanischer Lloyd (DNVGL) for World Sailing approval. Suitable FEA software is expensive,
typically over £20,000 plus annual mainte- nance, the time is expensive and the plan approval is not cheap either, but I cannot afford to be worrying at night about whether we’ve made an engineering mis- take with a keel. Nowadays if there are then any question marks on the quality of the construction we insist on a full bend- test of the fin, to loadcase equivalent to 150% of a 90° knockdown. Some years ago we were asked to design
a keel for a New Zealand-based boat that had a recessed T-connection. After sending a preliminary drawing of a cast and machined steel keel to their local keel builder, we were told that that method was particularly expensive in New Zealand and
could we not design a welded construction that would be much cheaper. The answer was that using the conventional welded approaches there was no way we could design a welded keel that would have suffi- ciently low stresses at the junction between vertical and horizontal parts, and even if we could then we were not comfortable about relying on a single weld. That Kiwi customer chose to have our
machined steel keel design built in China, so the problem was resolved, but after some further pondering on the subject I did come up with a solution to both the stress and the reliability issues… instead of joining horizontally at the T-junction, we could make slices in the transverse plane, each slice connected to a pair of bolts. There would still be stresses induced by
the welding process itself, but large radii could be incorporated to minimise stress generally and, by machining each trans- verse slice to an I-beam shape, the structure could be extremely efficient, for example the top 2m of the keel of a 46ft racer carry- ing a 2.6 tonne bulb weighs only 170kg. That method has now been used in
seven keels built on three continents. Simply as a proof test three of the keels have been tested to 150% of the knock- down loadcase. It is an expensive method of construc- tion because of the quantity of machining
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