IMS accuracy
 

I've been trying to find some information about the accuracy of the
time allowances in a IMS certificate. Haven't found any!

The boat I've been racing has it's certificate he
http://www.avomeripurjehtijat.fi/ims...003/L-9791.txt The
boat is a fast cruiser (http://www.se-ra.com/29cd/alku.htm), that has
been designed without any consern to IMS-rating.

How many sec/mi would you expect the individual time allowances to be
off? How many sec/mi benefit can a "good" IMS boat like X-332 have?
How bad can an non-IMS-optimized boat be?

Here is an example:

After the most recent race (36 nautic miles) I compared the time we
lost to our competitors in different conditions. It seemed that we
were OK in light (4-6 kt, 12,5 miles) beat and medium (6-12 kt, 12,5
miles) run. After these 25 nautic miles we were in a very good
position. Then started the problems. On a 3 mile ~90 degree (just
without spinnakers) leg in a 12 kt wind other boats near us (X-342,
X-332, Doufour 334, the new Dehler 34) were much faster. One actually
passed us rapidky on leeward side just about a boat lenghts distance.
According to IMS these boats should have been 0-40 sec/mi faster but
we lost 40-80 sec/mi to them, quite accurately 40 sec/mi too much to
all of them. There were only small waves and I think there was nothing
wrong in our sailing, just couldn't go faster.

On the following 3 mile beat in a 12-14 kt wind we still kept loosing
at least 40 sec/mi more than we should have. This I could understand,
since we had to keep the main quite far leeward in order to keep the
boat from not heeling too much and we were only three onboard (250 kg
vs 450 kg in certificate). I looked our GPS-track and calculated, that
we were doing about 900 sec/mi VMG instead of 808-828 according to
IMS. How many sec/mi would you expect to loose with those 200 kg
missing?

The last leg was in 45-100 degree 10-14 kt wind, most of which we
could keep the full power in our sails. We were passed by a number of
boats that should have been only 10-30 sec/mi faster and a boat that
should have been 15 sec/mi slower came closer all the time. According
to my calculations we lost 40-70 sec/mi more than we should have to
ALL the other boats.

There are a couple of things in our certificate that caught my eye:

1. The keel is defined as "FIXED KEEL with WINGLETS". The keel is a
modern keel with a round bulb (http://www.se-ra.com/29cd/alku.htm,
press MITAT). According to IMS regulations, this definition increases
the effective Draft (D) in VPP. I guess that would increase the
calculated L/D-ratio for the keel???

2. The mast has not weighted. The defaults are quite low, I would
guess the ast weights 70-100 kg. How much effect it has? On what wind
speeds and directions?

3. The RM of the boat is very low, 45 kgm/deg. A typical value for
other boats in the same size and sail area range is about 60 kgm/deg.
I have been told, that IMS VPP assumes a very little speed decrease on
a boat with a lower than "normal" RM. Any insights?

4. This boat is, according to IMS, surprisingly much faster than
Dehler 29 (IMS-certificate:
http://www.avomeripurjehtijat.fi/ims...002/L-9028.txt) ,
which has about the same dimensions and sails. Actually I don't know
any other 29-footer with a GPH around 670 at least with a displacement
of over 3000 kg.

Joakim

IMS accuracy
 

The message
from (Joakim Majander) contains these words:

I've been trying to find some information about the accuracy of the
time allowances in a IMS certificate.


All handicaps are spot on accurate - you accept that when you are rated
and when you enter a race under the appropriate conditions.

I know that it may be possible to say that in a particular condition
your boat is faster/slower than the rating you have... but I say again
you accept the rating when you enter an event under those conditions.

It is your objective to sail round the course more faster than your
ratings than the other competitors (or less slower) regardless of the
accuracy of the ratings.

What may be of interest is that is a rating results in a TCF
corrected/rounded to three decimal places then a change of 1 in the
lowest place requires the boat to be 3.6 seconds faster for each
corrected hour that she sails. In other words if two boats have
corrected times less than 3.6 seconds/corrected hour apart (eg 18
seconds for a five hour race or 259.2 seconds for a 72 hour race) then
if the TCF of the slower (on corrected time) boat was incorrect by 1 too
low in the last decimal place she should have won.

How far does your boat sail in 3.6 seconds - in dinghy racing it is
quite normal for 3 to 5 boats to finish in that timespan, and if they
are on handicap the differewnce in their ratings could be critical.

However remember that when you enter an event you does on the conditions
stated including the handicapping system and handicap stated being
assumed to be absolutely accurate.

Stuart

IMS accuracy
 

Stuart Cresswell wrote in message ...
The message
from (Joakim Majander) contains these words:

I've been trying to find some information about the accuracy of the
time allowances in a IMS certificate.


All handicaps are spot on accurate - you accept that when you are rated
and when you enter a race under the appropriate conditions.



My objective was not to complain about ratings. I just would like to
know how close they typically are to real life. If you are going 0,5
knots slower than you "should", you would like to know is that due to
poor sailing, poor trim of mast etc or is it just an normal variation
in IMS VPP.


I know that it may be possible to say that in a particular condition
your boat is faster/slower than the rating you have... but I say again
you accept the rating when you enter an event under those conditions.

Everybody seems to know that, but I would like to see some figures.
Remember that unlike other rating systems, IMS is able to take into
account different wind directions and speeds.

It is your objective to sail round the course more faster than your
ratings than the other competitors (or less slower) regardless of the
accuracy of the ratings.

Of course. Does this mean it is useless to discuss the accuracy of
ratings???


What may be of interest is that is a rating results in a TCF
corrected/rounded to three decimal places then a change of 1 in the
lowest place requires the boat to be 3.6 seconds faster for each
corrected hour that she sails. In other words if two boats have
corrected times less than 3.6 seconds/corrected hour apart (eg 18
seconds for a five hour race or 259.2 seconds for a 72 hour race) then
if the TCF of the slower (on corrected time) boat was incorrect by 1 too
low in the last decimal place she should have won.

I a agree, that 3 or even 2 decimals on a time-on-time rating is very
accurate. In my example we were slower on the last legs by 40
sec/mile. This would transate to ~250 seconds (= half a mile) in an
hour in those conditions. On a time-on-time correction (which IMS is
not), this would be a disprepancy of 7%.

Joakim

IMS accuracy
 

"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".

IMS accuracy
 

$author = "PSK125 " ;
"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".

AOLI agree/AOL

When I read the intial query my first thought was "Who was sailing the boat
while you were staring at the GPS so much?"

Sailing has an infinite number of variables and designing a rule that takes
into account every one of them is impossible. It's a matter of compromise to
come up with something that approximates a level playing field.

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

marty

--
"I yearned for her not in the way a warrior yearns to conquer, but in the way a
ship captain yearns to be out on the ocean." [1]

- rizzo

[1] http://www.kuro5hin.org/story/2002/11/4/21550/3557

IMS accuracy
 

Martin wrote in message
When I read the intial query my first thought was "Who was sailing the boat
while you were staring at the GPS so much?"

I wasn't looking at the GPS at all during the race. After I got home I
loaded our track (our position at 30 s intervals) to my PC and then
tried to analyze what had happened.



Sailing has an infinite number of variables and designing a rule that takes
into account every one of them is impossible. It's a matter of compromise to
come up with something that approximates a level playing field.

That was my question. How far off is it?


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


There aren't any one designs around here (Finland) that would fit our
cruising needs as well.

Joakim

IMS accuracy
 

$author = "Joakim Majander " ;

Sailing has an infinite number of variables and designing a rule that takes
into account every one of them is impossible. It's a matter of compromise to
come up with something that approximates a level playing field.

That was my question. How far off is it?

There isn't any answer to your question except, it depends.

For any handicapping system there will be conditions that will favour a
boat, and conditions that favour the other boats. You've discovered some
conditions where the handicapping system is hard on you. If you do that for
every race, you will also find conditions where you outsail your handicap.

marty

--
revery "They weren't the short little answers I was expecting." [1]

Llama King "You were expecting short answers? From a bunch of lawyers?" [2]

[1] - http://interviews.slashdot.org/comme...98&cid=6522542
[2] - http://interviews.slashdot.org/comme...98&cid=6524295

IMS accuracy
 

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.

IMS accuracy
 

"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

IMS accuracy
 

"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