[bigeasy1]

Thru bolting is definitely the way to go,when mounting riggers.
Another option is 1/4-20 well nuts.I run eight riggers on two boats,and while six are thru bolted,two couldn't be,because of the gunnel width on my Trophy.
Those two riggers,are mounted to the port and starboard gunnels with Well Nuts,a rubber sleeve wuth a threaded brass insert at the bottom.
When you tighten up the screw or bolt,it expands the rubber sleeve in the hole,similar to a molly bolt.
I've had these on those for several years,and they have held up extremely well.
I have five foot booms and run 13lb weights off them,and they see lots of use under constant salmon trolling for eight to twelve hours at a time.

On all the other riggers,i was either able to get to the underneath,or like Michael, I installed 6" deck plates to the back,so i could access the nuts,plus now i can easily get to some other stuff like wiring and trim tab hose fittings.

I'm going to be running four riggers on my 23 Tsunami,and fortunately i can get to the back as it's an I/O.