Keel design for shallow water
by Andrew Rice
Instead of a standard keel configuration with a lead bulb attached to the end of a long fin, a centreboarder sees lead ballast incorporated into the bilge to provide the necessary stability. A lifting centreboard then pivots up and down to provide the lateral resistance whilst sailing, fitting into a recess in the hull for minimum draught when not required.
Briand is well acquainted with this concept. We have done probably more than a thousand production boats like this so we know the naval architecture of this configuration. But this is not the kind of solution I have considered for large yachts, because compared with a lifting keel, it has a lot of downsides. It leads to a heavier boat. And the efficiency of the centreboard is also in doubt, because it requires an opening in the bottom of the hull and creates some drag. It is a much less interesting solution as far as the performance of the boat is concerned.
Nevertheless, there is a growing demand from owners who are prepared to compromise ultimate performance for the ability to reduce draught to its absolute minimum. Malcolm McKeon of Dubois Naval Architects relates the story of Nirvana, a 53.5m, 2007 Vitters-built ketch.
The owner wanted to go world cruising with his family, and in order to be able to anchor near the beach, he didnt want more than three metres of draught, he says. We thought, as the design developed, we could convince him that it was unusual to go that shallow, and that we would persuade him to increase the draught to 4.5 or five metres, which is a more conventional fixed draught for a boat that size.
When it became obvious that the owner really wasnt going to accept a draught of more than three metres, the design office started looking at a variety of lifting keel and centreboard ideas.
There are two ways of achieving stability with increased draught or increased beam. So with the extreme shallow draught, we opted for more beam and at the same time all the ballast was placed internally; in this instance we had to use 50 per cent more ballast than we would have done on a boat of this length.
Nirvanas generous 11.6m beam is about a metre wider than it might otherwise have been; with the pivoting centreboard down, the draught increases from three metres to a whopping 10m.
The efficiency of the yacht under sail is exceptional, says McKeon. The centreboard is a very high aspect ratio foil so it was made out of high tensile stainless steel to withstand the extreme loads. We tank-tested the design to confirm sailing performance.
One of the additional benefits of the centreboard is how well it dampens the seasickness-inducing roll of a large yacht downwind, and while the pure performance will never live up to a lifting keel alternative, the success of Nirvana has now led to a variation on the centreboard theme with the 57.5m ketch built by Royal Huisman,_ Twizzle_, and to a third-generation 56m centreboarder currently under construction at Alloy Yachts.