I've caused the destruction of our canoe.
The thick piece of fiberglass across the top of the bow gave way, allowing a bolt (and the nut fastening the bolt to the fiberglass) to pull free.
While the canoe was tied to the top of the car.
While the car was on the 115 heading south of Peterborough.
The crucial mistake in all of this was not using the nylon rope that I usually thread through the tops of the car doors and loop around the center strut across the canoe. That safety measure, which we've always used in the past, would have lessened the pressure on the bolt at the front of the canoe, and would have prevented the canoe from flying off the car.
In retrospect, the anti-migraine drugs I've been on for the past three months have been making me more than usually stupid. I similarly made a conspicuous error in judgment at my new job. I've now been off them since the incident and feel my brain returning to "normal". Too late for the canoe, of course.
The damage was pretty extensive. The rear seat, which we'd been using to hold down the rear of the boat, was torn to pieces. It was designed with wooden slats. I had to pick those up from the highway. There was a substantial puncture/impact hole where the thing slammed into the road. The bottom was in surprisingly good shape, to be honest, I think the incident was over so quickly that there wasn't much time for abrasion. But the worst damage was to the opposite side. The relatively thick fiberglass of the bottom seems to have been thicker than the stuff making up the side of the boat, and the impact stress was clearly released at that point because the side of the canoe was creased all along that side. I'm not saying that it wasn't repairable, but mum (who was driving behind us) agreed that it wasn't repairable by us (especially in this apartment).
So I phoned the cops. They've picked it up and will dispose of it. No fines or fees or other problems. The spot it happened was ideal: with a little-used on-ramp lane to my right, my mother behind me, and us in the right lane. Not a great way to end the trip but if we decide to replace the canoe we can get something better-suited than that flat-backed cargo canoe.