Geoffrey - agreed that experimenting is good, but rather than add a pulley I may just buy a selection of o-rings at varying inside ø's...

Eddy - yep, I had a look at a driving roller and a second idling roller design, trouble is that the motors I have run at 20rpm - so the tub would rotate at about 2rpm in that config...

I didn't want to get into bigger motor territory - 12v small and quiet(ish) was the aim!

At 60p/o-ring I may just buy a handful and report back on what works, just thought there might be a 'rule of thumb' for these things.

Wal.