IMS GP10 Rating: approximations and guesses

My inquiries and research suggest that to date no BCC28 has been measured and rated under the International Measurement System. That means that US Sailing, for example, does not have an IMS rating or a set of polars for the BCC28.

US Sailing now uses a laser device to measure hulls and has revised its propeller drag formula (see The Sam L Morse Co specifications and my own measurements are likely not as accurate as US Sailing.

There are a couple of ways to approximate or guess the IMS rating. Wayne Beardsley has published his IM-S-Timator web page ( which calculates an IMS rating for sloop rigs. WB claims that his approximator works for about 90% of boats, coming within 12 seconds/mile of the IMS General Purpose 10 (GP10, the speed in seconds per mile at a wind speed of 10 knots).

Using the figures for Zygote’s genoa and main (note that across the BCC28 fleet, the cut of sails varies: Zygote’s sails were made by Elliot Pattison), WB’s IM-S-Timator gives a rating of 728 seconds/mile. I calculate that as 4.94 knots.

In the January 2005 issue of ‘Cruising World’, Beth A. Leonard and Evans Starzinger give another way of approximating the IMS rating. BAL and ES had been peeved some time ago by George Day (editor of ‘Blue Water Sailing’) because GD used the idea that a cruiser could make 200 miles a day as a yard stick for judging boat designs. So BAL and ES did their homework (and show that you need a 75 foot LWL monohull to average 200 miles a day). As part of their homework, BAL and ES did a regression analysis of IMS ratings, coming up with a formula based on Sail Area/Displacement and LWL (WB’s approximator uses displacement, LWL and sail dimensions; WB recognizes that he ignores hull shape, pitching moment and more).

Plugging my own (and Sam L Morse Co’s slightly different) SA/D etc figures into BAL and ES’s formula gives better ratings than WB’s: I get ratings of 693.86 secs/mile to 694.5 secs/mile; with associated average speeds of 5.18 knots to 5.19 knots.

In her standard trim, Zygote is heavier than the design figure of 14,000 lbs. And in blue-water cruising trim, even heavier (I think - and confirmed by straddle crane operators) than the 14,327 lbs that has been used in stability modelling. So I don’t really expect to average 5.18 knots in 10 knots of wind. But it’s an interesting figure. And a target to try to beat.

Note that I didn’t do well in math at school, so my calculations could be out. I welcome any corrections and polar diagrams.