John, I read the ORR definitions of the stability index; it was the limit of positive stability plus (or minus) two other factors, so 125 degrees for the 121 and an adjustment. I'll see if I can dig out those adjustments later - related to beam, displacement and lwl I think.
Tim Mojito C&C 35-3 Branford, CT > On Mar 6, 2015, at 5:40 PM, John Pennie <j...@svpaws.net> wrote: > > > 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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