chainplates: round holes vs square holes

[billknny]

Some of the chainplates on my boat are mounted with what are essentailly flat head machine screws in a countersink. What makes them unusual is that the head has an undercut on the taper, making for a very large head, looks good, has held up without a prolem for 30 years.

You can have great fun flipping through a McMaster-Carr catalog for just the right bolt to meet your needs...

[Sean MacLeod]

My only problem with flat-head fasteners, especially those with a wider head than normal, is that any torque applied to the head of the bolt (as tension on the chainplate would seem to do) gets more leverage than on a normal pan or hex head bolt causing the likelihood of breakage to go up. One of the nice thing about hex or pan head bolts is the head is not in line with the load, all it is doing is not allowing the end of the bolt to go through the plate.
The other thing about flat head bolts is that, because of the inclined plane in line with the load, the force exerted by the plate actually pushes out on the head of the bolt. It seems to me that with a flatter inclined plane the mechanical advantage would be even more.
Granted they do look VERY nice all polished up and aligned, and in the instance of a bow plate, where you have sail all around and bolts in at a number of angles, I'm sure the would work quite well.