Installing Thru-Hull Fittings, Part 2

In the second part of our story on adding thru-hull fittings to a boat, we’ll cover the installation process and finishing touches. This includes some tips learned the hard way as well as the special tools needed to get the job done right the first time. 

Test Run

As we outlined in Part 1 of this story, we are using the example of installing an air conditioner with one intake thru-hull fitting below the waterline and a discharge outlet above the surface.

Always dry-fit the thru-hull/seacock prior to final installation. Remove the backing nut and insert the discharge thru-hull from the outside. On the inside, fit the backing plate over it, then install and snug up the flange nut.

The intake thru-hull will screw directly into the seacock. Remove the thru-hull nut (it will not be needed), insert the thru-hull into the hull, fit the backing plate, thread the seacock onto the thru-hull and tighten until snug.

For bronze thru-hulls, a special tool called a step wrench is used grip the ears inside the thru-hull and hold it securely during tightening. A wedge of plywood inserted in the thru-hull from the outside also works well in a pinch.

Proper Dimensions

Once inserted into the seacock and moderately tightened, there should be a sufficient length of thru-hull inside the seacock, but not enough that the thru-hull bottoms out. If the thru-hull is too long, cut it to length. If the thru-hull is too short, use thinner plywood (½” (1.27 cm) minimum) to construct the backing plate, or get a longer thru-hull.

Most flanged seacocks have three mounting holes to prevent the seacock from turning once installed. There are two acceptable methods of securing the seacock using these. 

The first is to drill holes through the backing plate and hull (using the flange as a guide) then install bolts through the hull and secure everything together. The downside to this option is, of course, three additional holes in your hull.

The second method is to screw the seacock flange to the backing plate only. This is the option we’ll go within our example. For this method, ensure that the length of the screws does not exceed the thickness of the backing plate.

Matching Materials

Regardless of the method chosen, ensure the mounting bolts or screws are of the same material as the seacock (bronze in our example). Either stainless steel or bronze is acceptable when mounting nylon seacocks.

Once the dry fit is complete, disassemble everything and seal both wood backing plates with 3 to 4 coats of epoxy and allow to dry.

Installation 

You’ll need a second set of hands to hold the thru-hulls in place during installation and while tightening. For both thru-hulls (intake and discharge), apply a good amount of marine grade sealant (polyurethane or polysulfide) around the inside of the thru-hull’s mushroom flange, then insert it into the hull.

On the inside, apply sealant to the hull (around the inserted thru-hull) and to the hull side of the backing plate. Slide the backing plate over the thru-hull, orient as required and press down, bedding the backing plate to the hull. For the discharge thru-hull, the next step is to add a thick bead of sealant to the top of the backing plate hole around the thru-hull, install the thru-hull nut and tighten snugly. You should see a ring of sealant ooze from beneath the thru-hull nut.  

For the intake thru-hull, apply sealant to the seacock’s flange (again, enough to see “squeeze out” all around once tightened) then thread the seacock onto the thru-hull and tighten. 

Once the seacock is properly positioned and tightened, use the flange as a template to drill the mounting holes into the backing plate. Be sure to drill into the backing plate only, not into the hull (use a drill stop or piece of tape attached to the bit at the proper depth to prevent this). Clean the mounting holes of debris, add sealant to the holes and threads of the mounting screws, then install.

Once the installation is finished, fit the seacock with an appropriately sized tailpiece / hose barb (using thread sealant) and attach the cooling system intake hose to it. Attach the AC discharge hose to the hose barb of the overboard thru-hull and it’s Pa'u Hana.

When is a Seacock Not a Seacock? 

Many builders utilize inline valves attached directly to the thru-hull, however there are a number of problems associated with this approach. To start with, in-line valves use NPT (National Pipe Taper) threads, which are not compatible with the NPS (National Pipe Straight) used in most thru-hull fittings (unless the thru-hull fitting is machined with “Combination Thread”). Installing an in-line valve onto a NPS thru-hull fitting creates a mismatch of threads, resulting in a weaker, less waterproof joint (due to less thread engagement).

As it has no mounting flange, an in-line valve can turn or loosen from the thru-hull fitting due to vibration or through normal use. 

Finally, unlike proper seacocks, in-line valves are not fitted with grease nipples, bonding wire tabs or drains, each of which can create potential maintenance and winterization issues. 

By Capt. Frank Lanier

Captain Frank Lanier is a SAMS® Accredited Marine Surveyor with over 40 years of experience in the marine and diving industry. He’s also an author, public speaker, and multiple award-winning journalist with articles on seamanship, marine electronics, vessel maintenance and consumer reports.