Yes, weather window was the phrase that I was thinking of...
But I'm certainly not the best person to recommend what is, or isn't possible. What I can tell you is that the smallest RIB on the cruise was a 6.5m (&150hp)...
I learnt an awful lot on this cruise. One thing was the safety of cruising in company. Not just the numbers, but the quality of experience. It was like being in a five-a-side team with Beckham, Gigsy, Owen and Seaman. It's not something that I had considered until joining BIBOA (mainly 'cos I didn't know anyone else with a RIB). One RIB suffered an engine fault off Cap de la Hague which could have been extremely serious on a solo RIB. This was resolved by a tow across the channel. Not a problem at all when the tow boat is a 10m with one of the country's most experienced RIBsters.
I don't have a RIB of my own, so I went over on Still Deep One, a 9.5m twin diesel engined Delta. The outward passage was at about 33-36kts, with almost no wind but a confused sea which slowed some of the smaller boats. I returned on the smallest RIB (when we left Carteret marina, some English people on a 36ft cruiser pointed at us and shouted "are you going across the channel on that???") and we lost no time in getting on with the passage as the wind was forecast to rise to F4-F5 during the afternoon. I took the helm for a short period of time and found the conditions difficult - not because of much of a chop, but just difficult to keep on course whilst maintaining speed. More experience needed, I think! We were able to keep up 32-33kts all the way back, but part of that was whilst travelling in the flat part of Still Deep One's wake whilst battling against the disturbed sea off the Cherbourg Peninsular.
Lessons? My skipper (Peter) was well at home on his 6.5 m Scorpion in a variety of conditions. However, with my limited experience, I was struggling when at the helm. But, having a crew member on board is very desirable. I could give Peter a rest, control the boat during a "comfort" break, use the VHF, all things that were impossible for a solo RIB skipper at speed.
However, to blow all my arguments "out of the water" Brian tells us that the Alderney Harbourmaster has done a cross-channel in a 4m inflatable in an F8! Just goes to show that you need to know your boat.
Longest answer to the simplest of questions? (don't answer that!
See you on the Weymouth cruise (or perhaps on a cross-channel?).