I was so surprised by these numbers I had to understand it better. Below is the best definition I could find on the IMS “stability index” but I’m curious how it is calculated. I’ve attached a link to the designer’s righting moment curve for the 121 (deep keel). It would suggest a much more stable boat than the IMS index. Any thoughts?
http://svpaws.net/Site/Resources_files/Right.pdf John Another single-value stability rating still frequently encountered is the IMS stability index number. This was developed under the IMS rating system to compare stability characteristics of race boats of various sizes. The formula essentially restates a boat's AVS so as to account for its overall size, awarding higher values to longer boats, which are inherently more stable. IMS index numbers normally range from a little below 100 to over 140. For what are termed Category 0 races, which are transoceanic events, 120 is usually the required minimum. In Category 1 events, which are long-distances races sailed "well offshore," 115 is the common minimum standard, and for Category 2 events, races of extended duration not far from shore, 110 is normally the minimum standard. Conservative designers and pundits often posit 120 as the acceptable minimum for an offshore cruising boat. Since many popular cruising boats were never measured or rated under the IMS rule, you shouldn't be surprised if you cannot find an IMS-based stability curve or stability index number for a cruising boat you are interested in. You may find one if the boat in question is a cruiser-racer, as IMS was once a prevalent rating system. Bear in mind, though, that the IMS index number does not take into account cabin structures (or cockpits, for that matter), and assumes a flush deck from gunwale to gunwale. Neither does it account for downflooding. On Mar 5, 2015, at 6:20 PM, Tim Goodyear via CnC-List <cnc-list@cnc-list.com> wrote: > Amazing - great information! It is interesting that you could enter one of > our older boats in this race, but not a C&C 110. From the 2014 scratch sheet: > > 110 108/109 > 115 112-116 > 121 110 > 33 118 > 33-2 123 > 34 117 > 35/1 114 > 35/2 121 > 35/3 117-122 > 37 120 > > Tim >
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