J/120 ORR  IRC Links & Specifications
With some of the new IRC type form boats that are now coming out, it is our recommendation it would be better to just buy a J/122 than rather to modify a J/120. Even though a J/122 is given time under IRC, a J/120 is at least 33 seconds per mile slower than a J/122 because it does not fit the IRC definition of a good typeform. However if the owner is determined to modify a J/120 to compete, another owner has modified their boat to race IRC. Here is a link to their web page:
http://www.avra.us/Boat.html
Here is a comparison for PHRF and ORR ratings
http://www.phrfne.org/page/handicapping/asdg
If the J/120 owner would like to get a fair rating in ORR here are some links below:
http://offshore.ussailing.org/ORR.htm
http://offshoreracingrule.org
IRC  US Web Site
http://www.usirc.org
ROYAL OCEAN RACING CLUB Rating Office in United Kingdom
http://www.rorcrating.com/
J/120 Preparation for Weighing for IRC (word document)
How to get rated
http://www.usirc.org/howtogetrated.htm
Comparison Chart for Different Boats Worldwide (unofficial)
http://www.navwiz.com
J/120 IRC US IRC numbers as of June 6, 2005
J120_IRC_list.xls
Comparison Chart for Different Boats in US (unofficial)
http://www.ussailing.org/offshore/irc/WEB%20IRC_TTCs.xls
PHRF New England IRC to PHRF Comparison (unofficial)
http://www.phrfne.org/page/823
IRC TimeDifference Table  Used to generate a timedifference and elapsed times table between you and your competition in IRC (Created by L. Kahl, Interactive Creations)
http://www.usirc.org/irctimetable.cfm
Understanding IRC Handicap Differences
ARE WE WINNING? Simplistically, each 0.001 of rating difference between two boats represents a corrected time difference of approximately 3.6 seconds per hour. This will however only be absolutely correct for boats rating very close to 1.000. So how can you be certain whether you are beating your rivals on the race course? Starting with boat A rating 1.010 and boat B rating 0.995, if Boat A sails for one hour, her corrected time will be 1.010 hours which is 1:00:36 (1 hour 0 minutes 36 seconds). Boat B will beat Boat A if her corrected time is less than that. To find out what this means in terms of time, we want to find Boat Bï¿½s maximum elapsed time. To do this divide Boat Aï¿½s corrected time by Boat Bï¿½s rating  1:0:36 / 0.995 = 1:00:54. In other words Boat B can sail for up to an extra 54 seconds before her corrected time is more than boat A's. An easier way of doing the same thing is to divide the rating of boat A by the rating of boat B ï¿½ ie 1.010/0.995 = 1.015 and 1.015 hours is 1:00:54. Notably, boats A and B have a rating difference of 0.015, thus confirming (54/15 = 3.6) the 3.6 seconds per 0.001 noted above. But if the boats are faster or slower, while the maths remains the same, the answer starts to vary from 3.6. If boat C rates 1.110 and boat D rates 1.095 (the same rating difference of 0.015 as A and B) the time difference for an hourï¿½s racing is 1.110/1.095 = 1.0137 which is 1:00:49 ï¿½ equivalent to 3.27 seconds for each 0.001. Similarly, boat E rates 0.910 and boat F rates 0.895 (again 0.015 less). The time difference for an hourï¿½s racing by Boat E is 0.910/0.895 = 1.0168 which is 1:01:00 ï¿½ equivalent to 4.0 seconds for each 0.001. For a given fleet of boats, this can then be dealt with by creating a table of time allowances for the fleet
Time Differences in seconds for various race durations 
1 hr 
2 hr 
10 min 
20 min 
30 min 
222 
443 
37 
74 
111 
154 
308 
26 
51 
77 
119 
238 
20 
40 
59 
51 
103 
9 
17 
26 
42 
83 
7 
14 
21 
35 
71 
6 
12 
18 
0 
0 
0 
0 
0 
0 
0 
0 
0 
0 
42 
83 
7 
14 
21 
42 
83 
7 
14 
21 
51 
103 
9 
17 
26 
58 
116 
10 
19 
29 
93 
186 
16 
31 
47 
The table above is the ideal way to deal with this. For occasions when you do not have this for whatever reason, another simple way of dealing with it is to divide 3.6 by your TCC. eg, 3.6/1.100 = 3.3 seconds or 3.6/0.900 = 4.0 seconds. The answer in each case will then be closer to the correct time allowance per 0.001 rating difference between your boat and a reasonably closely rated competitor. The answer will still be approximate, but rather better than just using 3.6! Authored by Mike Urwin
