The IRC rating offices have seen a growing number of instances and enquiries about additional cloth set under the boom, known by various names including lazy sweeper, deck sweeper, mainsail skirt or watersail.
In response to this the IRC Technical Committee has issued IRC Notice 2021/02 which notes that this cloth is defined as a sail and explains how to measure the additional area so it can be accounted for in the rig factor.
Notice 2021/02 Additional cloth set under the boom
