A qualified person must always be employed to check any critical repairs or modifications. What we say on Cruisers Forum is opinion, and must always be checked.
That said it sounds like you want to drill some small holes in the stern of your boat to mount a Hydrovane self steering
, and to cut some larger holes for navigational instruments.
Before doing anything that involves cutting into ferro you need to locate any framing members. These could be steel
rod, steel bar, steel reinforcing rod or even water
pipe. If you can get your hands on the original plans it will help, as will looking for clues in the surface, condensation
patterns after a cold night. Maybe even a sensitive stud finder? Anything that will guide you away from cutting into the framing or reinforcing members...
For the larger holes if you can avoid them it would be best. You could, for instance build a housing of wood to hold the instruments and bolt that to the cement using small bolts (say 5mm) and mastic. Other holes could be drilled for the cables
Ferro is not fibreglass and may not take kindly to large holes. If you really must drill a large hole a diamond saw is possibly the best option. Well made ferro is remarkably tough and while the cement part drills and saws sort of ok the reinforcing wire may not cut and may jam up the drill making for a real mess. It may be possible to drill a circle of small holes, and then somehow cut the reinforcing wire, but it's likely to be messy.
Once you have made a hole I'd suggest priming the bare cement with some warm thin straight epoxy resin, then if there is a bit of a mess, fairing with some epoxy filler.
Ferro also does not take kindly to point loads, so for your Hydrovane, which (if memory serves me right) will actually be steering
the boat you will need to seriously consider putting some substantial fairing pads (possibly made from epoxy coated wood and shaped to suit) on the inside to spread the load over as large an area as is practicable, possibly even to other structural members. These need to be well bedded down with good mastic. This is an area where expert opinion is essential.