3.5 How do those rating systems and all that stuff work?
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This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.
3.5 How do those rating systems and all that stuff work?
[Contributed by Roy Smith]
PHRF (pronounced "perf") is Performance Handicap Racing Fleet. Unlike
other rating systems (IOR, IMS, etc), PHRF ratings are not assigned
based on some sort of measurement, but rather on past performance of
similar boats. If you are racing in a club race or a local weekday
evening or weekend series, where different kinds of boats race against
each other, the odds are that PHRF is the rating system you're
using. In PHRF, boats are assigned ratings in seconds per mile. Your
rating is the number of seconds per mile your boat is supposedly
slower than a theoretical boat which rates 0. Most boats you are
likely to sail on rate somewhere in the range of about 50 to 250. All
ratings are multiples of 3 seconds/mile (i.e. the next faster rating
than 171 is 168). I think this is done as a recognition that the
rating process just isn't accurate enough to justify rating boats to 1
second/mile resolution.
Typically, a certain type of boat is given a stock rating based on
past experience. Just to make it a bit more interesting, ratings vary
somewhat depending on location; each YRA (Yacht Racing Association)
can assign its own rating to a class of boat depending on their local
experiences and conditions. For example, Western Long Island Sound,
under the jurisdiction of YRA of LIS, is famous for light wind, which
tends to give an advantage to certain types of boats, and YRA of LIS
takes that into account when assigning ratings.
On top of your regional stock rating, there are a variety of standard
rating adjustments depending on how your boat is rigged. The standard
PHRF rules allow you to have a 153% genoa. You can carry a larger
sail, but take a rating penalty for it. Likewise, you can chose to not
carry that big a sail and get a rating advantage. Having a
non-standard keel, extra tall or short mast, a fixed prop (the stock
ratings assume a folding or feathering prop), extra long or short
spinnaker pole, etc, all result in rating changes. Some boats have
several stock ratings for different common variations. For example,
there are 4 configurations of J/29's; masthead or fractional rig and
inboard or outboard.
Once you've got your basic rating, adjusted for location and
customizations you may have done, you still have the option of
petitioning for a rating change based on whatever evidence you might
care to present to prove that your rating is too fast (or the other
guy's is too slow), an area that quickly gets into politics and boat
lawyers. There are two flavors of PHRF, Time-on-Distance (TOD) and
Time- on-Time (TOT). TOD is the more traditional and easier to
understand, so let's start there. In TOD, you get a handicap equal to
the length of the race course in nautical miles multiplied by your
rating in seconds/mile. Thus, for a 6 mile race, a boat that rates 120
would get a 720 second handicap, i.e. her corrected finish time would
be 720 seconds less than her actual time to complete the race. What
people tend to do is think not so much about the actual rating, but
rating differences, i.e. if you rate 120 and the other guy rates 111,
he owes you 9 seconds per mile, so for a 6 mile race, as long as he
finishes less than 54 seconds in front of you, you will correct over
him and win.
The other flavor of PHRF is Time-on-Time (TOT). In TOT, it's not the
length of the race course that matters, it's the amount of time the
race takes. To do TOT, first you have to convert your normal rating,
R, in seconds per mile to a factor, F. The formula to convert R to F
varies from place to place, but it's typically something like F = 600
/ (480 + R). Actually, it's really something like F = 600 / ((600 -
Rav) + R), where Rav is the average rating of all the boats in the
fleet. Locally, we use an Rav of 120 which gives the formula with the
480 in the denominator. For reasonable values of R, you get an F which
is a number close to 1. For example, a J/24 rating 171 has an F of
0.9217, while a Newport-41 rating 108 has an F of 1.020. To score the
race, you take each boat's finish time, subtract their start time
(giving their raw elapsed time) and multiply by their F, giving their
Corrected Elapsed Time (CET). The theory behind TOT is that in a slow
race (i.e. light wind), the boats tend to spread out but since the
amount of time each boat owes the others is fixed by the length of the
race course (in TOD), slow (i.e. light wind) races tend to favor the
faster boats.
On of the problems with TOT is that there is no universally accepted
formula for converting R to F. With the sort of formula used above,
you can argue about what should be used for Rav. What we do locally is
use one Rav for the entire fleet, which is 8 divisions with ratings
ranging from 36 to about 250 or so. Some people think we should
calculate an Rav for each division, for example. Some people think TOT
is a total crock and want to go back to TOD.
Contributed by Stephen Bailey (sb)]
Sailboats racing under a "handicap system" have a function applied to
their elapsed time, producing a "corrected time," and the boats place
in corrected time order. This function, which differs among systems,
attempts to fairly represent speed differences among boats.
There are two major handicapping philosophies: "measurement" rules
which handicap based upon measurements, and "rating" rules which
handicap based upon observed performance.
The International Offshore Rule (IOR) is a measurement rule for racing
boats. The IOR evolved from the Cruising Club of America (CCA) rule
for racer/cruisers.
The IOR concentrates on hull shape with length, beam, free board and
girth measurements, foretriangle, mast and boom measurements, and
stability with an inclination test.
The IOR also identifies features which are dangerous or it can't fairly
rate, and penalizes or prohibits them.
The measurements and penalties are used to compute the handicap
number which is an "IOR length" in feet. A typical IOR 40 footer (a "one
tonner") has rating of 30.55 feet.
In a handicapped race, the IOR length is used to compute a "time
allowance," in seconds per nautical mile (s/M) which is multiplied by
the distance of the race, and subtracted from the boat's actual time,
to compute the boat's corrected time. Longer IOR length gives a
smaller time allowance.
The IOR is also used to define "level classes," where no time correction is
used. Every boat in a class has an IOR number less than some number.
The Ton Classes, (Mini Ton, 1/4 Ton, 1/2 Ton, 3/4 Ton, 1 Ton, and Two
Ton), as well as 50-footer, ULDB 70 and Maxi classes are examples.
To account for improvements in design and materials, boats are given an
"old age allowance" which decreases their IOR length as time passes. In
spite of the old age allowance, about 3/4 s/M/year on 40 footer, boats
over several years old are usually not competitive, which is why IOR
handicap racing is dead.
Peculiarities of IOR designs result from features which increase actual
performance more than they increase IOR length, or other odd rules; IOR
hulls bulge at girth measurement points; a reverse transom moves a girth
measurement point to a thicker part of the hull; waterline length is
measured while floating upright, so large overhangs are used to increase
waterline sailing at speed; the stability factor ignores crew, so IOR
designers assume lots of live ballast; after the 1979 Fastnet race excessive
tenderness was penalized; full length battens were prohibited to prevent
main sail roach area, but short battens became strong enough that the
IOR had to start measuring and penalizing extra main sail girth; main
sail area adds less IOR length than jib area, so new IOR boats are
fractionally rigged; The IOR encourages high free board, and high booms
and prohibits keels wider at the bottom than at the top (bulbs).
The Midget Offshore Racing Club Rule (MORC) is a measurement rule
for racing boats no longer than 30 feet. The MORC rule is similar to the
IOR. It computes a handicap length from various measurements, which is
used to define level classes and derive time allowances.
MORC seems to work better than IOR because the range of boats it
attempts to handicap is not as large, and it is more quickly modified when
problems arise. For example, the MORC recently adjusted their old age
allowance to permit older boats to be competitive.
The International Measurement System (IMS) is a measurement system
intended for racer/cruisers. The IOR was not fair to racer/cruisers, so the
Measurement Handicap System (MHS) was invented, in 1981, and
accepted internationally, as the IMS in 1985.
With a diverse collection of boats, relative performance varies not just
with design, but also with race conditions. A 33 footer can beat a 40
footer upwind in moderate wind, but the 40 footer will probably come out
ahead in heavier winds, or on a reach.
The IMS uses a Velocity Prediction Program (VPP) to predict speed on
different points of sail in different wind strengths. From the predictions,
and the distance, course type and wind strength of a race, a time
allowance is computed for each boat and subtracted from the boat's
elapsed time to give corrected time.
IMS rule designers believe the key to fairly handicapping diverse hull
shapes is measuring a large number of points all over the hull and
appendages, measuring sail area accurately, and using an inclination test
(which is the same as the IOR). The VPP uses these measurements to
account for heeling, crew on the rail, the immersed shape, and other
factors.
The IMS VPP doesn't yet account for dynamic drag of a boat pitching in
waves, nor for appendage shapes which result in reduced drag. Some
parameters are based upon incomplete experimental evidence. For
example, the VPP predicts a greater benefit from full battens than is
realized in practice.
IMS defines a "General Purpose Rating," which is a predicted time per
mile around a particular course, in 10 knots of wind. A typical IMS 40
footer has a GPR around 595 s/M.
The Performance Handicap Rating Factor (PHRF) is a subjective rating
rule. PHRF was developed to handicap monohulls that didn't fit under
the rubric of other handicap systems. It has since become the most
popular handicapping system in the US, being almost universally used in
club racing.
PHRF assigns a boat a rating, in s/M, which is multiplied by the length
of the course and subtracted from the boat's elapsed time to give
corrected time.
Ratings are assigned by a committee of the local racing authority,
formed from representatives of the member clubs. The initial rating
for a boat is based upon any information available, such as the boat's
rating in another area, ratings under other handicap systems,
information from the designer, ratings of similar boats, and a set of
standard adjustments to basic ratings (e.g. fixed prop, extra large
sails, etc.) All ratings are multiples of 3 s/M. For example, a J/24
rates around 171 s/M, and a J/35 around 69 s/M in many areas.
Since ratings are assigned and administrated locally, they may account for
local conditions. A good heavy air boat would rate faster in San Francisco
Bay, than in Long Island Sound.
A member may appeal a rating, presenting evidence, such as race results,
which supports the appeal. The local committee's decision may be
appealed to a committee of PHRF handicappers from all over the country.
Although PHRF is subjective, it still attempts to rate the boat, in racing
trim, with a perfect crew. Just because a boat never wins, or always wins
doesn't mean its rating should or shouldn't be adjusted.
Using this system, the slower the race, the smaller the percentage by
which a faster boat must beat a slower boat. To correct this, some PHRF
races are handicapped by multiplying a boat with rating R's elapsed time
by (C / ((C - Rav) + R)), where Rav is the fleet's average rating, and C
is a constant around 600-700, to compute corrected time. This system is
called "time on time", the previous, more common, system is "time on
distance."
The two systems only differ substantially when ratings span a large range
(> 30 s/M), or races are long (in time). It is not clear which system is
ultimately fairer.
The Portsmouth Yardstick (PY) is a statistically based rating rule. The
PY was developed by the Dixie Inland Yacht Racing Association to
handicap any boat, including multihulls, which are excluded from all the
previously described handicap systems, based on performance in races.
The PY begins with a boat which is well sailed, and ubiquitous, called the
"Primary Yardstick." This boat is assigned a Portsmouth Number (PN),
which is the time the boat takes to travel a fixed, but unspecified
distance. In the US, the Thistle the primary yardstick, and its PN is 83.
Elapsed times are collected for races. The fastest boat of each type in a
race is assumed to have sailed a perfect race. The ratios of the fastest
boat's time to the fastest yardstick boat's time, normalized by the
yardstick boat's PN are averaged over all races to compute that boat's
PN. Statistical techniques are used to discard outlying data points. A
class with a large quantity of data, and no recent change in PN may
become a "Secondary Yardstick," used in the same fashion as the Primary
Yardstick. The Laser and J/24 are examples of Secondary Yardsticks.
The usual way to handicap with Portsmouth numbers is to multiply
elapsed time by 100/(PN) to compute corrected time. This is a "time on
time" system (see PHRF).
In addition, PY has begun to compute numbers for different wind
strengths. The Primary Yardstick is defined to have the same number
for all wind strengths. Using these numbers, clubs can more fairly
handicap races in various wind strengths.
Since the PY data are not broken down by course type, it assumed that
boats racing under the PY are racing courses similar to an Olympic,
triangle or Gold Cup course.
Below are formulas for converting among different system's ratings.
Accuracy of these conversions may vary. (And indeed, the last one has
been called into question by one reader, so you should probably treat it as
suspect).
PN = PHRF/6 + 55 PHRF = GPR - 550 PHRF = 2160/sqrt(IOR) - 198
Since we know that the IMS GPR is the time taken to cover a mile (of a
particular course), in 10 knots of wind, we can estimate a boat's speed
over this course given its PHRF rating:
v = 3600 / (PHRF + 550)
So, a J/24's (171 s/M) speed is 4.99 knots, a J/35's (69 s/m) is 5.81
knots. The J/35 is 16% faster. Note that the standard PHRF increment
of 3 s/m represents around a 0.4% change in boat speed.
Using the IOR conversion, a one tonner might rate 72 s/M, whereas they
are actually much faster than that, rating around 54 s/M PHRF. This
illustrates the "advantage" designers can take of the IOR.
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