Originally Posted by HughN
Paranoid, I know, but circular to elliptical does not go.
True, but baffles are somewhat conical.
OP - I worry more about damage to PAX than to the tubes. It's a speed to seastate call, IMO. Divers tend to get driven around at 20kts (or they did in my day) and in reasonable conditions. I've travelled by "tube" in faster, bumpy trips and it's far from ideal, both from a safety point of view and an injury angle. To add some facts to the pot, commercial RIBs in Ireland are not allowed to seat passengers facing sideways (on real seats), never mind tubes.
IMO, fine for short hops in calm seas at speeds at and below 20kts. As regards damage to tubes, unless you do it every day, it won't damage OR wear them at all (at speeds that it's safe to do).