Langan Design Partners recently completed an extensive refit of the 40-year-old, 125-foot ketch named Axia. Ten years ago, we replaced the yacht’s original wing keel and skeg-hung rudder with a deep fin keel and spade rudder, significantly boosting performance for an owner focused on racing. Today, with the yacht’s use shifting to shallow-water cruising, the owner asked us for an efficient shallow keel that wouldn’t sacrifice too much performance. We agreed on a 3.733 m (12.25-foot) draft and designed an optimized bulb keel, which required substantial structural revisions to accommodate the new draft while maintaining performance.
To implement the new keel, we conducted a Computational Fluid Dynamics (CFD) study to optimize the bulb geometry. We analyzed various bulb shapes to determine which offered the required volume, maximum lift, and minimum drag. As the shallower bulb’s VCG rose, we slightly increased the yacht’s displacement to meet stability requirements. The result: a 26,480 kg (58,380 lb.) bulb, 8.537 m (28.0 feet) long, at the target 12.25-foot draft.
The original rudder had substantial span, but the new keel depth required a reduction to match. We removed the lower tip and added surface area to the leading and trailing edges to compensate for the resulting loss of lift.
The refit was executed at Astilleros de Mallorca in Palma, with a French keel foundry fabricating the new bulb and BM Composites handling the rudder modifications. The yacht’s draft is now 3.734 m (12.25 ft), down from 4.761 m (15.62 ft) with the deep keel — a reduction of just over 1 m (3.4 ft).