Port vs. Starboard tacks

[Zanshin]

Mainsail is in-mast, so no drag difference whether furled or not. The headsail was reefed, so a 3-4cm rolled section would disturb the airflow more on starboard tack, but just the leading edge wasn't laminar - I would be amazed if this causes a difference of over 20% though.
I've just spent the past couple of hours trying to repair the work that the yard did on my ship At least I'm attached to power for the first time in a week and have the air cranking while I'm head down in the bilges.

[sailingdog]

Umm... if it is like other mainsail in-mast furling systems, it will be biased a bit to one side when reefed... and essentially centered when not reefed.. so it too may have had some effect on the performance, since the sail shape will be slightly different.

[SailorMitch]

As others have suggested, use the GPS the next time conditions are good for a cross check. It the speed difference still exists, check the rig to make sure the mast is centered, is in column, etc. Yes, get a Loos gauge for the rig to check the stay tension side to side.

[Idiens]

Originally Posted by Zanshin

Y... I was using a log wheel, which is forward of the keel and only about 4 inches off the centreline (it is to port).

My log wheel is also offset and I get different readings on opposite tacks, though not as much as you are seeing. I theorise that, as the boat is (what's the word?) crabbing sideways while close hauled, especially in a seaway, the flow over the log wheel will be affected by the hull shape differently on either tack. GPS will give a good comparison but for any accuracy you really need recordings. Both the GSP and the log wheel electronics are averaging a series of measurements and pitching particularly introduces a different variation to both.

I think SD had a good point about the accuracy of the wind instrument, the head to wind setting has to match both the aerodynamic and hydrodynamic centre lines and the boat's centreline.