Are you using the fore and aft Grab Handles as your attachment points for these hold down lines ? It seems to me that these are not very strong attachment points.

Mark

Mark, I do use the grab handles on the plastic kayaks for the hold-down lines but they're mostly for for show to keep cops happy and to alert me if the boat is shifting. I'm not at all sure the handles would hold the boat if it came loose, but there is nothing else to tie a line to on plastic kayaks. The straps that hold those boats go over the hull and the cockpit rim slots into custom slots in a pair of wood cross-bars.

The wood canoe doesn't have grab handles - it's light enough I can carry it under one arm, so it has a painter eye instead.

For long trips I add an extra strap or two as a backup. For the canoe the hold-down line passes through the painter eye and back to the thwart which is fairly massive and I am reasonably sure that would hold in a bad situation.

A boat towed along behnd a car with just the bow and/or stern hold-down isn't going to last long of course: it's just to get the car to the roadside and keep the boat from tangling with traffic.