I have a heavy twin v8 engined (therefore back heavy) 26' sports boat on a Snipe triple axle trailer with 165 x 13 wheels and tyres. I tow it with a 110 TD5 CSW. Manouvering in tight spots is a pain, but towing is a dream. It just sits there a behaves, even in foul weather, heavy cross winds and pouring rain.
I also think that, although maintenance is more expensive, on long trips to the South of France, six tyres, six wheel bearings, six brakes etc will run cooler, etc than four (again I may be wrong, but it's the feeling I get). Each trip I stop after an hour and check the load etc, which includes checking tyres and bearings...and they're always running cool.
Even sitting on six decent sized 165 x 13 tyres you can see the trailer has some considerable weight on it!!
I wouldn't trust a twin axle trailer with a big boat sitting on top if I were towing for a big disctance (though I haven't tried it so I may be wrong).
