Re: Would like to know how this happened?
The tow vehicle being undersized does not cause a boat to come off the trailer. However, since it is more prone to fish-tailing, and lacks the weight and wheel base to correct it, it might contribute to some, but not all, load losses.
Boats, like any loads, can often be trailered without proper tie downs, with no mishaps, if there are no sudden stops, turns, etc. This is true with roller trailers b/c the winch is there to hold it on. So again, the lack of tie down itself won't cause the boat to fall off, and even sudden acceleration (of a minivan--big deal!) won't leave it behind. By the same token, if you don't have a safety chain, the boat, for that fact alone, will not come off the trailer.
The proper tow strap won't keep the boat on the trailer in extreme circumstances (as the pitcure shows).
Not every boat coming off a trailer is due to the operator not doing something right. Equipment fails unexpectedly. I had the whole bow eye pull out, leaving the winch cable and safety chain dangling unattached; the boat rolled back about a foot (roller trailer) but the strap (belly band) kept it on. So I resent the statement that anyone who loses a boat off a trailer is a moron.
here's the big BUT: first, you have the correct equipment (tow vehicle, tie down) not for the easy or uneventful trip, but for the inevitable minor problems: sudden stops, swerves, pot holes, inclines, etc. You have back ups like the safety chain because the winch cogs are likely to fail and are unreliable, and redundancy for the same reason. You have safety devices like straps that are strong enough for the typical problem, but no one can cover the extreme events. It's the same concept as motorcycle helmets--great at 25 mph; useless at 70.
So back to the original scene: the boat was clearly too big for the vehicle; he encountered a problem that would ahve been minor for a proper rig but was catastrophic for his; the boat slid off and spun around. That to me suggests fishtailing that broke whatever straps he had, and my guess is he didn't have all four devices employed: safety chain, winch lock, 2 rear straps. It is almost as likely that he didn't have anything but the winch; it popped loose as they are want to do, the boat slid back on each bump until it tilted, causing fishtails, slid off and spun when a chine dug in.
