On my GPS (a Garmin Map76CSx) I get 2 different top speed reading, an instantaneous reading on the screen, and an average per segment from the tracks. The instantaneous reading can be much higher as it's the fastest speed the GPS recorded that session where average per segment (my term) averages over however long and far it chooses before saving that segment.
With that said, NOHUHU and I hit
12.34 knots (14.2mph) instantaneous speed on the TI3 testing (
http://www.hobiecat.com/forums/viewtopic.php?f=69&t=38827&hilit=ti3) a couple of weeks ago. On other TI outings this year on Maui I've hit top speeds of
8,
9.3 and
10 knots sailing a TI.
I sail AI's more often, and have much more varied top speeds on them over almost 5 years of data, though the TI is a bit faster in most conditions. Here's my log page from my website
http://www.kayakingbob.com/sailing-log but I just noticed I didn't add which boat (AI/TI) I was in each trip. If you click on "track" on any trip (at least this year) it will display the GPS data with a description of which boat I was in. You can even "play back" the trip on the screen.
About once a year I hit very high instantaneous speeds in unusual conditions, but usually not lasting very long (5-30 seconds).
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Hobie Island Sailing since 2006