This exchange is a follow-up to a recent article entitled "Defining Seaworthy" which first appeared on SailNet last week. Here, noted authors John Rousmaniere and Don Casey further explore the abilities of modern boat designs. If you have not read Don's article, click on the following link to familiarize yourselves with the issues being discussed: Defining Seaworthy.
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John Rousmaniere writes:
Your new SailNet column on evaluating boats for their offshore survival capabilities is bound to both help readers and stir up the boat builders. I have two corrections.
First, it's not the "CCA Capsize Screening Formula." This quick-and-dirty way to estimate whether a boat might turtle and stay turtled was developed during the Capsize Project that was undertaken in the early 1980s in response to the 1979 Fastnet Race calamity. The project was co-sponsored by the Society of Naval Architects and Marine Engineers and the United States Yacht Racing Union (now US Sailing). While many project participants were members of the Cruising Club of America, which has long been a leader in offshore sailing, the CCA as an organization had no official role.
Second, to say that the screening formula is an "alternative numerical evaluation" to a calculation of a boat's range of positive stability gives the formula too much authority and the stability range calculation too little. The first uses "only data that comes readily to hand." That's a quote from Karl L. Kirkman and Richard C. McCurdy in one of their excellent chapters on stability in the book Desirable and Undesirable Characteristics of Offshore Yachts, which I edited and recommend to anybody interested in offshore sailing. These data are beam and displacement. The formula's simplicity makes it handy when getting a quick evaluation of a boat based on, say, a builder's brochure.
While the screening test is a helpful tool, a more full and accurate picture of stability takes into account other factors, including the boat's size, displacement, ballast, and especially her range of positive stability -- the angle of heel at which the boat stops resisting capsize. As you say, boats with high stability ranges are preferred offshore. It requires a more complex calculation. The average owner could use the capsize screening formula, but probably not find stability range. An International Measurement System (IMS) measurement certificate shows stability range, and a yacht designer can calculate it.
Don Casey's reply:
Thanks so much for taking the time to write. I am honored and more than a little pleased that you liked the piece.
In response to the issues raised, I think I knew that attributing the capsize screening formula to the CCA was not entirely correct, but I needed some label to identify exactly what I was talking about, and I have seen it described in other publications this way. In any case, my understanding is that this formula was an eventual result of CCA concerns following the Fastnet disaster, so if the CCA was responsible for planting the seed, it shouldn't be that incorrect to identify the formula this way. My intention is not to hold CCA accountable, just to give the formula an unambiguous name.
With regard to your second concern, I am cognizant of the limitations of the CSF. On the other hand, I think far too many sailors fail to understand the potential implications of additional beam. And far too many manufacturers are perfectly content to let that dog sleep. I am trying to get the reader to pay attention to beam, and the CSF does that. Who can read this piece and not want to make the calculation for his or her boat? It may well be the first time that the boatowner ever considered beam in anything but a positive light. I do point out that "this formula fails to take other factors into account, most notably ballast." Its greatest value may just be getting sailors to have some concern about beam, but I haven't seen many boats that yield more than a 2 with this formula that I would want to take offshore.
As for criticism from a boat manufacturer, my response is simple. Pre-empt this formula by including the positive range of stability in the boat's specifications. If a designer or manufacturer is proud of their boats' stability, put it in the advertising. Make it a selling feature. If the stability of every boat was listed along with draft and waterline length, the CSF wouldn't be an issue.
Because of your input, I took a lingering second look at what I say and why. I have decided that the reader is best served if I leave the piece as is. I hope you will not be offended and I sincerely appreciate your learned input.
Question from Len Keller to Don Casey
Monday, February 21, 2000 6:35 PM
I would like to ask Don Casey what, other than being greater or less than 2 is the significance between Capsize Screening numbers, say, 1.8, 1.9, 2.1 etc.?
Answer from Don Casey
The CSF is no more than an quick and dirty indicator. If the number is 2 or less, the boat passes the screen. If it is higher than 2, you need more information. You can get a more complete explanation of the logic behind this screen in Desirable and Undesirable Characteristics of Offshore Yachts.