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Everything You Should Know About Thru-hull Fittings

What is through hull fitting?

 Every piece marine thru-hull fitting is a latent cavity that can sink your boat in a few minutes, even if you can’t see them often and sometimes hard to reach. They need to be examined meticulously and routinely at least every 90 days.

A lot of skin fittings, for instance, engine cooling air intakes and sinks or cockpit drains, always remain open incessantly, and valves may get stuck in the open position. The valve is operated by you open and close it to ensure that water flow is blocked in the event of a failure of the attached hose.


The through-hull fittings are metal or plastic device composed of threaded pipes and washers. It connects pump hoses, outlets, sensors or other instruments or machinery inside the hull to the outside. It can be located below the waterline or above the waterline. A hole needs to be drilled in the boat hull so that the fittings can pass through. 

These fittings link the devices inside the hull to the outside. The applications  include different hoses, depth or logarithmic sensors, and underwater lights. No matter what building materials are used, they can be found on recreational sailboats and motorboats.

How to select the suitable material for marine thru-hull fittings?

  Through-hull fittings can be made of various materials. The most common are alloys designed for bonding strength and corrosion resistance, especially in salt water.

Brass: Common brass is an alloy made from a mixture of copper and zinc and is commonly used to make inexpensive ship accessories.While this may seem like ideal material for work, it’s not. However DZR(De-Zincification Resistant brass), this alloy also contains copper and zinc, besides small amounts of tin and arsenic are added to prevent dezincification and increase its physical strength, so DZR brass is the most oftenly used material for the production of yacht and marine accessories.

Bronze: Possibly the ideal material for metal thru hull fittings, it composes by copper, tin, nickel and other metals. Ordinary bronze usually contains about 88% copper and 12% tin. meanwhile, aluminum is constantly used in Marine applications as well, because it has good corrosion resistance of the alloy.

Stainless Steel: Stainless steel 316 skin fitting also has good corrosion resistance, but this mateiral is expensive and needs oxygen to work, the oxygen can be obtained from the surrounding sea water. But when the connector comes into contact with other materials (for example, a fastening nut), very narrow cracks are usually generated, and water can enter and stagnate, and there is no space for oxygen to flow freely. This result in the passivation film on the stainless steel to deteriorate quickly and causes the material to corrode as fast as ordinary low carbon steel.

How to install boat thru hull skin fitting?

1.  If you are installing a through-hull fitting beneath the waterline of a solid GRP hull, It is best to apply some pure epoxy resin to the inside of the hole and allow it to cure before installing the fitting itself to make sure that the laminate is completely sealed. If your hull is below the waterline (uncommon, but present), it’s a good idea to drill a smaller hole first, move away the area twice the diameter of the core material as the diameter of the fitting, refill the cavity and hole with solid GRP, and re-drill to the appropriate diameter to fit your through-hull fitting after fully curing.

2. After applying sealant to the flange, such as epoxy, insert it into the new hole and tighten it from the inside.Excess epoxy around the hole will now be used as a compression ring so that the core material in the hull laminates will not be crushed when the flange ring is fully tightened. Please make sure and check the installation instructions, as some parts, such as plastic fittings, may require other sealants.

3. Please note that if the seacock is going into a particularly tight area, it is best to temporarily screw the seacock onto the skin fitting by hand before the sealant solidifies to make sure that the handle is in an easy opening and closing position after completely assembled.

4.  Eventually, remember to provide some seasonal inspection and maintenance.

Here are different sizes and styles boat thru-hull fittings available at Fanovo Industries, please visit our website to find the suitable one for your applications, for more detailed information, please contact us: