Above: the 40-footer Showtime, originally designed by Ker Yacht Design but which has since then had a locally designed and built replacement keel fitted in Australia. During the delivery back from this year’s Hobart finish the keel snapped off at the hull and the boat capsized in the middle of the night. The crew survived through some quick thinking and the bravery of crew Christian Charalambous who, in pitch dark and with the inverted hull slamming violently, removed his lifejacket to dive back under the boat to retrieve the liferaft. Below: there are some keel failures that are less life threatening than others… if you are lucky. This is Marc Pajot’s French Cup challenger France 3 which lost its keel off San Diego in the 1995 Louis Vuitton Cup. … And others that are deadly. When the keel snapped off the 100-foot Rambler while approaching the Fastnet Rock in 2011 (opposite and above) it was a miracle that no one was lost. Shamefully eight years later the engineering report on the Rambler failure has yet to be released
In the pitch black of the early morning
and, as with all hollow metal structures, the thinner walls mean particular care must be taken to avoid corrosion. So not the right solution to every case, but certainly a solution to ‘light and hollow’ that we are prepared to provide. The introduction of the Plan Approval
process was a big step forward but it should not be seen as a guarantee that a structure will not fail, but rather that according to some simplistic checks it complies with the letter of the ISO struc- tural regulations. The ISO loadcases are generally ade-
quate when it comes to keels, but for hull pressures in slamming zones some modern- shaped boats are clearly seeing greater loads, with many racing boats that pass ISO requirements then experiencing cracked ring frames and sheared cores. For keels it is easy to assess the ‘big
picture’ numbers at the sections up and down the span, the flange thickness, the bolt sizing, but quite hard for the approval body to properly assess local stresses from a set of 2D drawings. It is also hard to really assess from a set of 2D drawings how the structure inside the boat and the keel itself deform together and load each other, as that can have a large effect on the load sharing of the bolts and the stresses in both the boat structure and the keel. For the boat owner commissioning a
new boat, or just a new keel, there is no absolute guarantee that failures of engi- neering or construction will not happen. DNVGL can be expected to perform a good service within the scope of the plan approval process, potentially picking up low-hanging fruit of obvious errors. For greater reassurance the yacht owner’s best options are: 1. Choose a design/engineering office with the capability to analyse the keel system in detail, and the experience to spot if the analysis is producing results that can be reasonably expected. 2. Extend the scope of DNVGL’s remit, with agreement from the design/engineer- ing office, and require them to apply detailed FEA to the full keel and internal structure assembly.
Showtime incident The Sydney-based Ker 40 Showtime had a replacement keel designed by a local Australian naval architect, accommodat- ing IRC changes since 2011 by increasing draft and reducing fin and bulb weight in a similar manner to several previous and subsequent optimisations by our office for its sisterships. We are informed that the keel was of a welded hollow construction similar to other keels by that naval archi- tect but connected by a T-flange to the original boat structure.
of 6 January Rob Buchanan, the skipper on the return delivery from Hobart, was sleeping on the windward bunk of Show- time when he woke on hearing a ‘ping’ noise; he was immediately thrown to the leeward side of the boat as it capsized. After adjusting to the new inverse environ- ment he realised that the inverted hull was holding air pressure. When later outside the boat it became clear to the crew that the vertical part of the keel fin was not visible. Putting those facts together, we deduce that the horizontal top plate of the fin remains intact and bolted to the boat, while the fin had failed at the junction between the vertical and horizontal parts. An Australian Sailing inquiry has been
announced with a remit to learn from the failure and the survival of the crew.
Keel methodology Casting – has the advantage that the material can be close to the final desired shape. Potential risks include porosities in the casting. Hollow shapes are possible through the suspension of ceramics in the mould. Two common casting routes are: l Ductile iron with a T-flange – such as SG 500/7 with 500mPa ultimate strength and 7% elongation. This material is easy to cast and offers one of the lowest cost methods of keel construction. Thousands of keels have been made of ductile iron, often with lower grades that have higher elongation, but 500/7 grade is common for more performance-focused applications. l Cast and heat-treated steel – such as 4340 steel. Usually machined from a casting that is sufficiently oversize to accommodate the bends that are prone to happen during the cooling process.
SEAHORSE 49
GILLES MARTIN-RAGET
RICHARD LANGDON/OCEAN IMAGES
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