Right, found it - or at least a similar reference at last... from the advanced setup manual for the Silva NX2 Race Software - if you're not re-trimming the amount of twist in your sail between tacks, or if, like mine, your sail is a little old and baggy, and you can't flatten it properly on port, you may be getting more boatspeed on starboard than port. See below:
1.5 Wind Sheer
When there is wind-speed, there is wind shear!
What is wind shear?
The main reason for wind shear is due to the Coriolis force. The wind will rotate counter clockwise on the North Hemisphere around the centre of the low pressure.
Then, there is a friction between the sea surface and the free air above. This friction will slow down the airspeed at sea level and gradually up into the free air above. This friction will also decrease the effect of the Coriolis force, so the wind will shear (to the right) from sea level up to the mast top (and above). The wind transducer will only measure at one altitude, so you need to understand and consider this sheared angle to adjust the sail accordingly down to deck level.
Note! Several meteorological effects will have impact on the size of the sheared wind. When cold and warm air is mixed with faster winds from higher levels, gradients and sheared winds may locally change very fast.
Generally, by applying sail trim according to the size of wind shear, you can get more power out of the wind on starboard by adding more twist to match the shear (on starboard only). This will reduce the top force from the wind, move the pressure centre downwards and allow for a more forward pointing and efficient wind force.
The wind instrument will "show you" that you are sailing lower than port side, but it is only a relative illusion since the reference is from your average attack angle and sail trim including wind shear.
On port side, the sheared wind is "negative", and it requires more flat sail trim. The instruments will tell you that you are sailing high and fast on port tack, but this is also a relative illusion, but opposite from starboard tack. You have less wind force in the top of the sail, so the efficient wind pressure centre is moved downwards. Then, for a given heel angle, you will have a wider wind angle, which mean that you are actually not sailing as high as the instrument says!
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