I think I'm lacking some info. I see guys with 20 ft inboards using single axel trailers and then see 21 ft outboards using tandem. The outboards should be lighter in weight than the inboards so whats the skinnie? Are you required to have tandem if you exceed a certain lenght?

The cutoff must be more than 21',my dad's Bayliner is 21' and rides on a SA trailer.My uncles' Bayliner Trophy is 24' and rides on a DA trailer. They are both I/O 5.0ltr Chev engines. My neighbor has a 21' Campion it also rides on a SA trailer,again it has a I/O 5.0ltr Chev engine.

I think it is weight not length. It is about 4,000# they go to a double