After my short post about the stability calculations of my Willard Horizon, I received several anxious e-mails from trawler owners about the stability of their own vessels. In addition there were a couple of additional e-mails about the equations used to calculate stability. The calculations for reserve or ultimate stability of BALLASTED DISPLACEMENT hulls are fully described in Dave Gerr's article in the October 2004 issue of Sail magazine and in most texts on boat design. These calculations are largely irrelevant for most of the trawlers owned by members of this list - so not to worry. RESERVE STABILITY is defined by Gerr as the number of degrees a boat can heel and still right itself. If tipped beyond this point the boat will continue to capsize. The angle at which capsize occurs is called the angle of vanishing stability (AVS). If you see the glass as half full, it is often referred to as the range of positive stability. The AVS is of great concern to owners of ocean going sailboats because they are subject to sail forces which cause them to heel and unusually strong wind gusts which might lay them flat on their sides. To provide a restoring force, sailboats carry heavy ballast placed as low in the hull or keel as possible. To be considered safe, ocean going sailboats should have a range of positive stability of 115 to 120 degrees or more. More is better. Trawlers, unless they have an unusually high tophamper are generally not subject to the same wind forces, nor do they carry a high percentage of their weight in ballast. Semi-displacement trawler hull stability comes largely from the relatively flat shape of the underbody which tends to strongly resist tipping. However, once knocked down to 90 degrees, few modern trawlers will recover. Such a situation is rare unless the boat is hit broadside by a breaking wave with a height greater than the trawler's beam. That's what small craft warnings are for. It is possible to design unballasted hulls which are self righting (i.e., USCG surf boats) however not many recreational trawler owners would put up with the design compromises necessary to do so. Trawlers with pure displacement hulls (i.e., Nordhavns, Willards, and Krogens, etc.) have soft chines and carry some ballast so the stability calculations are more meaningful. Basically the calculations consider the draft of the hull excluding the keel, the displacement, the weight of the ballast, and the overall beam. I'm not going to give the equations here because I remember the flap caused when I discussed Keith's formula for determining engine power required to achieve a given speed. If you want to do the stability calculations for yourself, do the research to find the formulas and measurements you need. More meaningful to trawler owners is the CAPSIZE SCREENING NUMBER. This simply estimates the resistance of a hull to capsize when hit on the beam by a sudden transient force like a wave or wind gust. The capsize number is simply the boat's overall beam divided by the cube root of the displacement volume in cubic feet. C = Beam/((displacement in lbs/64)^.334) Boats with screening numbers under 2 are considered resistant to capsize. Quite simply, the heavier and beamier a boat, the more resistant to capsize. It stands to reason that canoes and kayaks are more likely to capsize than garbage scows. None of this predicts how seakindly a boat will be in rough weather. Indeed, boats which have very high degrees of stability can give a rather unpleasant ride. It's like the springs on your car. Stiff springs are best for cornering but you feel every bump in the road. Nothing like my old Buick three hole Roadmaster. Larry Z