Eric
In my early days of using Sailwave I started using one file for all fleets.
In theory this is OK but if you start having different numbers of races for
fleets (say incomplete programme for the day for one or more fleets) you may
have some difficulty.
Sailwave is great either way but I later found that the KISS (keep it simple
stupid) principle is the best advice and subsequently have always set up one
file for each fleet and only used fleets within one file when I need to
produce combined as well as fleet by fleet results.
For the Youth Worlds, no-one will thank you if there is a delay or error in
the results system because you did not anticipate a particular problem
Best of luck & could you please set the program to ensure that the Brits win
at least the 29ers (as if we needed any outside help) 
Regards
Ralph Tingle
Ogston SC & 29er UK Class Association