PHRF Time on Time help!

Imported from Yahoo
1

Tried to score a race using PHRF time on time last night, but was not
able to determine what the "Rating System Coefficient Defalut A Value,
and Default B Value" should be. Anybody know how to determine what
these values should be.

Thanks

2

Eric at RYC we use 650 and 550.

Cheers,

Wayne Hatfield

Sailing Director RYC
http://www.rothesayyachtclub.com/racingframes.htm

···

using PHRF time on time last night, but was not able to determine what the "Rating System Coefficient Defalut A Value, and Default B Value" should be. Anybody know how to determine what these values should be.

Thanks

3

Do you know why those numbers are used, should I use those, or do we
need more information to determine what we should use? I guess I
just don't understand what they accomplish.

Thanks, Erik

Eric at RYC we use 650 and 550.

Cheers,

Wayne Hatfield

Sailing Director RYC
http://www.rothesayyachtclub.com/racingframes.htm

>using PHRF time on time last night, but was not
>able to determine what the "Rating System Coefficient Defalut A

Value,

>and Default B Value" should be. Anybody know how to determine

what

···

--- In sailwave@yahoogroups.com, Wayne Hatfield <kriver2@...> wrote:

>these values should be.
>
>Thanks
>
>
>
>
>
>
>
>
>
>

4

PHRF ratings are commonly expressed
in terms of time on distance (TOD), that is, in seconds per mile. A boat with a
rating of 100 (seconds per mile) racing against a boat with a rating of 120,
owes the 120 boat 20 seconds per mile. In order to calculate your relative time
you need distance. If the race is 10 miles long then the difference is 10*20 = 200
seconds, or 100 must finish 200 seconds before 120 to win.

Time on time (TOT) makes a
correction based on the elapsed time of each boat and is independent of the
distance sailed. Using a formula of Corrected
Time = Actual Time * TCF (Time Correction Factor) where the TCF = (A / B + PHRF Rating).

The constants A
and B which are used to compute
the TCF vary with the scoring organization. However, all are similar in that
slower boats have a smaller TCF than faster boats. In practice, when computing
a corrected time, slower boats have their elapsed times decreased, while faster
boats have their elapsed times increased.

The constant A
= 600 in the numerator has the least effect on the outcome of corrected times.
The purpose of this constant is to come up with TCFs that give corrected times
that, on average, will approximate the expected times of the race. It should be
picked so that a boat in the middle of a class or fleet will have a TCF of 1.

The constant B
varies by scoring organization and is in the range of 480 to 650. The denominator, B + PHRF, is the number of seconds it takes
to sail a nautical mile in the expected conditions. Another way to look at it
is that the denominator divided into 3600 is the average rhumb line boat speed
in knots. If the races are sailed in about 10 knots with about a
third of the distance sailed to windward, 550 might be a good selection. In
general, choosing a lower number for this constant favors slower boats and
choosing a higher number favors faster boats.

US Sailing recommend the following B factors.

B Factor

When used

480

Heavy air or all off the wind

550

“Average” conditions

650

Very light air or all windward work

If you have historical data, I would strongly recommend that
you rescore several races using different B
constants and see how the results change.

···

From: sailwave@yahoogroups.com [mailto:sailwave@yahoogroups.com] On Behalf Of mann.erik
Sent: Monday, August 21, 2006 4:45
PM
To: sailwave@yahoogroups.com
Subject: [sailwave] Re: PHRF Time
on Time help!

Do you know why those numbers are used, should I use
those, or do we

need more information to determine what we should use? I guess I

just don’t understand what they accomplish.

Thanks, Erik

— In sailwave@yahoogroups.com,
Wayne Hatfield <kriver2@…> wrote:

Eric at RYC we use 650 and 550.

Cheers,

Wayne Hatfield

Sailing Director RYC

http://www.rothesayyachtclub.com/racingframes.htm

using PHRF time on time last night, but was not

able to determine what the "Rating System Coefficient Defalut A

Value,

and Default B Value" should be. Anybody know how to determine

what

these values should be.

Thanks

5

Erik this link should help explain why the values are used. The former race director also used these values in his excell spreadsheets before I took over so I just transfered them to Sail Wave.

http://www.ussailing.org/phrf/TOT.htm

Wayne

mann.erik wrote:

···

Do you know why those numbers are used, should I use those, or do we need more information to determine what we should use? I guess I just don't understand what they accomplish.

Thanks, Erik

--- In sailwave@yahoogroups.com, Wayne Hatfield <kriver2@...> wrote:

Eric at RYC we use 650 and 550.

Cheers,

Wayne Hatfield

Sailing Director RYC
http://www.rothesayyachtclub.com/racingframes.htm

using PHRF time on time last night, but was not able to determine what the "Rating System Coefficient Defalut A
     

Value,

and Default B Value" should be. Anybody know how to determine
     

what

these values should be.

Thanks

6

Erik:
Check with your local PHRF organization, different PHRF regions use
different numbers. Some will change numbers based on wind strength etc.
On Lake Ontario we use 507.411 as the B number, the A number is the B
number plus the scratch boat rating we use 685.411 for a scratch boat
of 177.
PHRF Lake Ontario specifies the numbers to be used.

Yours
John Crawley

···

--- In sailwave@yahoogroups.com, "mann.erik" <mann.erik@...> wrote:

Tried to score a race using PHRF time on time last night, but was not
able to determine what the "Rating System Coefficient Defalut A Value,
and Default B Value" should be. Anybody know how to determine what
these values should be.

Thanks

7

Hi Mark, thanks for that answer. I am scoring a time-on-time series
for the first time, and I have a basic ToT question. After reading
the USSailing site and your answer below, it seems reasonable to me
that the B variable should be chosen on a race-by-race basis
(especially where our wind speeds are so variable from one race to the
next). Yet, it seems to me that everyone chooses one B factor for the
entire series (a season for us). That's the only way I can find to do
it in Sailwave as well. Yet, how can you choose a B factor at the
beginning of the season when you don't know what conditions most of
your races will be sailed in? It seems for that to work, you would
not be able to choose a B factor until the series was over and then
retroactively figure an average wind speed.

Does anyone apply different B factors to each race? Anyone know why not?

Thanks!
Jeff

--- In sailwave@yahoogroups.com, "Mark Townsend" <s_mark_townsend@...>
wrote:

The constant B varies by scoring organization and is in the range of

480 to

650. The denominator, B + PHRF, is the number of seconds it takes to

sail a

nautical mile in the expected conditions. Another way to look at it

is that

the denominator divided into 3600 is the average rhumb line boat

speed in

knots. If the races are sailed in about 10 knots with about a third

of the

distance sailed to windward, 550 might be a good selection. In general,
choosing a lower number for this constant favors slower boats and

choosing a

higher number favors faster boats.

US Sailing recommend the following B factors.

B Factor
When used
480
Heavy air or all off the wind

550
"Average" conditions

650
Very light air or all windward work

If you have historical data, I would strongly recommend that you rescore
several races using different B constants and see how the results

change.

···