Here's the brief recap:
Go to pick up "new" used boat. small brand name i/o equipped, perfect for fishing. three hour drive, partner had seen the boat and executed sea trials. Boat old but reasonable condition given price. trailer is two years old and 'slightly used by previous owner.
To the marina. Find trailer in field, in fact has been used once (maybe twice) and that was two years ago. Check brakes, tires, lights and get the truck wiring working with the trailer wiring...all good (once we find an air compressor) and the trailer is towed to the marina.
The boat is on the hi & dri rack. The lift operator shows up ~ 15 minutes later, gets directions from the boss, and comes back with the boat. lowers boat to trailer and backs out. Not too bad....walk towards the back and Oh S--T!!! the transom is sitting forward of the last set of rollers by at least a foot with the bow in the stop chock.
Get the yard boss and ask WTFO? He shakes his head until he sees it and then gets redfaced. Go to the cafe and have a late breakfast and we'll have it fixed when you are done. Off to the cafe. An hour later, the transom is balanced on the end rollers. (The yard was recognized as the local volume dealer for the big name trailer the boat was sitting on and yes, they sold the trailer to the previous owner.)
Start on the trip home. The local roads are fine and the truck handles the "new" boat load a bit better than expected. The hydraulic brakes are functioning but not well behaved. We suspect drums are not round due to age and rust. On to the state road. All is fine until about 10 miles from the marina. A long downgrade followed by the rise on the other side...things felt fine until about halfway down the hill. Both of us in the truck's cab knew something was 'different' but wasn't sure what the cause was.
A few miles down the road another hill but a bit steeper. A few hundred yeards down the hill, I looked over my shoulder the the boat looked weird. the Yawing event had started and proceeded to increase as we proceeded down the hill. Looking forward was even uglier as the truck was now gyrating back and forth between both sides of this two lane road. Good thing there were no other cars on the road at that time. Somewhere around the bottom of the hill the strap holding the hull to the trailer broke and the hull climbed up on the fender. As we started up the other side, the speed was reduced and then the yawing subsided.
We came to a stop on the shoulder of the opposing lane of traffic. The chine was dragged along the asphalt at least a 1/4 mile and was much thinner than when the trip started. Fender on the trailer rubbing on the tire but no serious damage. Cars started appearing very frequently and the very helpful nature of that community's residents was obvious (yes, before cell phones were common).
Conclusion - the "idiot" in the marina that "adjusted" the trailer to fit the boat only moved the bow stop back a foot or so. Nobody, including us, checked the tongue weight (less than 100# post accident and the boat alone is over 3000#). No matter how "prominent" (or other adjective) the "dealer" is (or is rumored to be), ALWAYS do a detailed verification on a "new to you" rig before hooking the coupler over the ball....Not only your life but all the ones around you could be at stake.