Features June 2017 Issue

Plastic Through-hull Warning

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

through-hull breaks
Photos by Frank Lanier

When the flange on a through-hull breaks, the resulting hole can be large enough to flood the boat.

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 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.

stainless-steel stem bolt
Photos by Frank Lanier

The stainless-steel stem bolt (circled) on several Groco valve models—IBV, FBV (shown above), FV, TWV—made between 2004 and 2006 was out of spec. Groco issued a service notice, saying it will replace these free of charge.

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.]

Comments (9)

Thanks to everyone who commented. Frank accidentally submitted general notes collected from various sources at the bottom of his article he submitted, and our editors assumed it was part of the article and his original work. Sorry about the confusion. Thanks for keeping us alert. A published correction will appear next month's issue. Editor Darrell Nicholson.

Posted by: sailordn | March 16, 2018 3:43 PM    Report this comment

Now I am even more concerned about the editorial standards of these articles. On March 11, my post (see below) points out that some of the content appeared to be authored by a commercial entity and not an independent surveyor. Now, that content has disappeared, with no clear explanation or transparency. There is only a note from Mar 10, that says that it was 'edited for clarity'.
So, if the time stamping is correct, the text I was concerned about was edited out of the article with no explanation or documentation. Or, if the time stamping is not correct (and the editing was done before my comment posted), someone is being more than a little coy about the justification for such editing.
I recommend that the entire article be taken down and the topic should be re-addressed by someone who is not selling through hulls (or copying material from those that sell them) and address the substantive errors addressed by other commenters.
A lot of folks rely on PS for unbiased and accurate information on boats and related products - it is essential that the content be squeaky-clean w.r.t. conflicts of interest, ghost writing, etc. and to the accuracy. The value of PS is totally dependent on our ability to trust you in these respects.

Posted by: lesliegb | March 15, 2018 2:01 PM    Report this comment

I agree with the comment by "Hall S." Reduce the number of emails by 50% and put the time into more testing or research. Up the quality of the testing and research and subsequent recommendations. Decrease the quantity of email reports.

Posted by: mark2 | March 13, 2018 1:09 PM    Report this comment

PLEASE, Practical Sailor editors, your content is slipping!! I've subscribed since sometime in the 1980s and quality testing and evaluations have been your hallmark. Your legacy of hard working staff conducting thorough and impartial testing is at stake when you print material that lacks the research you're known for. I don't need something from you in my in-box every day, but I do rely on you for factual information to assist in building and repairing boats. Quality, not quantity is what sets you apart from the rest.

Hall S. (builder of water toys for 50 years)

Posted by: ventura multihulls | March 12, 2018 12:28 PM    Report this comment

Thinking about this article more makes me even more concerned. It starts with "Plastic Through Hull.." then refers to "Nylon" which implies they are the same.
It infers that plastics should not be used below the waterline yet many logs and depth sounders use plastics as do many rudder bearings.
It then goes on to say "Try fittings of some other, nonmetal material such as nylon"!
And what is a GRP hull if not Plastic?
It seems to me that a poorly designed/manufactured item - probably not intended for marine use has made the author jump to all sorts incorrect conclusions and contradictory dictats.

Posted by: bobgarrett | March 12, 2018 9:22 AM    Report this comment

I am confused by the authorship of this article. The byline is claimed to be by a marine surveyor. Yet, the article includes this sentence: "Try fittings of some other, nonmetal material such as nylon or Marelon, both of which we carry."
This sentence reads like it was written by a store or chandlery that is selling stuff.
Which is it?

Posted by: lesliegb | March 11, 2018 7:26 PM    Report this comment

Are those through-hulls in the picture even actually nylon?

Posted by: bobgarrett | March 11, 2018 4:46 PM    Report this comment

Agree with Chrisli the article misrepresented acetate and nylon.

Posted by: Kiwidoug | September 30, 2017 4:41 AM    Report this comment

While the recommendations seem appropriate, the material descriptions are incorrect.

Do not confuse polyacetal with nylon. They are different polymers. Acetal, or polyacetal, is, basically, polyoxymethylene , a polymer of formaldehyde. Nylon can be one of several types, typically, nylon 6,6, nylon 6, or nylon 12, polymers based upon carbon-amines, combined with carbon-acids to form straight-chain polymers with repeating segments of carbon atoms, connected by amine-acid groups. Marelon(c) is a carbon black, glass-fiber composite of nylon 6,6.

So, you can see that the recommendation to avoid nylon would, technically, exclude Marelon(c) -- not what the author intended. Both polyacetal and nylon are available in several different compounds as different molecular weights, copolymers, and composites with various fillers.
When singling out specific plastics, one has to be careful to describe the exact compound that is evaluated.

Posted by: Chrisli | June 11, 2017 12:29 PM    Report this comment

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