Originally Posted by Gwozhog
Fantastic question and I would love to answer it. I pretty sure nobody is doing what the general lee is doing when they trailer. Now I'm not sure how you ride around on the water but when I'm WOT and I pass another boat that throws a pretty big roller my SIB occasionally has looked like the general LEE in the pic below. Not sure if you have ever pulled a skier behind your SIB either but it has quite a bit of force on the transom too. So if your boat is properly inflated on the trailer and you are keeping all six of your wheels on the ground and not breaking any laws on the roadways you should not have a problem trailering with an outboard on the transom. Hope that answers your question.
You've not seen British roads! Its certainly possible that the wheels on a sib trailer bounce over potholes, kerbs or speed bumps even with sensible driving.
When it is on the water the boats is supported over a relatively large area, and when landing on water it has a relatively cushioned landing. When on a trailer, many sibs have poor support at the actual transom.
Some SIBs will actually spend longer on the trailer travelling to the water than being used afloat; many of those miles will be at 2-3x the maximum speed on the water and so the frequency of the vibration from towing will be quite different.
The engine itself is partially supported by the water (both through bouyancy and by lift) most of the time, only momentarily is the entire weight of the engine ever acting vertically on the transom.
That said, I'd tow a sib with an engine on the transom, with some extra support
that (a) helped to stop it bouncing around and (b) transferred the load direct to the trailer rather than via the tubes.