Keel design for shallow water
by Andrew Rice
The resulting FC Cube 100 would seem an obvious candidate for a canting keel, but as Finot Conqs David de Premorel explains, they decided against it. We did look at having a keel that was both lifting and canting, but it would have been a big weight penalty.
Since the intention for the boat is to do some of the big offshore races, you need a minimum AVS an angle of vanishing stability of about 105 degrees for a boat of that size. Its something that is indirectly specified in the sailing instructions of these races, and also a basic safety feature for the boat. The problem with a canting keel is that, once youre canted, your capsizing angle decreases.
If you want a canting keel and the same minimum capsizing angle, you need either a deeper draught or more bulb weight to compensate.
In the end the canting keel option was rejected on safety grounds, but also with the problems of maintenance making it less attractive too. Instead, the FC Cube 100 is being built with a lifting keel giving a 5.4m maximum draught.
We would have loved to have an even deeper maximum draught, says de Premorel, but the lifting movement of the keel is limited by the height of the hull and deck. If you limit yourself to a certain figure when the keel is up, it mechanically limits you to a certain draught when the keel is down.
The only other option would be to install the lifting ram above deck, and apart from the technical challenges, for most superyacht owners this would be too much of an aesthetic sacrifice.
So what other options are out there? If you cant achieve sufficient righting moment with one keel, what about having two? After all, were used to the idea of two masts. Briand doesnt dismiss it as such a silly idea.
Twin keels have been done in the Americas Cup for some time, and its a configuration we studied when weve been involved in design projects for the Cup. So we know a bit about this, and yes, this could be a solution for bigger boats.
Maybe one time we will do it; the only problem we have with that is the ability for tacking and manoeuvring, as having one keel forward and one aft has a big effect.
However, as Briand acknowledges, if the tandem keel was worth considering for Americas Cup racing, and all the tight manoeuvring that this kind of racing entails, it should be good enough for the more sedate world of blue-water cruising.
I wouldnt be surprised to see this in the future, he says. We certainly wouldnt rule out the possibility.
While the tandem keel remains just a concept at superyacht level, one alternative configuration that is already making its way in the superyacht world is what Briand refers to as a centreboarder, otherwise known as a whale body.