IRC Bulb Weight refers to the Bulb as defined by the ISAF Equipment Rules of Sailing. This document is provided as clarification that all under-fin spacers and infills shall be included in the total Bulb Weight.

The ERS defines Bulb as
E.1.2(e) A hull appendage containing ballast at the bottom of another hull appendage primarily used to affect stability.

Ballast is defined as
C.6.3(e) Weight installed to influence the stability, flotation or total weight of the boat.

E.1.1 defines Hull Appendage as
Hull Appendage Any item of equipment – including the items listed in E.1.2– which is:
• wholly or partly below the sheerline or its extension when fixed or when fully exposed if retractable,
• attached to the hull shell or another hull appendage, and used to affect: stability, leeway, steerage, directional stability, motion damping, trim, displaced volume.

And clarifies that:
Any of the following shall be included in the hull appendage:
corrector weights,
• integral ballast, and
• associated fittings.

Therefore, all under-fin spacers and infills shall be included in the total Bulb Weight.

Addition in reply to a specific question:
Q. We have read the latest IRC bulb weight clarification were it clearly states that all under-fin spacers and infills shall be included, but we are not clear about the bolts/nuts/washers. Do you mind clarifying if we should consider these items as part of the weight?

A. As any retaining bolts and attachments, as well as infills and adjustable weights, are only there as part of the bulb and would not be there if it was not for the existence of the bulb, these would be included in the bulb weight.

RORC Rating Office
March 2016

PDF version

IRC - Single Roller Furling Headsail allowance – Maximum size for a Heavy Weather Jib

Link: Help with mathematical formulae

Under IRC boats may elect to rate with multiple headsails or with a single roller furling headsail. Boats rated for a single roller furling headsail then have the option of declaring that they will also use an ISAF heavy weather jib (HW Jib) and receiving the rating credit at a reduced rate. For all boats that elect to use a single roller furling headsail + HW Jib and are eligible ie. rated headsail LP >1.3*J, the maximum area HW Jib is then noted on their IRC certificate.

For information, and to help other boats in deciding whether to adopt this option, the IRC Rating Authority calculates the maximum heavy weather jib area from the following formula:

Maximum HW Jib area (m^2) = 0.135*(Forestay length^2 – J^2)

ie 0.135 multiplied by (Forestay Length squared – J squared) with the answer being in square metres.

The actual area of a boat's HW Jib is then calculated in accordance with IRC Rule 21.7.1, ie:

HW Jib area (m^2) = 0.0625*LL*(4*LP + 6*HHW + 2*HTW + 2*HUW + 0.09)

Both of the above areas are rounded to two decimal places.


While the formula above for the calculation of maximum heavy weather jib area is a close approximation to the formula in ISAF Offshore Special Regulations for the calculation of heavy weather jib area (area not greater than 13.5% of the height of the foretriangle squared), it is not identical for the reason that IRC data does not include height of the foretriangle. The IRC formula above may therefore give a slightly different answer to that in ISAF OSRs. Compliance therefore with IRC Rules in respect of maximum heavy weather jib area for the purposes of single roller furling headsail allowance does NOT necessarily confirm compliance with ISAF OSR heavy weather jib requirements.


If your boat is not a standard production boat with which we are familiar, or your boat has had modifications to the design, we usually ask for photos so that we can assess the boat in the fairest way possible.  In order to help us do this, we have put together some guidelines to show the type of pictures that are useful to us:

IRC photograph guidelines

If you are not familiar with mathematical formulae, they can appear very confusing!  This page is intended to help you with the various formulae used in the IRC Rule:

*  Multiply

<  less than

>  greater than

^2   squared  (eg. 4^2 = 4 squared = 4 x 4)

^3  cubed (eg. 9.5^3 - 9.5 cubed = 9.5 x 9.5 x 9.5)

^0.5  square root  (eg. 25^0.5 = square root of 25 = 5)

m^2   square metres

( )  calculations shown iithin brackets are solved first  eg. 4*(3+3) = 4*6 = 24

If you would like clarification of any the formulae in the IRC Rule, please contact RORC or UNCL who will be able to help you.


Headsail HUW default size

Those of you revalidating your 2012 certificate for 2013 will have noted that it is acceptable to use a default HUW (Headsail Upper 7/8 Width) for existing headsails. The default size depends on the overlap of the headsail, and is related to the HTW (Headsail 3/4 Width), and is calculated as follows:

For overlapping headsails with LP/J greater than 1.10, the default is HTW x 0.50.

For non‐overlapping headsails with LP/J 1.10 or smaller, the default is HTW x 0.56.

These default sizes are based on actual measured data for a significant number of sails, and any variation from the default will have a very small effect on rating.

For all newly measured sails we now need the HUW to be declared.

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Header images by:

John Crawley (CAN IRC)  
Jinno (c/o JPN IRC)
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