boat repair · how-to · hull fitting · marine plumbing · through hull · thru-hull fitting ·Sep 25, 2026

How to Install or Replace a Thru-Hull Fitting

What Is a Thru-Hull Fitting?

A thru-hull fitting (also written "through-hull") is a plumbing connector that passes through the boat's hull to allow water to enter or exit — for bilge pumps, livewell intakes, cockpit drains, raw water cooling, and more. Because these fittings penetrate the hull below or at the waterline, they must be correctly sized, properly sealed, and made of materials that resist corrosion in the marine environment. A failed thru-hull can sink a boat, so this repair deserves careful attention.

Safety First: Below-Waterline Work

Before starting any thru-hull replacement below the waterline, the boat must be hauled out of the water or the fitting must be plugged from the outside while you work. Never attempt to replace a below-waterline thru-hull while the boat is in the water without proper precautions. Keep a tapered wooden bung nearby as an emergency plug in case water ingress occurs unexpectedly.

Identifying the Right Replacement Fitting

Thru-hull fittings are sized by their inner diameter (the hose size they accept). Common sizes include 1/2", 5/8", 3/4", 1", 1-1/8", and 1-1/2". Measure your existing hose ID carefully before ordering. Also note:

Flange style: Standard flanges provide a broad seating surface; narrow-flange versions suit tight spaces or shallow installations.
Material: Marine-grade plastic (glass-filled nylon) is corrosion-proof and widely used for above- and below-waterline use. Bronze and stainless are also options for specific applications — always verify compatibility with surrounding hardware to avoid galvanic corrosion.
Thread type: Thru-hull fittings typically use NPT or BSP threads; match your existing seacock or valve if one is installed in-line.

Tools and Materials You'll Need

Replacement thru-hull fitting (matched to hose ID and flange style)
Marine sealant — 3M 5200 or Sikaflex 291 (permanent) or 3M 4200 (removable for future replacement)
Large slip-joint pliers or thru-hull removal tool
Screwdriver or Allen key for interior nut
Acetone or isopropyl alcohol for surface prep
Sandpaper (80 and 220 grit)
Wooden bung as emergency backup

Step 1: Remove the Old Fitting

Disconnect the hose from the barb and remove any seacock or valve attached to the fitting.
From inside the boat, use pliers or a strap wrench to hold the interior flange nut and prevent the fitting from spinning.
From outside the hull (with the boat hauled), turn the fitting counterclockwise to unscrew it from the nut.
If it's bedded in 5200, expect resistance — work carefully to avoid damaging the hull laminate. A heat gun applied to the outer flange area can soften old sealant.
Once removed, clean the opening thoroughly: remove all old sealant, sand any rough edges, and wipe with acetone.

Step 2: Prepare the Opening

Inspect the hull laminate around the opening for any signs of water intrusion or delamination. Sound the area by tapping — a dull thud instead of a sharp tap can indicate moisture in the core. If you find delamination, consult a boatyard before proceeding; re-bedding a thru-hull into a compromised area will only delay a bigger problem.

If the hull looks solid, lightly sand the exterior seating surface to a dull finish for better sealant adhesion, and wipe clean with acetone.

Step 3: Install the New Thru-Hull

Apply a generous bead of marine sealant around the underside of the new fitting's outer flange.
Insert the fitting through the hull from outside. Have a helper inside to thread on the backing nut.
Tighten the nut snugly from inside while holding the fitting in place outside — do not overtighten, especially on fiberglass.
Wipe away excess sealant that squeezes from under the flange.
Allow sealant to cure fully — 5200 typically takes 24–72 hours for a full cure; 4200 is faster.

Step 4: Reconnect and Test

Once cured, reconnect the seacock (if fitted) and the hose. We recommend double hose clamps when possible and for any application near or below the water line. When the boat is relaunched, inspect the fitting immediately from inside for any seepage. Monitor it again after the first hour underway. A properly installed and sealed thru-hull will be completely dry.

Shop Thru-Hull Fittings

We stock thru-hull fittings in a full range of sizes from leading marine brands including Perko and Attwood:

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