I can't stand the answers you've been given, so maybe I can give something
more helpful.

In the US, we have what I call an observational rating system, PHRF, that
looks at some basic parameters and historical performance of sister boats
and assigns a rating. They will never give you a change to your rating in
an increment of less than three seconds a mile - this is the minimum
accuracy they can promise. And keep in mind that even at the highest
levels - say the Star Worlds with the best sailors, there may be a
difference in finishing times of up to 40 seconds per mile. Good sailors,
good boats, good sails, bad day.

I think the rule of thumb for IMS is that it's accurate to about a second
per mile or so. There will always be things that are hard to account for
(waves, sails, etc), and of course an optimized boat will have an advantage.
Designers like Farr, who can spend lots of time and money on their own VPP
and see where the weaknesses of IMS may lie, will probably be tough to beat
as well. But for an accurately measured, non-optimized fleet IMS shouldn't
be off by the 7% or so you saw. Sorry to say but I think your problems may
have been with sail inventory, trim, boat prep, or your light weight.

One of the keys to IMS is finding where your boat might be outperforming the
predictions and those areas where it falls short.

Martin wrote in message

If it all gets too much for you, find a one design class.

And for those of us without a local one design class that appeals to our
cruising side what are we to do? We're stuck with some sort of rating
system, warts and all, but still want to do well.