what would happen if I bolted RIB tubes to side of cabin cruiser?

Would it:

1) ride dryer?
2) be nice extra cushy bumper
3) be nice place to sit with toes in the water or walk/stand?
4) increase seaworthiness and stability?
5) without any noticeable loss of performance or extra weight?

 

Is this the actual boat that you have in mind?
I see on the photo that she is a Noah Thompson design - it would be worthwhile asking for his opinion?
Noah Thompson Design https://www.noahthompsondesign.com/
I cannot see any advantages (only disadvantages) in having RIB tubes in the position shown on your attached photo.
But if the tube was attached at the gunwhale, rather than at the chine, then she should ride dryer, and it would be a nice extra cushy bumper.
BUT - it would be extra weight.
And if the boat is seaworthy and stable as she is, and works well, then methinks best to leave her alone and don't try to 'improve' her.

 

I think thats a cat, notice the blunt bow and most of his boats are power cats.

I was thinking of at the waterline when at rest to be big bumper/boarding area for smaller craft for mothership with high freeboard and with stern taken up by outboard and trolling motor, and to mitigate roll at rest, and for extra seating without hurting stability, and to help buoyancy when over-loaded, and be a place to strap oversized lightweight cargo like kayaks or camping chairs.

 

what would happen if I bolted RIB tubes to side of cabin cruiser?

Well, if you mount them like RIB tubes they won't touch the water at rest and especially not on plane.
But they will make your boat more costly, heavier and likely harder to sell when the time comes.
Oh, and more costly to maintain.

 

One assumes inflatable tubes on rigid hulled boats are in vogue because it acts as a buffer that reduces the chance of injury to people being rescued etc. The fundamental stability of any boat should not need tubes added as an afterthought, though obviously it can be enhanced by adding them, but at the design stage they should not be there for the reason of stability. That should be designed in to the hull to start with.

 

You would be laughed out of the yacht club.

 

RIB tubes are really heavy. It would not surprise me if adding them to certain designs would result in failure to plane, certainly fuel consumption on the plane would go up noticeably.

If they were touching the water at rest then added drag at low speed would be significant. This might be enough to prevent a boat from getting over the hump and onto the plane.

Assuming they ended up just above the water on the plane then slamming in waves would be increased, but they might redirect spray making the ride drier.

They might make a nice place to sit and would certainly act as a bumper. I would caution that pulling yourself out of the water onto RIB tubes is not trivial, so they would not be a benefit in a MOB situation.