Installing A Seacock

Safety and reliability depend upon the right gear.

Seacocks are required to shut off the flow of seawater supplied via a through-hull fitting to onboard equipment, including engines, gensets, livewells, air-conditioners and the head. A seacock is not simply a ball valve. Its threads mate with those of the through-hull fitting, which has NPS (National Pipe Straight) threads. In-line ball valves have NPT (National Pipe Taper) threads. If you screw a valve onto a through-hull, you won’t get a good seal and won’t be able to screw the valve down flush with the hull, and since valves aren’t fitted with grease ports, bonding wire tabs or drain valves, winterization, service and maintenance are problematic. A “proper” seacock is fitted with a wide flange, is capable of being fastened to the boat so that the assembly can’t spin, and features a hole in the handle into which you can insert a socket wrench to extend your reach and operate seacocks deep in the bilge. Seacocks may be made of bronze or Marelon. Here are the steps for a solid fiberglass hull.

Installing A Seacock
Installing A SeacockBoating Magazine
Installing A Seacock
Installing A SeacockBoating Magazine

1. Mark, Epoxy and Drill
Drill a pilot hole through the backing plate and hull, dead center in the diameter of the circle representing the seacock's barrel, which you traced inside the hull.

-Drill square to the hull's deadrise angle, not vertically. Swap the bit for a hole saw matching the through-hull's outside diameter and cut.

-Glue your backing block down with epoxy resin.

Installing A Seacock
Installing A SeacockBoating Magazine

2. Chamfer Edge
I bevel the sides of the hole with a router or chamfer bit. This ensures the sealant doesn't just squeeze out when all is tightened down.

-Dry-fit the through-hull and seacock. When the 'cock bottoms out, mark the threads on the through-hull and cut the stem to length. You want the seacock flange to screw down tight to the backing plate, not be suspended above it.

-Be wary of obstructions that can limit the lever's throw.

Installing A Seacock
Installing A SeacockBoating Magazine

3. Screw Down Seacock
Bed the through-hull's "mushroom" flange with sealant and also bed the seacock's flange with enough goo that you can see "squeezeout" all around after all is reassembled for good.

-Use the step wrench, which engages the "ears" inside the through-hull, to prevent the through-hull from turning. You'll need a partner to help.

-Tighten the seacock, making sure it ends up tight to the backing plate. A strap wrench is helpful for this task.

-Pre-drill the holes for the flange screws, using the flange as a template. Use a drill stop or piece of tape on the bit to avoid drilling all the way through the bottom of the boat. Just drill into the backing plate.

-Apply sealant to the screw holes and insert the screws.

-Thread the appropriate hose fitting for the equipment being served onto the top of the seacock.

Build a Backing Plate
You can use plywood as a backer, but I prefer to make one using fiberglass cloth, mat and resin. In either case, use a large hole saw to cut the finished plate out round. This more evenly distributes stress, is less of a toe-stubber and is easier to clean around.

1. Scrounge a piece of plate glass.

2. Apply alternating layers of fiberglass cloth and mat and wet out in resin until you have achieved at least a half-inch thickness. If you apply these layers "wet-on-wet," they will all cure together, overnight for epoxy resin or within a few hours if you choose polyester resin.

3. When the sheet has cured, pop it free of the glass — it won't be stuck fast — and clamp it onto a scrap of wood. Use a hole saw to cut out a disc larger than the footprint of the seacock's flange.

4. For good measure, bevel the top edges of the plate with a file or by rubbing the disc on 40-grit sandpaper glued to a workbench.

For Cored Hulls

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; 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 (, 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.