Plastic Through-hull Warning

Some cheaper materials can fail within the first year of use.

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through-hull breaks

Photos by Frank Lanier

Cracked nylon (acetal plastic) 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 theres nothing left to keep the through-hull in place, meaning itll eventually be pulled inboard, leaving a gaping hole in its place.

If located near the waterline such a hole can reduce the vessels 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 thats bad and the others are of the same vintage, play it safe and replace them all.

stainless-steel stem bolt

Photos by Frank Lanier

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.

Nylon should not be confused with Marelon 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.

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.

Editor’s note: [3.10.2018. This text has been edited for clarity.]

Practical Sailor has been independently testing and reporting on marine products for serious sailors for more than 45 years. Supported entirely by subscribers, Practical Sailor accepts no advertising or any form of compensation from manufacturers whose products we test. Testing is carried out by a team of experts from a wide range of fields including marine electronics, marine safety, marine surveying, sailboat rigging, sailmaking, engineering, ocean sailing, sailboat racing, and sailboat construction and design. This diversity of expertise allows us to carry out in-depth, objective evaluation of virtually every product available to serious sailors. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser with more than three decades of experience as a marine writer, photographer, boat captain, and product tester. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.

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