"Joakim Majander" wrote in message
om...
"Matt/Meribeth Pedersen" wrote in message
hlink.net...

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.)?

I think a whole course will be more accurate, just by the law of
averages. For individual values IMS is pretty close but most of
its weaknesses come from dynamic issues - waves, surfing
ability, changes in apparent wind due to motion (things that are
hard to quantify).

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.

Right, I think IMS is very accurate for light air and windward sailing,
what some people call displacement mode. In these cases I'll bet
it's hard to exceed IMS speed predictions. Most of the differences
will come on the downwind legs - can you surf a little every once
in a while, or if you head down an extra 10 degrees from the optimum
course will you go faster than IMS predicts? The most accurate
way will be to have wind and boat speed data collected over time
(and a big hard drive), or you can do it the old fashioned way.
We used to go out for a practice or even during a race, and record
these measurements on a blank polar chart. The helmsman would
hold the boat on a steady heading for a couple minutes, one person
would watch the wind meter and mentally average the wind speed
and another would average the boat speed. By doing this for a couple
of seasons we got a lot of good performance information. Make
sure you have clear air when you do this, we had some data
anomolies until we figured that out.

Also, don't forget that IMS assumes your sails are always perfectly
set and trimmed. A deck sweeping genoa, cracked off a little bit on
close reach is really a performance killer.

Matt