: The magnet size and placement I now use holds the hatch very securely.

: You must lever your fingers against the deck under one side of the hatch
: cover
: to start to dislodge the force of a couple of the magnets and pull firmly to
: release
: the cover. Many people don't have the finger strenght.

: The internal lip makes the diameter of the hole smaller so any gear pressing
: against
: the hatch interior will distribute that force over more magnets.

: Any shock associated with running through waves will apply downward forces as
: the
: hull hits the water surface after cresting a wave. This will not apply force
: to dislodge the magnets

: I'm confident that the hatches will stay in place.

I am pretty confident you are right. My scenario is definitely an unlikely occurrence, but it also a situation where a failure could be catastrophic. One of my concerns is many magnets loose their magnetism when exposed to mechanical shocks. But I found http://www.swri.edu/3pubs/IRD2000/15-9203.htm which subjected rare earth magnets to 3,000 g accelerations and they did fine. But there are impacts that kayak may experience which will bend the hull and deck slightly. If the deck flexes while the hatch cover does not, while at the same time there is an acceleration or a hydraulic force trying to lift the cover, the closing force will decrease and could continue to decrease.

While it is true that webbing can fail, it has a breaking force of several 100 lbs, and if two straps are used both would have to fail before the hatch fails. With magnets the separation force is only a few pounds and as the hatch starts to lift this force decreases rapidly.

While camping in my kayak I tend to pack a lot of stuff in my hatchs, I often need to push down on the hatch to close it over my gear, so there is already some lifting force created by the gear. I always seem to be putting my Teva sandals in last when there is no room left and having to push them down with the hatch cover. The gear I pack near the hatches also tends to be the heaviest stuff in an effort to keep mass out of the ends. As a result the there are objects in the area of the hatch which are already close to lifting it, and with a sharp impact, they may well shift and be enough to lift the cover free.

Your idea of something inside the hatch to keep gear from pushing against it is a good one. From a practical, ease-of-use stand point I think a couple straps over the top would keep the hatches in place, while the magnets serve to compress the gasket and provide a good seal. It would not be as clean and nice as the pure-magnet solution, but it would add a lot of security. I don't like plastic buckles but they work pretty well.

I think your magnet idea is one of the best hatch-holder-downers I've seen. I am just trying to imagine what could go wrong. I am not saying it is likely, but that it is worth considering what the failure modes may be to see what could be done to improve an already good idea.