Tips for Using Below Waterline Thru Hull Fittings

The failure of any thru hull fittings below the waterline can lead to the flooding, sinking and total loss of a boat, which is why they’re so crucial to boating safety. 

Thru hull fittings failure

Here’s a quick guide and some tips for using below waterline thru hull (or through hull) fittings. 

Buy extended length thru hull fittings

Buy bronze thru hull fittings

Through hull fittings are made of metal or plastic, each with its advantages and drawbacks.

  • Metal thru hull fittings: Strong and resistant to impact, but prone to corrosion.
  • Plastic thru hull fittings: Not as durable as metal fittings, but resistant to corrosion.

Metal Thru Hull Fittings

Thru hull fittings are available in a variety of metals, including bronze, brass, stainless steel and aluminum. However, only bronze should be used for through hull fittings below the waterline, because it’s highly resistant to corrosion, unlike the other metals.

Perko bronze thru hull fitting

Perko bronze thru hull flange

Plastic Thru Hull Fittings

These through hulls come in a variety of plastics, including nylon and polypropylene. Marelon, a polymer composite reinforced with fiberglass specifically developed for marine use, is a popular choice of material for thru hull fittings. However, fittings made of plastics aren’t generally recommended for use below the waterline. Bronze fittings are considered best for below the waterline use.

TH Marine thru hull fitting white

TH Marine thru hull fitting black

Thru Hull Fittings and Seacocks

A seacock is a valve that can be closed if the thru hull fitting fails, and one must be attached to every below waterline fitting. Seacocks have bodies that mount directly to the hull, as opposed to a typical ball valve that only mounts to the fitting itself. This makes the seacock more sturdy and secure. 

Below waterline thru hull fittings

A seacock’s screw threads must match the NPS (national pipe straight) threads of a thru hull fitting. Seacocks are operated by a handle that opens and closes it. In an emergency situation, the seacock can be closed with the flick of a wrist. All seacocks attached to below waterline fittings should be cycled open and closed a few times (at least once a month) to move the internal components and prevent them from getting stuck. 

Below Waterline Thru Hull Fitting Additional Tips

  • When choosing a new seacock, make sure it’s UL Marine rated. 
  • Know the precise location of every below waterline thru hull fitting in case of emergency. 
  • Have a plug tied to every through hull fitting in case a fitting or seacock fails. 

Below waterline thru hull fittings tips

Finally, all through hull fittings and seacocks should be periodically inspected and maintained to ensure their durability and reliability. 



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