Pick Your Location
This photo is showing the spot for the new seacock & thru-hull as chosen from inside the vessel.
1- Once you have chosen your spot drill through the hull from inside using a 1/4" drill bit. The reason for the 1/4" bit is that it matches the size bit used on most hole saws.
2- You then drill into the hull from outside using the proper sized hole saw to match the thru-hull. Start the hole saw in reverse as I have done here and you get no gelcoat chipping. Once through the gelcoat switch to forward and drill up through the hull making sure you are perpendicular to it.
Tape, Knife & Thru-Hull
The next step is to tape off the hull and then use an X-Acto knife or similar to cut out the hole in the tape.
Tape The Hull & Cut The Hole
Here is the hull taped off and the hole cut out cleanly with the X-Acto knife.
Sand Away Bottom Paint
1- With the hole cut in the tape press the thru-hull tightly into the hull and lightly trace it with the X-Acto knife then peel away the tape.
2- Once the ring of tape has been pulled off sand away the bottom paint that would be under the thru-hull. Sand until you get to gelcoat or a barrier coat as shown here. This will create the best watertight seal between the hull & thru-hull fitting.
Install Your Backing Block
Install your backing block following the instructions in my other article. Here it is shown epoxied into the hull.
Goop The Thru-Hull
With the seacock installed and bedded lather up the thru-hull fitting. The beveled mushroom head of the fitting is what creates the water tight seal and is the most important place to put sealant. Putting some on the threads is only a secondary measure and adds some thread lubrication when tightening it down.
Thread It In
The thru-hull is then threaded into the seacock. It's important to dry fit test everything first to make sure you've cut the thru-hull to the proper length and that everything fits perfectly. Do this before applying the marine sealant.
I generally use Sikaflex 291 for below water applications.
Seat It All The Way To Tight
In this photo the thru-hull has been fully tightened using a thru-hull step wrench and a large adjustable wrench. I torque it down fairly well into the seacock.
Once tight simply clean up the excess sealant and remove the tape..