I fitted the galvanised rectangular type on a grp Wanderer and was concerned about the stresses around the fitting. Therefore I recessed them into some 8mm (ish) hardwood first to make a bigger fitting to spread the load (which is what you maybe concerned about). I actually surface mounted these plates over the existing (plastic and inadequate looking) rowlock hole, but removed them prior to selling the boat (not comfy to sit on and an easy gel coat repair). However if you are seriously going to row you do want strong enough fittings flush in the gunwhales. The best design I had seen was the galvanised type let into wood which is then resined/fibfegassed into the gunwhale flush. This is a long way of saying I bottled hacking into the gunwhales myself! Good luck with your choice. Dave