Hi there Joakim on the other side of the "Pond",

On IMS or VPP accuracy I have some first hand experience that is quite
relevant. My 30 ft custom MORC racer (also not a bad IMS boat) is
equipped with the Ockam System (same system as some of the America's Cup
boats have), including a built in VPP that I display all the time when
racing. Doing mostly windward leeward courses we live or die by the
target speed that is derived from my custom VPP, very similar to the IMS
polars. The target speed display is constantly updated by the system,
so even minute changes in wind speed is reflected within seconds, and we
immediately adjust what is necessary to maintain our targets. We sail
more than 90% of the time very close to our upwind and downwind targets
(within 0.1 knot) and our boat speed is displayed in 0.01 knot steps, so
watching the trend is easy. Our wind speed display is in 0.1 knot
steps. Very few boats in our area have such accurately calibrated
system, so they don't know how fast they are supposed to sail to be at
optimum. We usually do very well against this fleet.

Last year, after assessing our performance by the local PHRF
organization in many regattas and series, we sailed about 40 sec/m
faster than our PHRF rating which seems to support the reality of your
observations in your fleet. (Our PHRF rating was not adjusted because
our sister ships were not quite so fast.) Of course we were not faster
than our VPP, but others were even slower.

The bad news for you is that trying to use GPS for proving/disproving
your IMS rating with roughly averaging speeds will not tell you how well
you sailed your boat at any moment, it is too slow and inaccurate, so
you can not conclude that your IMS rating is inaccurate. IMS is much
more accurate than what GPS can show you, certainly orders of magnitude
better than 40 sec/m. Sailing to the IMS polars you need much better
calibrated instrumentation, instantaneous data display and absolutely
excellent crew work

The IMS VPP is not based on the average sailors ability to sail the
boat, but sailing the boat at it's optimum at all times. This takes
some very significant effort that is not necessarily obvious to the
average club sailor. Top sailors can sail their boats faster than their
VPP by utilizing other opportunities such as wind shifts, currents,
smart tactical moves etc.

I hope that this helps with your reality check.
There is much literature on this all around.

Adam
(Toronto, Ontario, Canada)


Matt/Meribeth Pedersen wrote:

"Joakim Majander" wrote in message
. com...


"Matt/Meribeth Pedersen" wrote in message


thlink.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