I had a guy tie to me last year with one and it rolled around everywhere and made a ton of noise and left marks on my boat, they came off but it still sucked.
This is what I do.First always try to raft to a boat that is the same basic size and shape. I use 10 and 12 inch round and anywhere from 24 to 34 long bumper's and I have the cover's on them so they don't make noise. Some people have told me they scat ch the paint but I have been using them on three boats and have never seen any scratches. Tie as tight as you can in the rear and just snug in the front, it seem's the bow's never go up and down the same speed. I have a friend that has the flat fender's that tie's to me, the boat's are a lot closer than I like but they never hit or need to be adjusted like round one's do some time's.