Power cat capsize, slamming and general seaworthiness issues: Having maintained that the catamaran is just as appropriate for power as for sail for over 3 decades, I welcome the chance to comment on this question. It is not however, one that can be covered in just a few sentences, so please bear with me. Australia and to some extend NZ is the home of the power cats, but rapidly expanding in the rest of the world. I am not aware of any power catamarans having capsized - which to me is an amazing statistic considering the way that cats were viewed by virtually everyone when I first started sailing multihulls in 1960, though part of the explanation could be the relatively small number that make ocean crossings and the ability of a fast cat to move away from a storm. However, it does fit with the conclusion that I reached about 20 years ago. There were a number of instances of sailing trimaran capsize where the crew were helpless to avoid it. The wide sailing catamaran, on the other hand, is usually capsized by the crew becoming over confident or unprepared or just pressing too hard. Take away the sail power moment and well designed power cats should be almost totally free from capsize in offshore operation, but do not be tempted to reduce overall beam too much. I believe the instances of catamarans surviving, sometimes after the crew have abandonned ship, in conditions where monos have not, bear out this view. We have all seen the sketch of the cat on a wave illustrating the widely held theory that the cat slides sideways in front of the breaking wave. My limited experience in storm conditions, the situation a number of my clients have related and a tank test video I saw, all, I believe contradict the theory. In one 3 day storm we were able to run with it. We picked up speed on the wave front, ahead of the break and ran in front of the worst of the break. However, imagining lying ahull to those waves did not fit the picture. A couple of clients have described being in a situation where they expected to capsize from the size of the wave and the angle of heel, but then suddenly found the cat was back on its feet again. One of these clients decribed the waves passing while lying ahull and of the most disturbing part being the fall of the windward hull into the trough as the wave passed. The video demonstrated the situation as I understand it. The tank test was done in Southampton comparing a Lock Crowther catamaran ferry with a comparable mono, in large breaking waves and high wind, with the models lying ahull. As the wave hit the cat, which was still in the water, the windward hull was thrown up into the air, looking like an immediate capsize, but before the angle of heel had gone to 30 - 40 degrees, the wave had passed to the lee hull, lifting it just as rapidly to bring the cat level and then fall as the wave passed. The cat never capsized. The mono rolled every time. This situation actually applies whatever the orientation of the catamaran to the waves. Of course, there are situations of confused seas which could produce a very different result and is probably the explanation of a few of the sailing cats going over. There is an area around Bermuda which is noted for confused seas and which has claimed a number of cats and tris. For the theory of sliding sideways to be true, the cat would need to accelerate from about minus 1kt. to over 20 kts and it would need to do so in half a second!!. If the sliding theory was correct, all of the sailing cat designers would be introducing an unseaworthy feature in fitting permanent low aspect ratio keels. There is a danger here of talking about power cats and lumping them all under the same heading. There are power cats which are not intended for serious offshore and those that are, currently come in all varieties, shapes and sizes. There is some coming together of views, such as most designers using displacement hulls but on other aspects there is a lot of diversity. To me, what we have learnt over the last 4 decades of sailing catamarans is the starting point for designing power cats for offshore. Obviously, not all designers share my view. There are cats that are high and wide and with end to end and side to side accommodation on wide hulls with high bows and probably with the bridge-deck extending forward and aft of the waterline. They look like big powerful commercial craft. If they are big enough they are probably OK offshore, but will have none of the advantages of performance and easily driven hulls, which is the reason that most of us choose catamarans. At the other end of the scale there are lightweight cats with a small accommodation space on long hulls extending well forward and aft of the bridge-deck. Between we have; the tunnel cats which are like monos with a tunnel down the middle; some wave piercing styles which look like monos with training wheels and we have the power cats, which are not that different from the sailing cats in their general arrangement. Slamming is obviously a very important question, and particularly when the cat can be powered straight into waves. Most cats will lift their bows at speed, but slamming is likely to be worst at the time when it is essential to reduce speed. Bridge-deck shapes which soften the slam are a good idea on a cat with that forward bridge-deck, but surely better to let the wave pass without hitting the structure of the boat or at least give it some chance of doing so by keeping the bridge deck both short and high in the forward area most likely to slam. I watched the Americas Cup from the TV catamaran - a 50 footer with 16ft. beam. In the lumpy seas, we and 90% of the dozens of cats around us were slamming. With just one or two exceptions an extra 6ft. on hull length would have solved the problem. Weight is the most significant factor in all cat design. It is the starting point for every aspect. Very slim hulls are attractive for low wave making resistance and when matched by the appropriate low all up weight there is no better craft. However, those slim hulls are much less tolerant of overloading. Making the hulls deeper in order to carry weight is not in my view a satisfactory solution. The high initial stability is a factor to be considered in the detail of the design. The cat follows the pattern of the waves to a greater extent than the equivalent mono. When meeting a wave the cat rises to it more quickly giving a rapid motion. A beam sea will lift the windward hull quite rapidly, which can make life in a high wheelhouse or flybridge very uncomfortable. Longer hulls and less superstructure make better catamarans. I noted one comment on aluminium welds breaking. Metal and welding is a poor structure for joining a hull and bridge-deck. A thin plate is going to flex. A chamfer plate helps. Fiberglass and foam sandwich structures are ideal, particularly as it is easy to increase scantling as needed. Power catamaran development lags slightly behind the sailing cat development but there can be no doubt that the current explosion of interest in these craft is justified by the numerous advantages and will continue to expand. My advise to anyone considering a cat for its performance and ease of driving that may encounter a storm is --Watch the weight factor in relation to the hulls and general arrangement, very closely. --Ensure the cat design is a good compromise between allowing the waves to pass as unhindered as possible under the bridge-deck and the height of structure and windage. --If going for ultra slim hulls, ensure the cat is ultra light and stays lightly loaded. --Leave the high enclose wheelhouses for the inshore cats. --Do not be tempted to make max use of all the available space on lengthxbeam. and do take along that sea anchor. I will take the opportunity if I may, of calling attention to our latest, but I think perhaps most exciting development for all catamarans. All our designs are now drawn for modular assembly. A very very simple idea, but the effect on build time and hence on cost will be spectacular. See http://www.kelsall.com. There is also more detailed comment on power cats. Derek Kelsall Kelsall Catamarans R.D. 2., WAIHI, NZ PH: 07 863 3332 FAX: 07 863 7915 EMAIL: Derek@kelsall.com WEB SITE: www.kelsall.com