Put a piece of masking tape over the area to be drilled to keep the gelcoat from cracking around the drilled hole as you drill. Mount the tube per the manufacturer's instructions if you have them. Six inches to either side of the keel centerline but not in the area directly in front of a drive as a start. Put some epoxy down in the hole and let it partially cure to seal the transom coring(probably plywood). Use 3M 4200 urethane adhesive on the outer threads of the mounting screws(last 3/8" under the head)and on the underside of the plate that you are mounting. Some will say that this is overkill, but you will get no water intrusion into the plywood core if you do it right. NEVER use silicone on a boat, unless it is on an interior surface that has no connection to any underwater fitting and never below the waterline, but better still NEVER use silicone sealant on a boat. There are better sealants such as the urethanes.