As you can see, we use the Optiparts one,
www.ukmirrorsailing.com/index.php?option...le&id=597&Itemid=339
and I would say they are very effective at stoping inversions. However, there is a limit. If both sailors remain on the 'cockpit' side of the hull and hang on the top gunwale etc. for long enough, even with a float, it will invert (but is still easier to right from that position than without a float)
Thanks - that is really helpful - all I want is a slowing down if it's a dip to windward to enable me to release kicker etc before swimming round to right the boat. thanks again, Eleanor