In terms of punting along at a decent lick, suspect Thumper could maintain 20knts - bit unsure as never taken her up river. With regards to launching, I'll have to examine where would be best with regards to range as currently have underconsole 25l tank but would of course carry extra, pos as much as same again in cans stowed in bow.
May be best to launch relatively up river and join the flotilla (or whatever colective term is for RIBs?!) there.
One option to avoid night navigation would be to head up in daylight and for me to return to launch site before dark - partly misses the point of seeing xmas lights
but pos prudent given first "cruise"...safe than sorry etc.