Maybe a dumb solution.



I noticed that my boat speed had suffered on the day you are talking about, when a huge tumbleweed looking thing was on my leeward rudder. When I tacked, it was so big, it didn't budge, had to clear them. If you had something similar near the top of the board or rudder, might explain it. If near the top on the windward side, it wouldn't be in the water much. just a thought.