This sport of sailing never ceases to amaze in how you never stop learning. I'm embarrassed to admit I worked something out recently I probably should have known a long time ago, and that is wind shear.
Wind shear is described as a difference in either wind angle or wind speed over a short distance in the atmosphere. Real world explanation : It can lead to your masthead instruments and your telltales telling you different things.
Its always there, and the variety of meteorological conditions that contribute to can cause wind shear to becoming more or less noticeable. Upshot is we all sail in it, with cooler water and hotter air making it more evident.
I confess to being a bit obsessed with data, so I enjoy mucking around with sensors such as the masthead, impeller and Nav. Calibrating those sensors to be always accurate for all conditions is not an exact science, for your masthead vane and anemometer the wind from the sails can disturb the readings, and this may be different when going up wind as compared to downwind etc etc. I still like to try and get it close to reality as I can whilst acknowledging the limitations of the system.
So the other weekend we're out, and I noticed a calibration issue appear. It's intermittent, but every now and then I'll see a big difference in our port and starboard tack angles. In this case , it was not tide, (as a running tide will definitely cause a difference in TWA between the two tacks also)
Conditions: flat water, turn of the tide, 12-14 knots breeze. Boat was trimmed exactly the same for both tacks.
-Starboard tack we were 35 TWA. Boat speed was down a bit, and my heel angle was less than what the wind strength suggested.
-Port tack we were at 55 TWA. Boat speed was too high, and my heel angle was fine.
???????????????? I just calibrated the *&%$@ thing!!!!
After lots of number crunching and research, I discover this is the classic symptoms of wind shear. A simple guide:
In the southern hemisphere, wind will always veers to the right as it slows down.
As the wind gets closer to the surface, it slows down,
So for us Aussies, the wind at the surface
will always veer right compared to the wind at the top of your mast. This can vary, and it can vary a lot.
So, if you see, like I do, a marked change in your tack angles, it could be wind shear your experiencing and not tide. The giveaway seems to be heel angle, if your instruments are reporting a wind strength and your heel on starboard is weirdly more upright, that's wind shear. Your wind instruments are living in a different wind pattern (one more shifted to the left as its faster wind up the top) than the majority of your sail plan, so you can't trim to suit your wind instruments .
Outcome.
In the prescence of wind shear, flatten the upper sail plan when on starboard to suit the decreased TWA up the top.
When on port, ease the jib cars back to open up the top leach, and ease mainsheet to twist off the top leach to suit the increased TWA.
The reason this is particularly annoying is that the false reading screws up not just your helmsman getting TWA numbers that are out, but all your navigation routing, laylines, polar performance and sail selection. If we are aware of it, it's easy enough to do your own revised calculations (tack before the port layline by the half the degree difference you see when on the opposite tack) if you're calling lay lines or doing the nav.
I hope this helps!