VMG = V cos(TackAngle / 2) is correct only of there is no leeway or current. There is always leeway, more on a cat than a monohull. A charter cat will have as much as 10 degress of leeway. In that case, it would be VMG = V cos((TackAngle + 10)/ 2).
VMG is reduced by downwind current, increased with upwind current.
This is a good point (though the mathematician in me bristles at inserting an arbitrary constant into the formula instead of a variable), and further illustrates why it seems pointless to sail this boat to weather.
Lots of time for musing at the helm, so I concocted an analogy (rationalization?) to skiing: The cruising cat is like downhill skis. Yes, you could use them for uphill work, but they are really ill suited for the job. A performance mono, on the other hand, is more like cross country skis, suitable for both uphill and downhill travel but with their own limitations. In the cruising catamaran, he ability to sail well upwind has been largely sacrificed to livability.
Likewise, in the age of sail, there were boats like sloops and skiffs that probably sailed upwind reasonably well, and vessels designed for carrying large amounts of cargo for which even a shallow bay in a blow could spell a death sentence. Fortunately, I have an engine.
