The full IRC Rule text can appear long and complex to the untrained eye! While there is no substitute for being familiar with all the rules, while you are examining them this simplified version might be helpful. However, this version is not definitive and there may be exceptions – do not rely on this document in a protest meeting!
To race in an IRC class, the boat must hold a current IRC rating (Rule 8.2)
It is not OK to use the rating of a sistership, last year's rating, or one made up by the Club secretary based on your personal handicap number. Firstly a boat's rating is based on its physical data, so a sistership's rating will not be accurate. Secondly, the IRC program is developed each year so ratings often change (for better or worse). Thirdly, not having a current certificate based on the boat's own data means that the quality of IRC rating deteriorates, and lastly, loss of funds required to maintain the system to the level that is required by owners and clubs means that the prices go up for everyone, which is a vicious circle!
So, make sure you (and your competitors) have a current rating for your own boat. The list of rated boats can easily be checked at www.ircrating.org.
Change of ownership or sail number invalidate the certificate (Rule 8.8)
If you have just bought a boat with a current IRC certificate you need to re-register it in your name, which is very simple and cheap, especially if you've made no changes to the boat.
If you change the sail number then this will need to be amended on the certificate (and see below regarding approved sail numbers).
Changes to the boat that may affect the performance must be declared (Rule 8.9 and 22)
If you change the anything on your boat (including but not limited to sails, rig, hull, deck/coachroof, keel, rudder, interior) then you must tell the Rating Authority. Some changes may not affect the IRC rating but most will invalidate the certificate unless declared and rated. If you are not sure then get in touch with your Rule Authority and check.
Removing standard items such as the saloon table, doors or other standard fittings must be declared and will increase the rating.
If your boat is rated as a One Design then it must comply with the relevant Class Rules (Rule 13.7)
If you apply for a One Design IRC certificate for an approved design class, and declare that the boat conforms to the relevant Class Rules, your IRC certificate will state 'ONE DESIGN' after the TCC and you and the boat must comply with the Class Rules when racing IRC. Also see below regarding crew limits.
The boat must have an approved sail number (Rule 14)
Sail numbers are important not only for race committees so you get your correct result, but in the case of an incident involving search and rescue they are vital for identification. You can get an approved sail number from your National Authority.
Builder or hull numbers are not usually approved except for International One Designs. If you are not sure, check with your National Authority.
If you have press-button power for your backstay or other running rigging such as electric sheet winches, this is allowed but must be declared and rated (Rule 15)
If you have a hydraulic backstay that you pump by hand, that does not count as 'stored power'. You do not have to declare a powered mainsail halyard, reefing system or sail furling.
If you are rated for a single furling headsail, you cannot use other headsails (Rule 21.1.5) except ISAF sails.
Your certificate will say whether the boat is rated for a single furling headsail, single furling headsail plus ISAF Heavy Weather Sail or multiple headsails. The exception is that you can use an ISAF Storm Jib at any time.
If you are doing a regatta held on consecutive days (including lay days) you cannot change the sails you carry (Rule 21.1.5 (d))
Unless the race organiser changes this rule in the Notice of Race, you must keep the same sails aboard the boat for every day of the event. That means not leaving some sails on the dock if it's light, or changing which ones you carry each day.
The total crew weight must not exceed the IRC crew limit (Rule 22.4), unless this rule is changed in the Notice of Race.
Many events change this rule, so check the Notice of Race. If not changed, then the total crew weight is limited to the crew number x 85kg.
Boats rated as a One Design must comply with their Class Rule crew limits, unless freed from this by the Notice of Race.
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Full IRC rule text: http://www.ircrating.org/technical-a-certification/rule-a-definitions