"Accuracy" is exactly why it makes no sense to develop rules that are difficult
and expensive to use, like IMS. There are too many variables. The rating may
be exactly-spot-on accurate to twenty decimal places in ten knots of wind with
no waves and a wing-sail that doesn't bend and a helmsman who steers razor
straight. Last time I went out, however, it was blowing 25, there were five
foot waves, my sails had all sorts of strings to adjust their shape, (along
wiith the mast's), the boat heeled, I had one fewer crew than I would have
liked on board, and my course changed at least 3degrees when I got handed a
cup of cocoa. PHRF - perhaps with an adjustment to cover "condidtions" as is
being called for in IMS - would be a lot cheaper and easier to administer and
determine. And perhaps just as "accurate".