I looked at these long and hard a little while ago, and thought they were very expensive for what they are. I ended up going down the scrappy and getting a commercial van roofrack (much better qualirty than halfords stuff, for a couple of quid, added a couple of planks that I secured with cable ties, and bob's your uncle! /forums/images/icons/smile.gif
I have one, love it. Transported our old 3 metre GRP pram dinghy (God knows how heavy...) from Gillingham to Chichester. Fabulous. Wobbled ever so slightly initially, but just tightened up the ratchet straps and worked a treat. You just need to make sure it's a) blown up enough and b) on tightly enough. Definitely worth it. Inflatable would be dealt with easily I'd think, compared to what we put on it..! Of course, if you're talking a 15 foot RIB...
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Don't waste your money or the paint job on the roof of your car.
They tend to move and scuff the paint not to mention wobble. They have to be tied through the windows - (safety hazard) or through the door opening (strains the door frame).
Get a proper one from a scrappy or a "dealer" one from your local car dealer.