Remember your GPS gives you speed over ground, while Hull Speed is speed over the water. A one knot favorable current can easily explain the difference.
Gary S/V Kaylarah '90 C&C 37+ East Greenwich, RI, USA ~~~~~~~_/)~~~~~~ On Fri, May 19, 2017 at 1:16 PM, Ronald B. Frerker via CnC-List < cnc-list@cnc-list.com> wrote: > GPS is usually in miles per hour IIRC. Knots is faster by about 15%, so > 6.7kt times 1.15 would be about 7.7mph. > Unless your GPS is set in kt in which case this is all wrong. > Ron > Wild Cheri > C&C 30-1 > STL > > > > > ------------------------------ > *From:* RANDY via CnC-List <cnc-list@cnc-list.com> > *To:* cnc-list <cnc-list@cnc-list.com> > *Cc:* RANDY <randy.staff...@comcast.net> > *Sent:* Thursday, May 18, 2017 3:52 PM > *Subject:* Re: Stus-List Fun Race Last Night > > I'm just as surprised as anyone. I know that 1.34 times the square root > of 24.75 (Grenadine's waterline in feet) is 6.67 knots. But apparently > that's only a very general rule - see http://www.boats.com/reviews/c > runching-numbers-hull-speed-boat-length > <http://www.boats.com/reviews/crunching-numbers-hull-speed-boat-length/#.WR4GQccnuq0> > and > https://en.wikipedia.org/wiki/Hull_speed. At 9,000 pounds displacement I > guarantee I'm not planing :) > > But I assume GPS doesn't lie. On June 22nd last year I was using RaceQs > during a race. I forgot to switch it off after finishing, so it recorded > Grenadine sailing for fun in the 25-33mph gusts that piped up after the > race, under full main and 150% genoa (which tore that night, before I could > get a rail in the water). If you watch this replay from 19:42:50-19:43:00 > local time, you'll see Grenadine going 7.5 knots over ground according to > RaceQs GPS-based iPhone app: > http://raceqs.com/tv-beta/tv.htm#userId=1032518&divisionId=4 > 1508&updatedAt=2016-06-23T03:05:38Z&dt=2016-06-22T18:08:05-0 > 6:00..2016-06-22T21:05:54-06:00&boat=Grenadine > > I'm not one to look a gift horse in the mouth :) Maybe at 25 degrees of > heel my waterline length is appreciably longer than 24.75 feet. Or maybe I > just have to give credit to George Cassian, George Cuthbertson, and Rob > Ball for designing a faster-than-predicted hull form. > > Cheers, > Randy > > ------------------------------ > *From: *"David Knecht via CnC-List" <cnc-list@cnc-list.com> > *To: *"CnC CnC discussion list" <cnc-list@cnc-list.com> > *Cc: *"David Knecht" <davidakne...@gmail.com> > *Sent: *Thursday, May 18, 2017 1:25:19 PM > *Subject: *Re: Stus-List Fun Race Last Night > > Theoretical hull speed of a C&C 30 is 6.7 knots (based on 25’ water > line). How are you hitting 7.6-8? Foils? Dave > > > > _______________________________________________ > > This list is supported by the generous donations of our members. If you > wish to make a contribution to offset our costs, please go to: > https://www.paypal.me/stumurray > > All Contributions are greatly appreciated! > >
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