"Faure, Marin" <marin.faure@boeing.com> At 12:59 PM 2/20/04 -0800, you wrote: >example, when the winds are 20 knots and the wind-waves are 4-5 feet, >the potential for amplifying a roll to the danger point in beam seas is >pretty high. It would be nice to be able to say, okay, the winds are I know the area, the nature of the winds and wave conditions where you mention. You will be screaming for mercy long before your 36' Grand Banks will capsize in the conditions of 20 knots, and 4-5 foot seas. At 10' feet you can start worrying. In the meantime, the boat can take more than you can. Maybe not the contents. It just so happens that 10' is about 35% of your LOA. Which also happens to be in the range from the tank tests discussed in Heavy Weather Handbook, which was part of my post about a week ago. Intelligent handling will keep you upright at far more than 10 feet, if need be. In practical terms, in a protected (short) fetch environment, assuming any practical wind speeds (say less than 50 knots, above which you are very unlikely to be out in), at 36' the only place that you would be likely to have waves high enough to capsize you, would be in shallow water, say less than 40'. If there are hard walls nearby, the wave reflections could make things less certain. What is far more likely to happen than capsize, would be taking a breaking wave in such a way as to do major damage, like breaking out a window. Followed by another wave bringing solid water on board, through the breach. You can imagine where this sort of thing can lead. Such a breach must be protected at all costs. Best choice would be to run off downwind, if possible. It would have been better to have headed there, before breaking out the window. Don't ever continue with the present course and speed where there is serious risk of having unacceptable damage. Change course and/or speed and pick another destination. Plan ahead so that you don't get into a situation where you can't change course and are forced to accept a pounding. One last comment. You wonder if the roll can be amplified until the boat rolls over? Assuming that your liquid tanks are not contributing to instability, broken or missing baffles, tanks partially slack with free surface effects, then there is almost no chance of synchronized rolling causing a capsize. You can pretty safely assume that lacking a breaking wave, your boat can't be capsized, period. And that it would take a broadside on breaker to do the job. But the amplified rolling is a problem and should be dealt with immediately, since if it were to couple with a breaking wave, it would take a smaller breaker, to over-you-go. This may seem like I am contradicting myself, but in any situation where you are encountering breaking waves, the chances that the roll rate of the boat will couple with the waves is a bit remote as the waves are not coming at the same rate, nor is the next wave after. This assumes the open ocean, not some surf zone on a beach. The theory looks like this. A few numbers first, from typical boats. 60' boat, 6 second roll rate. 30' boat, 4 second roll rate. 10' boat, 2 second roll rate. To achieve coupling the waves must come at the roll rate of the boat. Therefore a 60' boat requires waves coming at 6 seconds. Another wave period will not couple. It is very difficult to produce a wave 20' high, 6 seconds apart. And we know that it takes a breaking wave about 20' high to capsize a 60' boat. Such a wave is only likely on a shallow entrance, during the ebb, or in the surf zone. A 30' boat requires a breaking waves 10' high, 4 seconds apart. Such a wave is not common. But if the wave period begins to pass 4 seconds, then the coupling effect is likely to cease. And periods less than 4 seconds, are at any size over about 8 feet, are also very unlikely. A 10' boat requires a breaking waves 3' high, 2 seconds apart. Such a wave is fairly common. But if the wave period begins to pass 3 seconds, then the coupling effect is likely to cease. And periods less than 2 seconds, are at any size over about 2 feet, are also unlikely. Boats in between 10 and 60 feet will have roll rates proportionally. All these numbers and tables are just approximations, but give you some idea of what is actually involved in constructing a rule of thumb. My best estimate is that the numbers I have used are not far off the mark. In case you missed the salient point here. As wave period diminishes the chances for production of a wave of increasing height go down. Carried to extremes, try to imagine a wave of 1 seconds period, 10 feet high. It just won't happen. I was going to refer you to some technical explanations of all this, but I can't seem to locate a really appropriate chapter or page. Oceanography & Seamanship by Van Dorn would be my best suggestion. Mike Capt. Mike Maurice Wilsonville, Oregon (Portland).