ORCV Safety - Cat1 and Cat2 Stability

Which way is up?


The following information has been designed to assist you with getting the correct information together regarding the stability of your boat.

'Aspect Computing', © and with thanks to Richard Bennett, Sydney Hobart 1998

The refinements to the pre-existing Special Regulations that came into effect on July 1st 2009, had a major effect on the ORCV and they way that skippers must provide documentation demonstrating resistance to capsize for their yacht. We will be requiring all competitors to comply with the current 2009-2012 Special Regulations, as the new rules will apply from the 2009 Melbourne to Stanley (M2S) race and forward from there on.

The refinements (technically from the rules of the last eight years) require you to provide a declaration and proof of stability by one of the following methods for,

Category 1 Races:

  • IMS or ORCI Stability Index of 115 or greater, OR
  • ISO or IRC Category A exept that the STIX number shall be increased to a minimuim of 35

Category 2 races:

  • IMS or ORCi stability index of 110 or greater, OR
  • ISO or IRC category A, OR
  • IRC SSS base value of 28 or greater.

Please note that for an IMS/ORCi certificate to be a valid proof of stability, it needs to meet the following requirements:
• For Cat2 races or above, that the boat is still owned by the holder of the certificate
• The certificate has been issued in the last five years
• The Boat has not been modified
• The current freeboard measurements have been confirmed as being consistent with the certificate.

Using the IRC Base Value

Importantly, some boat owners will find that their yacht has complied with stability requirements in the past, but that the IRC SSS Base value is below 28 and you will need to find an alternative means of proof. Please note that, an IRC SSS base value of 28 can only be used for Category 2 or below races. Also, the SSS figure found on IRC certificates is determined by the RORC rating office and is an automatically calculated value from all the other measured data for the IRC certificate and it’s a seaworthiness factor as a opposed to a stability index.

What do I need to do if my boat does not currently comply?
The first step is to find out if your boat has one of the following:
• ISO Category A statement from the designer or other appropriately qualified person (typically a naval architect) issued in accordance with ISO 12217-2
• IRC Category A certificate assessment shown on IRC certificate (this can be found on some IRC certificates, and added by way of an application)
• IRC SSS number of 28 or greater for Cat2 races and below
• ORCi certificate in your name that is less than five years old.

If you have one of the above and you are happy to declare that no changes have been made to your boat, which affect its stability, then you will meet the new requirements.

NB: A GZ curve provided by the yacht designer is shown on an ORCi Stability & Hydrostatic Datasheet and is produced as part of the ISO 12217-2 calculations. By itself, a GZ curve does not meet the YA Special Regs for stability!!! In other words, the ORCV will not accept a GZ curve provided by a yacht designer on its own. Supporting documentation from an inclination test will need to be provided to demonstrate that the boats resistance to capsize matches the designer’s data provided.

Further Information (Should you do not have one of the above):
1. Revalidate your boat's IMS certificate, by contacting Yachting Australia on 02 8424 7400 to discuss the process required.
2. To have STIX information added to your IRC certificate, please visit the RORC Rating Office website at www.rorcrating.com, where a your boat’s designer is able to complete an application form to have the Inclusion of Designer Calculated STIX, AVS, and Design Category, on a Boat’s IRC Certificate.

To revalidate you IMS certificate you need to do the following:
1. Check that Yachting Australia has a record of your boat's data
2. Update any changed measurements, freeboards and incline your yacht if necessary

What will it cost me?
• Inclination and freeboard measurements $300. Keep in mind that this is a measurer determined fee and it may change depending on how much time it takes the measurer.
• Yachting Australia revalidation fee $450+gst

To obtain your IRC certificate you need to do the following:
Contact an IRC measurer to discuss measurement activities required for your IRC certificate application. For an IRC certificate to be Endorsed, boats and sails must be directly measured and be hull and rigs weighed.

What will it cost me?
In essence, visit Yachting Australia’s website for further information on new and revalidated ORC & IRC certificates. Fees are applicable for the ORC & IRC measurers time to conduct inclination tests and then process the application forms to Yachting Australia.

As a guide you may find the measurer's will need approximately four hours at around $80p.h. Weighing the boat individually may be very expensive, as you need a suitably sized crane and a load cell. Look for a group weighing day, such as those held by YV, which are much better value. The cost of a new IRC certificate is LOA by $28.50 per metre. To re-validate a past IRC certificate is $27.50 per LOA metre and to amend a current one is just $10.50 per LOA metre. Please visit YA's website, as prices are subject to change.

Obtaining your boat's GZ Curve.
A GZ curve is calculated by the yacht designer or naval architect. It is easily done if you or the yachts designer has an electronic hull file. You will then need to provide an inclination and the boat's weight. If you have the lines plan of you yacht, some naval architects will convert these plans to a hull file. We cannot give you an accurate cost for this process as you will have to negotiate with the yachts designer or a naval architect. Please remember and as mentioned above, providing a GZ curve by itself will not meet the requirements of the Special Regs. It is one of several bits of information that are part of the calculations behind ISO and IRC category, and the ORCi/IMS data.

The GZ or stability curve is a graphic presentation of a boats static stability. As the boat heels, it develops a righting moment, which is the force created by the ballast and hull buoyancy that works to resist the heeling. The stability curve shows this righting arm (righting moment divided by displacement) as a function of heel angle. When the righting arm turns negative (at 123 degree for this diagram) the boat will no longer resist the heeling force and will capsize. An important measurement on the righting curve is the relative areas under the positive and negative parts of the curve. In this curve the positive is 3.96 timers greater then the negative. That means it takes 3.96 times as much energy to turn the boat from upright to capsize as from capsize to upright.

How can the ORCV help you?

We can arrange to incline your boat and measure your freeboards and we will happily arrange a weighting day for a collective group to get all boats done. Please call Simon Dryden 0418 145 909, or email This email address is being protected from spambots. You need JavaScript enabled to view it. if you need any further information.

Click HERE to see what doing the inclination tests looks like.


By Simon Dryden and John Curnow, with many thanks to Glen Stanaway and Susan Veal from Yachting Australia for their keen eye, attention to detail and sensational tips.


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