Number 10 machine screws is what the great majority of boat builders use.
Quarter inch hardware is a bit more robust. Ed's actually drills their seat hangers for 1/4" hardware, but this is more to allow some wiggle room to avoid binding the bolt when placing them through the gunwales, hangers and seat frames.
It is a bit hard to get everything to line up exactly. You have a hole in the top and the bottom of the gunwale, then a 4 inch long channel in the seat hanger, then another channel extending through the seat frame and the straight machine screw has to go through all of them.
You might find that you have to enlarge or even redrill the holes in your seat frames to line up with the holes in the hangers. You might also have to enlarge the channels in the hangers a bit by running a drill bit through them and wiggling it around a bit to allow the machine screws to go through at exactly the right angle.
4-place Boat Trailer
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