How thick it it..........really?
Dan, Grab a small drill bit and make a test hole through the starboard bulkhead to the outboard end of it. Far enough outboard that the settee cushions will cover it. That should have you ending up with a hole into the head beneath the locker and behind the locker door, well out of sight. Then you can measure it accurately. Now out boat is slightly larger but that bulkhead is comprised of two 1/2" teak veneered plywood panels glued together. Maybe??? yours is that thick too. But the only sure way is to do as suggested above , to grill a small hole. Best of luck and let us know, Former owner of E25+ hull #515 and for the past 23 years, E31 hull $55, Glyn Judson, Marina del Rey CA