"Matt/Meribeth Pedersen" wrote in message hlink.net...

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.

Yes, we have a similar system called LYS in the Nordic Countries. It
is a time-on-time coefficient around 1, with the accurcy of two digits
(- ~5-10 sec/mile). The problem is with lot of one-offs and short
series of many boats, which means most of the LYS-numbers are just
guesses. With one number time-on-time and very different kinds of
boats it is also evident that there will be clear hard wind etc.
boats. The difference in the fleet are very much larger after time
corrections in LYS than in IMS.

Even though it is normal to have 40 sec/mi differences in one designs,
I believe that they are mainly due to better route choises, not boat
speed.



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.

Thanks a lot for this comment. As I understood it, In your oppinion
some boats may have an advantage of a few sec/mile, but not even close
to 40. That was what I kind of expected. There probably are still
many things to check in our boat. Do you think these values are valid
for individual values (say 75 degrees and 12 kn) or just for a whole
typical course (windward-leeward, circular etc.)?


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

How close to the "target speeds" should you get? A few sec/mil is very
close and hard to verify. That would mean down to accuracy of your
instruments.

Joakim