hey,,I'll beat it till it's not moving,, you're exactly right on about the advantage of a coaming recess removing the limitation that can come with stacking plywood off the deck, that way designing thigh brace height, adequate foot room and aft coaming height can all be done without compromising hull shape. I don't think having 3" of total coaming width is much of a problem except for big people in skinny boats,,my Express has a coaming width of 15.5". For me that's comfortable minimum, so adding 3" for the coaming and 2" for no good reason other than the coaming will be narrower than the hull gives a 20.5" wide hull,,,I'll be honest,,that's too skinny for me. For the standard 17" wide (big butt) coaming that leaves a 22" wide hull as an acceptable minimum, no problem there. But if a person is stuck with building the coaming on the deck then the options start narrowing down as to the extremes in design. What happened to the Pax18 is somewhat related, starting with a 18'x21.5" gross dimension for the 10%beam/length category for citizen races the 11" radius deck beam that was used for the Pax20 was deemed the only deck beam possible. With an 11" radius deck beam and a 21.5" beam the foredeck height would have been huge were the hull shape for four panels been anything like a conventional hard chine hull,,so the bottom panels were designed for a VERY shallow hull to bring the deck height to a more conventional height of 11.5". But with a very shallow hull, very little rocker the hull displacement was relegated to 175lbs max paddler,,,,except there's no 175lb/10%B/L race category and no ability to handle waves. With the result that the very low coaming limits hip bracing and the high curved deck messes with making sturdy thigh bracing unless there are wood ones backing the minicell. It's ok that it's a flat water race boat,,if that's what someone wants. On the Coho the thigh bracing required wood bracing curved down into the cockpit for better purchase as the high deck without a keyhole spread my legs WAY too far apart for comfort. Even with the Merganser 16 which is a tad lower displacement than my belly needs the foot room was good, the thigh braces had a good angle and the coaming was more protected from the tumble bashing a boat can get because the forward edge of the coaming was almost to the height of the deck.
Speaking of bashing, get this, I wrapped a 1" bias cut of 4oz around the coaming, and two layers of 4oz s-glass on that,,after so many summers of dragging boats across my lap I'm done with bashing the wood,,so one day the wind blows a 7' aluminum frame ladder onto the coaming,,,nada dents.