MULTIHULL YACHT STABILITY
Not for the first time the question was asked about the stability of multihull yachts, i.e. catamarans and trimarans.
Let’s start with a very general statement that, if we exclude record-breaking, racing machines from this discussion, the chances of capsizing are negligible.
First of all, over 90% of mass-produced catamarans have a “fuse” incorporated into their design. This means that if you end up with too much sail area for a given wind, the rigging will fail and the mast with sails will have gone overboard (which of course I do not wish to anyone, but it is a much more desirable solution than capsizing the boat), before the force on the sails reaches a level that allows lifting windward hull out of the water. This applies to static conditions in flat water. Situations in which production catamarans have overturned involved the wind and waves so extreme that potentially even a monohull could capsize. Of course, the obvious con of any multihull in this case is that it will most likely remain upside down. Once again I would like to remind you that we are talking about extreme conditions, that are entirely avoidable if you couple a decent forecast with a reasonably fast multihull. Most cruising monohull yachts are too slow to effectively position themselves to the weather systems, so they are rather and hopefully build to withstand harsh conditions and endure them rather then escape them as in the case of fast multis.
With trimaran the situation is similar to catamaran, except that the force needed to raise two hulls is significantly greater than that required to fly one hull in a catamaran. It is even more so since one of those hulls is the central one in which about 80% of the whole yacht’s weight is concentrated. Hence, one hull in a trimaran with enough dihedral flies relatively easily and safely, while the other two are much more stable at than a catamaran with both hulls in the water, because our other two hulls are now on the leeward side and the righting moment increases radically, while the weight of a windward ama (outer hull on a trimaran) airborne only adds even more to the righting moment. Of course, greater beam of the entire superstructure of a typical trimaran has a dramatic effect on the hull stability and increased righting moment.
To summarize, we often sail on two of the three hulls, but never on one. We sail fast enough to choose the weather we intend to be in. A few times we consciously chose forecast with winds above 50 knots, but only after positioning ourselves to the weather system so that we safely sailed at 7-12 knots on a bare mast on an autopilot in comfort and perfectly stable. Of course, I do not recommend conscious choice of such weather without adequate experience of the crew and equipment, which is not only suitable for this in terms of design, but also in terms of condition and preparation.
There is a niche group of production catamarans for enthusiasts such as Gunboat, Outremer, or Catana, which have purposely designed strong enough rigging and mast to allow safely to generate loads that allow to fly a hull. Remember, however, that even in this high performance regime any mutihull is far from the tipping point. The righting moment drastically increases in the multi-hull yacht up to 45 degrees of heel on average depending on the design and load distribution. Only when we insist on continuing to load the rigging past the peak of the righting moment curve and do not let go of the sails are we destined to capsize.
Again please keep in mind that this is a highly averaged out simplification and generalization applying to flat water conditions. With large waves on a beam this safety margin in terms of large righting moment to high heel angles drastically decreases. So don’t try this at home without proper instruction and practice.