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Old 02-19-2005
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Bluewater defined?

About the value of STIX as a useful tool to assess a boat stability I have found of interest the opinion that Bob Johnson (president of Island Packet Yacht) wrote in a letter published in the last special issue of Sail Magazine:


...."As the U.S. participant on the international work group of designers and naval architects responsible for developing this particular ISO standard, I''d like to provide some background on its development and shed some light on why I believe it will grow to have considerable value to both the design community and the boat owner."

" As Dave''s article illustrates, the accurate stability assessment of any sailboat is a complex process. Numerous design characteristics influence the ability of any boat to safely resist or recover from capsize, ranging from downflooding openings to the shape of its righting-moment curve. In all, the ISO work group found that seven different calculated factors encompassed all significant stability concerns and, taken together, could establish a single stability index value (STIX) that allows meaningful stability comparisons between various designs."

"The minimum STIX value for each category was created and validated from a study of a large number of existing sailboats, ranging in size and type, that had experienced capsize events, with and without successful recovery. Good correlation was eventually obtained between theory and practice, and the final STIX standard, I think, encompasses all critical stability factors in an analytical method that is based on established principles of naval architecture, sound methodology, and correlates well with real-world experience for real boats. No other single stability-assessment method, to my knowledge, is as comprehensive as STIX. "

...."Lastly, I’d like to comment on the Capsize Screen method mentioned in another sidebar. This was proposed in the 1985 USYRU and SNAME Joint Committee Report on Safety from Capsizing (prompted by the 1979 Fastnet disaster). As Dave correctly states, and I heartily reinforce, by itself the capsize screen evaluation method has virtually no value in the assessment of a yacht''s stability. It''s really addressing a concern that lightweight designs with a wide beam are suspect regarding their susceptibility to capsize, justifying further stability evaluation. One of the seven STIX factors addresses this very issue.


Bob Johnson, President
Island Packet Yachts
Largo, Florida





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