If you enjoyed our treatise on installing seacocks but have a cored hull, you will need to take a few additional steps.
1. After cutting the hole for the seacock, you will need to seal the now-exposed core material between the hull’s inner and outer skins.
2. Excavate the core material between the skins out a distance beyond that of the seacock flange’s footprint.
3. Fill the cavity created with epoxy resin mixed with Cab-O-Sil (silica) filler to create a “compression ring.” BoatLife makes empty caulking cartridges ($2.88 each; jamestowndistributors.com). Get them at better hardware stores and marine suppliers and fill them with your thickened goo-glue to shoot it into the crevice with a caulking gun. This ensures there’s no weakness where you’ve breached the core and also ensures water doesn’t enter the core material.
4. You can use a router, or a specialty tool like a Dremel or Fein MultiMaster (themultimastersystem.com/index.jsp), to ream out the core material and create an epoxy compression ring. Or, make a reamer by chucking the long leg of a hex wrench into a drill, using the short leg to excavate the core. A manual reamer can be made by heating an old file and bending its tang in a vice to form a right angle.