You'll knacker your boat roof-racking it inflated.
Its also going to be heavy. I mean REALLY heavy. 3.5m Glass Rib will be 80-100kg with nothing.
You may well be over limit for the roof - would have thought it'll be 50-75kg. If not getting it up there will be a challenge. See Camp4Ever - 3.5m Kimberley Hypalon Alloy RIB
- this is a ALLOY rib not glass, so is lighter and they still use a fancy rack for it, and use the 3.1m.
You engine will fill the Ka. How many people are you moving? By the time you've but oilskins, life jackets, anchors etc in the boot is there room for the engine without seats down?
That roof rack is designed not to put pressure on the toobs. a standard roof rack will cause wear on the toobs.
You aint going to enjoy moving it through the house.
If you don't want a SIB is a RIB and storage on a trailer elsewhere possible? (Caravan storage place?)