It is almost certainly the trailing edge of your rudder/daggerboard shedding vortices and it slows you down. Make the trailing edge a 45 degree angle instead of a flat and it will disappear.
rob
>I seen speed recorded on my gps unit of 12mph sailing on the outside open ocean with no problems just some vibration and humming.
i've heard such humming on my Weta and a Farrier that i crew on when going at similar speeds. everyone gets excited when this happens because they think it means we're planing. perhaps it's correlated, but i'm thinking that it's really due to the foils cavitating. true? if so, that's bad, right? more drag less lift?? short of replacing the foils and/or slowing down, is there anything i can do to stop it?
ben arthur
weta #358, "gray matter"
ithaca, new york