Breezing, let me beter clarify
Regardless the current, TRUE wind is a measure of speed over ground, not over water !! Suppose you're sailing dead downwind at 6knots against a counter-current of 6knots: your wind instrument will show you ZERO true wind, but reality is, since your are still relative to the ground (SOG is zero in this example), the reading should be the real 6knots of wind. So, your Raymarine instrument will fool you exactly by the current. And for the others not caring about this, I always read the instrument to decide for a spi, and as a single handed, I push my engine along with wind to create a 1-2 knots apparent. This makes setting the spi a breeze, and once I'm done, I just kill the engine and the spi fills; very safe right ? Unless you use Raymarine instrument. Since GPS is a commodity for boats nowadays, SOG is plenty available over the cables. Why simply Raymarine doesn't take SOG instead of SOW for true wind calculation ?