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

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

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

"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".

$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

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