I dunno, but just a SWAG, as I'm not an NA or certified anything....
Construction regs state that a load bearing member
*may* be penetrated/cut/drilled to 1/3 of the beam/joist's web dimension before reducing the effective load carrying capabilities.
This is assuming ( the engineering "assume") a round hole; or one that the corners have been radius'd enuff to prevent stress cracking @corners.
I'd venture a guess that IF yer new port size is at least one diameter from the nearest "edge", you'd be OK, as long as there's no point load at the apex. Additionally; I'd consider the port to be non-structural and of no consequence in retaining "hull" integrity. IOW; IF there was no fixture in the hull
, would a hole that size/place cause a lessening of integrity, not counting ingress of water
Think of it as a round-top window in a bearing wall and don't forget to calculate/consider wind
(water pressure) loading.
I need to "grow" my cabin
by an inch or so to get new opening ports
in. No worries,as there's still at least one width remaining top and bottom.