As far i remember from my BW 23 Outrage the fishbox hatches had two short bolts screiwng in a treaded copper insert in the wood panel and it rotted away.
I wanted to replace them with SS bolts going thru the hatch., but the "gutter" was in the way and the hatch would not close, so i had to invert the bolts, countersunk V shape bolt head on the inside and the nut on the hinge. Worked, but the nut was higher and hurt when standing on it.
Check that before you do something with thru-hatch bolts.
Also, seal well between the hatch and the hinge and around the bolts , otherwise humidity will creep in the wood? core of the hatch.
Chris