Originally Posted by Tims Osprey
Infact in one respect the tubes being out of the water has it's advantages, If for some reason there was a failure or accident involveing the tubes then no worries there, on the other hand if you were to split the hull open and flood it the tubes are there to stop you going down to the sea bed so in some respects the best of both worlds i think, The only disadvantage is that it's not as stable at rest.
That is one way to think about it. However, even the smallest RIB or SIB has 3+ air chambers, and a multiple chamber failure is exceedingly unlikely. A RIB or SIB is highly unlikely to sink due to any damage, particularly in comparison to a hard boat damaged in collision or gounding. Been there, done that, TWICE! One skipper put a J/24 sailboat on the rocks, breeched the hull and we had to be rescued by the local Harbormaster, in >5 knots
I've also rolled a J/24 180 degrees and sank it in <35 knots, an altogether different story
If you're seriously concerned about sinking a hard boat, there is a better way to protect it. There are inflatable bladders you can fit below decks, which are equipped to inflate upon submersion, much like an auto inflating life jacket. May or may not be more cost effective than adding tubes as above, but certainly better looking!
The added stability of having the tubes immersed at rest is probably the #1 benefit of a RIB, IMHO. But once you're planing, there's no real detrimental effect. For my use, being able to bump & snuggle up alongside a sailboat under sail with no damage is a close #2. Love those giant permanent fenders!