OK, so is it the type of hull with a flat underside, or one with a shaped support, like the back end of a trawler?
If its a flattie, cut a slot and set the tube in place with filler and any other support you deem necessary. If its a trawler type stern, you will need to drill out using a long bit of the appropriate diameter using plenty of support, a steady aim, and if you do it lik me, a bit of tongue sticking out the corner of the mouth.