Plastic Through-hull Warning
Some cheaper materials can fail within the first year of use.
Cracked nylon thru-hulls are a common problem, as a walk in almost any boatyard will bear out. Unlike fittings constructed of industry approved materials (bronze, Marelon, etc.) nylon thru-hulls are not recommended for use at or below the waterline. Age often plays a factor in the failure of nylon thru-hulls, but ultraviolet light is the main culprit. While different brands vary widely in their susceptibility to UV damage, some are so poorly made they can fail within the first year of use. The stress placed on the thru-hull by an unsupported hose can also cause failure, with the weight of the hose acting like a lever as the boat bounces around while underway.
Nylon thru-hulls typically fail where the body of the fitting joins the outer flange, which can result in a crack or even total failure (both of which are shown in the accompanying photo). Once the flange shears off there’s nothing left to keep the through-hull in place, meaning it’ll eventually be pulled inboard, leaving a gaping hole in its place.
If located near the waterline such a hole can reduce the vessel’s effective freeboard from feet to inches, meaning a boat needs only to settle slightly before it begins to take on water and sink.
If the failed fitting happens to be your bilge pump overboard discharge, then the pump just continues to recycle any water that enters the bilge as your boat gets lower and lower in the water until…well, you get the idea.
Nylon fittings should be inspected at least annually. If you find one fitting that’s bad and the others are of the same vintage, play it safe and replace them all.
Nylon thru-hull fittings like this are culprits in more than a few sinkings, when used as cockpit drains that ended up underwater and were cracked due to age and UV damage. Nylon is most definitely not for below-waterline use.
The difference between bronze and some types of brass in the world of plumbing fittings concerns their relative potentials for galvanic corrosion. Dissimilar metals in contact can set up an electric current in an electrically conductive fluid (an electrolyte, such as saltwater). One of the metals will be “eaten away” or “sacrificed” in the process.
Sacrificial anodes are made of zinc because it is one of the least “noble” (i.e., highly active) metals and is thus sacrificed to protect your engine, rudder, propeller shaft, refrigeration condenser, etc. from damage due to galvanic corrosion. However, with thru-hulls, seacocks, etc., dissolving fittings are not an option. You want all the fittings to remain intact.
To avoid this potentially hazardous situation, make sure you are not setting up the conditions for galvanic corrosion by connecting dissimilar brass fittings to bronze fittings. Generally, you can tell them apart just by looking at them. Bronze fittings have an outer surface that is rough in feel and appearance, almost like it’s covered with sand. That’s because bronze fittings are sand cast, and then machined.
You should continue with what has always been sound marine practice: use bronze fittings for the thru-hull mushrooms, seacocks, strainer bodies and valves. If one of the pipes must be turned through an angle, use a bronze” “L,” either 90° or 45°. Once the bronze fittings are in place and aligned, thread a bronze hose barb into each, and join them with sturdy hoses and stainless steel clamps. Don’t confuse bronze hose-to-pipe fittings like those made by Groco and Perko with small brass hose-to-pipe adapters sold in hardware stores.
Bronze fittings used for seacocks are fine for use in saltwater. But what if the application absolutely demands other fittings? Try fittings of some other, nonmetal material such as nylon or Marelon, both of which we carry. Brass should never be used in raw-water plumbing due to the risk of galvanic corrosion.
Marelon is a high strength, polymer composite, developed as a solution to electrolysis and corrosion problems associated with bronze/brass plumbing fittings. It is UL/ABYC approved and ISO certified for use below and above waterline.
Acetal plastic (nylon) is an inexpensive plumbing alternative that doesn’t belong on the boat.
All approved seacocks and through-hull fittings, metallic or composite, should carry the UL approval number 1121 or 618C, for “Marine Through-Hull Fittings and Sea-Valves.”
Capt. Frank Lanier is an accredited marine surveyor with over 30 years of experience in the marine industry. His website is www.captfklanier.com.