The blocks are connected to the leech of the mainsail with webbing, and reduce point loading on the mainsail when reefing. The small diameter sheaves are suitable to very high loads. The centre hole can be used as a safety connection to the boom when reefed. The small version (D=50 mm) is for boats to 50 ft, larger (D=120 mm) for boats to 100 ft. If a larger sheave is required and if higher weight and larger sizes are acceptable, then the clew blocks described above can be considered.