Heavy Weather Jib – Maximum Size

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.