How do you hang curtain rods inside a sailboat cabin with fiberglass?

No you can't just screw it in because the holes will loosen with time and leak. About the only way you can do it is to get some nicely finished pieces of TEAK the length you need to curtain rods to be and then EPOXY them to the fiberglass. Once they are completely mounted, then you can drill the holes in that wood to mount your curtains. You need to put rods ABOVE and BELOW the port holes otherwise the curtain swill hang down form the sides of the cabin and it just doesn't look right.. you need to stretch the curtains Between the rods so the follow the lines of the side of the cabin.

There are choices to be made. You do not put any hole or fitting into any GRP hull without knowing what the core material is. In any case you never put a hole in any boat it doesn't need. If the hull is a cored structure using balsa, foam etc. you need to drill oversized holes without penetrating the outer skin, and fill these with a thickened epoxy mixture - into which you can set threaded nylon bolts, or you can just drill and tap directly into the cured epoxy plug and use nylon bolts. This is fiddly and time consuming, drilling and tapping directly into the core even though inside and above the waterline is still something I would look at with very long teeth. Better to attach nicely shaped and finished timber mounts to the inner gel coat using a structural adhesive such as Sikaflex - note the white stuff is very good, but it yellows with age, done right, and with good masking and trimming the black gear is quite classy behind a nice bit of Spotted gum or mahogany etc. - just looks like a gasket. Then you drill and attach your curtain wires or rods to the timber sections - easy. You need both top and bottom on boats, houses don't lay over or roll much so they only need the top one....... Another option with portlights is to fit drop in or drop down shutters - these could be opaque rather than solid for light entry and privacy. With more classic designs there are storm covers fitted externally for ocean conditions, and while a curtain or three can look good quite often, it is just more mould food if you have a humid environment or have to close the boat up a lot when storing or mooring. Depends very much on your situation and location but it might pay to look to the alternatives before committing to the curtain idea. Curtains are more commonly used across large glassed areas rather than small ports, as the large areas are much less able to be covered by a simple shutter or board. Good luck, there are some very good books out there regarding the fit-out and internal trimming of boats, sail and otherwise.

http://www.itapestore.com/ i'd lose the rod and just apply velcro tape to the curtain and the hull itself. no drilling, quite possible to remove and clean any residue and have it like it was never there. when you want the curtains, stick them up. when you dont, pull them off.

You cannot use screws in fiberglass. The gelcoat will shatter and cracks will radiate out from the hole. I successfully used a drill and tap method to attach the shades in my sailboat. It took great care and patience but I have no rods or slides to deal with. I drilled the holes and then carefully tapped threads into the fiberglass. I used nylon machine screws to attach the shades. The nylon is softer than the fiberglass and will not cause cracking or crazing of the surface and they will adapt to any irregularities in the tapped hole. The shades the are pull-down pleated type that I made using redi-shades.