Join the Cat Crowd
There's no shortage of good reasons to climb aboard a catamaran
Loads and Righting Moments: A cat bears heavier loads because of its inherent stability
If cruising catamarans have failed to live up to the mayhem predicted by early critics--oceans littered with upturned boats--it's due in large part to lessons learned from accidents with early racing machines. Many capsizes were the result of the lee bow immersing under the dual influences of wind pressure and wave action, which caused the boat to pitchpole. A key factor proved to be the relationship between length and beam. It's no coincidence that the maximum beam of a cruising cat rarely exceeds 55 percent of its length. And high freeboard forward isn't there simply to provide headroom but to create reserve buoyancy in that all-important lee bow.
It's rare for a catamaran to capsize under wind force alone. To render the chance as unlikely as possible, designers limit the sail area of vessels (such as charter boats) that might end up in the hands of less experienced cat sailors. A typical situation that might catch the unwary is a tropical rain squall, in which the wind can gust from 15 to 30 knots in seconds. Even with sail areas small enough to minimize such a threat, most cruising cats have adequate sail power--sail area-to-displacement ratios in the low 20s--for all but the lightest conditions.
The reason for employing such a cautious approach to design is simple: Catamarans don't give the same clues as monohulls do when they're overpressed. Most important, they don't heel.
At small angles of heel, a monohull has a small righting moment. As wind strength increases, a monohull responds by heeling, which dampens the shock load in much the same way as a stretchy nylon line absorbs energy. A monohull's righting moment increases until the heel angle reaches about 60 degrees, and it remains positive well past 90 degrees, at which point the heeling force becomes minimal, and the boat begins to right itself.
A cat's righting moment is the resistance to immersion offered by the leeward hull. It starts out as a measurement much greater than a monohull's, so the boat is unable to absorb wind gusts by heeling. A wind force on the sails that would cause an average 45-foot cruising monohull to heel 15 degrees would heel our average 40-foot cat only three degrees.
When a cat does heel, its righting moment increases until the leeward hull carries the entire weight of the boat and the windward hull is flying. For practical purposes, designers consider this the angle of vanishing stability because from this point on, as the boat heels farther, righting moment diminishes rapidly as the center of gravity moves closer to the leeward hull. On our theoretical average boat, this angle is about 16 degrees. It's fairly general practice among naval architects to design cruising cats so that in theoretical static loading conditions, this point won't be reached in winds under 35 knots.
As discussed in "More Righting Moment Means Less Heel" (see the sidebar), the maximum righting moment for an average 40-foot cat is approaching twice that of an average 45-foot monohull. Since this is the starting point for calculating rigging loads, it follows that spars and standing rigging have to be substantially more rugged. Here, the cat's wide platform works in its favor: The wide shroud base reduces the shroud load needed to support the spar, which in turn reduces the compression loading on the spar. Nevertheless, working loads are high, and standing rigging tends to be heavier on a cat than on a monohull.
Moreover, the compression load from the mast must be carried in an unsupported area, literally in the center of a beam, and the headstay load likewise. These loads and the racking forces generated as the boat moves through a seaway demand structures meticulously engineered to be stiff. Any flexing will generate damaging cyclical loads in the hull and the rigging.
However placid a catamaran may appear at rest, once it gets moving, it's a powerful creature and deserves respectful handling by the crew.