Seacock Removal and Replacement

 My boat was built several years back so it was not a surprise to me that it had several types of seacock valves installed on the thru-hulls. What did surprise me however was finding myself alone in the boat with a seacock valve in one hand and trying to stop old faithful from sinking the boat with my other hand. The valve had broken off at the thru-hull fitting as I was closing it. Luckily there was a toolbox nearby. I retrieved a screwdriver from the toolbox with my foot and with my free hand a towel that was located on top of the galley sink. I could barely reach it. Suddenly I began to laugh because it reminded me of the Twister game that kids play. I wrapped the towel around the screwdriver handle which I forced into the thru-hull, handle first. This seemed to slow the waterflow down to a trickle and it gave me time to wrap the broken thru-hull fitting to the screwdriver shank with duct tape. With a feeling of great insecurity I hurried to a local marine store to see what could be done. I was handed a set of tapered softwood plugs and was told, “that is all you can do until you haulout.” I rushed back to the marina and to my relief the boat was still floating-it hadn’t sunk! I decided to put the plug in the thru-hull under the boat instead of on top. I didn’t have confidence that the plug would stay in the top position for any length-of-time. With the wood plug and hammer in hand and in my underwear I made a dive over the side and drove the plug into the thru-hull. Cold and embarrassed, I thought to my self “talk about not being prepared” and maybe a few other unfriendly words.
The seacock that failed on my boat was a gate type valve which is considered to be a garden variety valve and was not marine grade. The problem with gate valves is that you cannot determine by looking at one weather it is open or closed. Any piece of debris caught in the gate gives the impression that the valve is closed, because the handle won't turn any further, in fact, the gate may still be half open. I found out later that this type valve was used to save the builder dollars. If you have gate valves, especially below the water line, consider replacing them immediately.
There are several types of marine grade valves. The three most common seacock designs are tapered-plug type, expanded-rubber-plug type, and ball-valve. The ball-valve is the latest design and practically maintenance free. A dab of waterproof grease once or twice a year and occasional turning of the handle, will prevent it from seizing. I made it a habit to close all the valves when I leave my boat. This solves two problems, 1) it eliminates seizing and 2) the boat will not sink if a hose fails. The other type valves require annual disassembly maintenance. Even if you have ball-valve seacocks it is a good idea once a year while the boat is out of the water to remove the tail piece and inspect it for foreign material and with a dab of grease on the end of a wood dowel grease the ball. Turn the handle several times to make sure the grease covers the ball. Repeat the same greasing procedure through the thru-hull from out of the boat.
Every thru-hull should be mounted through a backing block that is shaped to fit flat against the inside of the hull which has a surface area greater than that of the seacock flange or the thru-hull flange nut. For improved strength, the flange of the seacock should be thru-bolted to the hull or blindly fastened with screws to the backing block to prevent it from turning. All seacock tailpieces should have barbed ends to prevent the hose from slipping off. If the valve is below the waterline the hose should be double-clamped to the tailpiece.
To remove a seacock you will need to borrow a step wrench from the boat yard (Figure 1). This tool is tapered to fit into the thru-hull from the outside of the boat. It fits snugly against the two knobs protruding inside the thru-hull. With a large wrench unscrew the thru-hull fitting out of the boat. If the flange is not thru-bolted or screwed onto the backing block a second hand might be needed on the inside to hold the valve from turning.
If for some reason you cannot locate a step wrench try a metal wedge that will fit into the thruhull so that it will lock onto the knobs. If the knobs are gone due to corrosion you will have to cut the thru-hull out with a reciprocating saw. Remove the valve before cutting, place the blade inside the thru-hull fitting from the outside; and cut to the edge. Make several cuts to ensure that the thru-hull will collapse inward try not to cut into the fiberglass. Another method is to drill the thru-hull out because the hole will require a large bit and you will most likely have to borrow a bit and drill from the boat yard. With all this borrowing going on it would be a gracious gesture to take whom ever you are borrowing from out to lunch. It’s a minor concession if you need to borrow again.
Once the thru-hull is out, remove the seacock or flange nut from the backing block. If the flange is thru-bolted to the hull, scrape off the bottom paint in the area around the thru-hull and find the plugged holes. Chip the filler out of the holes to expose the bolt heads and remove them. If the flange is not thru-bolted and will not come off the backing plate or the hull, try applying heat to the flange with a heat gun and slide a putty knife under the flange to break the bond.
 Before installing the new seacock and thru-hull, inspect the backing block for rot. If you need to replace it, cut the backing block out of a marine grade plywood with a surface area larger than the flange. Bore the hole for the thru-hull in the middle of the block. The marine plywood is fine as long as the end grain is sealed with epoxy resin to keep moisture out.
There are two types of thru-hull fittings; 1) mushroom the most traditional and requires no special tools for installation and 2) flush mounted thru-hull primarily used for racing. A special counter sink tool is used to bevel the hole for the thru-hull to fit flush (Figure 2).
Some thru-hull fittings are sold with a flange nut that is separate from the seacock (Figure 3). If you decide to use the flange nut to install the thru-hull the installation is quite simple. Apply a generous amount of underwater bedding compound to the inside of the thru-hull flange between the backing block and the hull, and between the flange nut and the backing block. Because the thru-hull is not thru-bolted, some owners have used 3M’s 5200 as a bedding compound for extra strength. The only problem using 5200 as a thru-hull bedding compound is  it increases the difficulty for future removal. Use plenty of bedding compound, clean-up is easy with a razor after the bedding has set. To install the thru-hull, have someone outside holding the thru-hull from turning with a step wrench,  and another person in the boat tightening the flange nut with a wrench. Once the thru-hull is tight, and with the step wrench still held in position, install the valve. Use some plumbers tape on the thru-hull threads this will assure that there will be no leaks.
A more stable and reliable installation method is to use a flange that is part of the seacock valve and is thru-bolted through the hull (Figure 4). In most cases, the thru-hull will be too long and will require cutting. Measure the thickness of the hull and backing block. Mark this measurement on the thru-hull starting from the inside face of the thru-hull flange. Then thread the thru-hull into the seacock as far as it will go and put a mark on the thru-hull. Unthread the thru-hull and count the threads it took to bottom out. Add this number of threads, less two to the initial mark on the thru-hull. By subtracting two threads, you will be sure that the assembly will tighten up before the thru-hull bottoms out inside the seacock. After cutting off the excess from the thru-hull, clean the threads and make sure the threads will start into the seacock. On a work surface, set the seacock on top of the backing block with the thru-hull in place to keep everything centered. Next mark and drill the holes for the thru-bolts. Back at the boat hold the backing block against the hull, centered on the hole for the thru-hull, than mark, drill and countersink the holes for your thru-bolts. Before applying the underwater bedding compound make sure that everything assembles properly. With this type of installation do not use 5200. After applying the underwater bedding compound insert the thru-bolts through the hull, backing block and the seacock flange. Start the thru-bolt nuts, but do not tighten until the thru-hull has been partly threaded into the seacock. This will make sure that everything will align. After the thru-bolts have been tightened, turn the thru-hull with a step wrench until tight.
I learned a few things from this adventure. Seacocks are the sort of mechanical devices that are reliable and simple enough to invite neglect. It is a good idea to routinely inspect all seacocks and thru-hulls annually. Make sure they are marine grade. Check to see if each seacock is mounted on a backing block and fitted with a proper tailpiece. Evaluate how easy they operate and make sure that no leakage occurs anywhere in the fittings. Keep them in good condition all year, know their locations and have a softwood plug available in case of failure.

Tools, Hardware and Supplies

Thru-hull fitting
Thru-hull bolts
Plumbers tape
Basic hand tools
Putty knife
Step wrench
Caulking gun
5200 compound
Underwater bedding compound
Wood dowel
Waterproof grease
Calking gun
Softwood tapered plugs