I put a Synchro 61 mm snap shackle on the top mainsheet block on my Hobie Tiger & am pleased so far. This particular shackle has no swivel, which appears to reduce the lenght by maybe 0.5 inch. The top block has a swivel, so the swivel on the snap shackle is redundant. Besides, I hear that some F18 sailors prefer to lock the swivel to prevent twisting. Synchor Block Snap Shackle - 61mm 22mm Opening Dia. 2,431lbs. SWL West Marine No. 3734415 Lewmar No. 29925040 Here's the URL for the West Marine catalog, copied from my web browser address window: http://www.westmarine.com/webapp/wcs/stores/servlet/producte/10001/-1/10001/81255/377%20710/0/Synchor/Primary%20Search/mode%20matchallpartial/0/0?N=377%20710&Ne=0&Ntt=Synchor&Ntk=Primary%20Search&Ntx=mode%20matchallpartial&Nao=0&Ns=0&keyword=Synchor&isLTokenURL=true&storeNum=2&subdeptNum=323&classNum=324