Stability
Considerations Part 3


So far I have only talked about static or steady state conditions. In this article I will talk about stability in cross seas and in gusty winds  conditions that in practice are the most dangerous. Wind gusts are dynamic forces, again I won't dwell on the mathematics, except to say that it is the DYNAMIC stability of a boat that counteracts the effects of wind gusts, and this is proportional to the area under the stability curve (see Part 2 for more details of stability curves). In the graph below I have shown two cats, one heavy and narrow (Cat 1), one light and wide (Cat 2). Using conventional formula the wide cat appears to have less stability than the narrow one, but I hope you can see that the area under the stability curve when sailing in 25 knots of wind is nearly double that of the narrow cat. Thus the wide boat is actually the safer boat when sailing in gusty conditions. Everything I have said so far assumes that the catamaran is sailing at normal angles of heel. However, there are times when the boat might come very close to capsizing, either due to a wind gust or maybe in the "ultimate storm" with no sails set. So what happens at high angles of heel (6070 degrees) is vitally important. There are two factors that will help decide whether a cat can survive such extreme heel angles. These are hull flare and the buoyancy gained by having a large cabin that extends out to the gunwales. To help illustrate this, the following graph shows both the static stability curves for our 35' Mira design (which has both good hull flare and a long cabin) and the Tornado type beach cat hull shape with vertical topsides and flat deck. The bump in the Mira's stability curve shows, quite dramatically, how much extra large angle stability can be gained. Also shown is the well known curve from the 1979 Fastnet Race report of the 30 ft Grymalkin, which capsized in the race and stayed inverted for several minutes with the result that several crew lost their lives. Since Gymalkin and Mira have very similar displacements, sail areas and waterline lengths, a direct comparison of their stabilities is reasonable. As you can see, the Mira has considerably more dynamic stability (the area under the curve is obviously more) while its static stability is only less than Grymalkin's at over 70 degrees or more. From practical experience we know that it is virtually impossible to heel a cat to this sort of angle, so I would always prefer to be in a cat than in a monohull. This graph also shows that a Mira is stable to about 95 degrees of heel (depending on loading), whereas many offshore monohulls are only stable to about 105 degrees. The recent race results (Vende Globe, Around Alone etc) as well as cruiser disasters have shown that its not only multihulls that have stability problems. Before concluding this series of articles I must mention a bit of a cross over between theory and practice. Some years ago the Wolfson Unit at Southampton University conducted some model tests in their wave tank of both a power cat and a conventional deep V power boat to see which was the more stable in waves. People accept that a modern powerboat, like a Princess or Bayliner does not capsize, yet the test showed how EASY it was for a model powerboat to get rolled over by even relatively small waves. In comparison, NOTHING the Wolfson Unit could do would make the powercat capsize! Believe me, they tried everything! So, how does all this theory affect you in practice? Well I think the main thing (which should be obvious to everyone) is that multihulls are not the dangerous unstable vessels that many people think. In fact they have proven more seaworthy than many "conventional" sail boats. Perhaps the most obvious statistic is that, to my knowledge, NO cruising catamaran has EVER capsized when under bare poles. To capsize with sails up implies a fair degree of "operator error", whereas its only when under bare poles that the boat MUST look after the crew. Theory also shows that the most dangerous time is when close reaching in a big cross sea. Speed can help throw you over when on the crest of a wave, while if you bear away the boat speed and hence apparent wind speed will increase, and with it the heeling forces. Similarly if you luff up the apparent wind will also increase. So the only way to increase stability is to ease the sheets. But that's often hard to do unless the deck layout is well organised because the sheet loads will be high. In fact the major conclusion from these two paragraphs is that just because you "never race" is no reason why you should not have a well sorted deck layout and be able to reef readily and release all sheets at will. Finally, sailing catamarans is SAFE! otherwise I for one wouldn't do it! 