Marelon vs Bronze

The owner of a 16 year old Tartan 3500 was told by a couple of his know-it-all friends to replace his Marelon through hulls and valves ASAP or the world could end.  He went online and found forums to be inconclusive.  No, really? Everyone on the internet doesn’t agree with each other?  He asked for my opinion of the decades old boaters’ debate, Marelon vs. Bronze.
For this article I evolved into a “real journalist”.  I spoke with Mr. Gerry Douglas, of Catalina Yachts and Mr. Bill Hanna of Forespar, the distributors of Marelon through hulls and valves.
Forespar purchased a bankrupt R.C. Marine (New Zealand) in 1983 after several years of purchasing and distributing R.C. Marine made Marelon. Marelon (pronounced “mar – i – lawn”) is a carbon reinforced plastic, or per their web site “Marelon® is a proprietary formulation of polymar [sic] composite compounds using composite reinforced polymer and additives” and Marelon is:
  • U.L. and A.B.Y.C. approved
  • Forespar “93” series valves and thru-hull fittings meet all design criteria and exceed all mechanical property requirements specified by the International Standards Organization
Mr. Hanna refers to the material as glass reinforced nylon and calls it a true engineering plastic.  Forespar purchases material from DuPont, having tried other suppliers.  They have tried various combinations of material and tweaked their materials over time.  They currently sell through hulls and valves to 325 boat builders worldwide.


Forespar’s website includes ASTM testing results for Marelon ( showing the material is very resistant to corrosion, UV degradation and marring.  It also shows the tensile strength and flexural modulus compared to bronze.
Mr. Gerry Douglas admitted to early skepticism but said Catalina was an early convert to Marelon, having switched from bronze almost 25 years ago.  He has been through several generations of the product, and a few problems, but the Marelon products do not corrode, require no bonding and he is a believer.  Most importantly he reports happy customers.
Like Gerry, I was initially skeptical.  I was use to bronze through hulls and valves.  I preferred seacocks (valves mounted directly to the hull) over ball valves (valves mounted on the stem of the through hull away from the hull) and, with my pen, prohibited the use of gate valves below the waterline (with a few commercial exceptions).  Then I began seeing the Marelon products.
I have seen in many less expensive and cheaper plastic through hulls fail.  Many are susceptible to UV damage, crack around the inside of the mounting flange and fall into the hull, turning a bilge pump into a recirculating pump.  But I have also broken more bronze through hulls and valves than I have Marelon; I twist a lot of through hull valves.  Both types of through hulls and valves require maintenance.  Forespar sells Marelube to prevent valve seizing and both types of valves should be exercised regularly.
The forums are full of stories of failing plastic through hulls and valves and old salts who don’t want to change.  I respect these experiences but have converted to believing in Marelon, primarily for the lack of corrosion.  Corrosion is what I see as the most common cause of through hull failure and the plastic through hulls and valves don’t corrode.  
The strength of the Marelon components is less than bronze.  So use bronze in high traffic or exposed areas, otherwise I choose plastic.
In the interest of coming together and showing that we can all get along, here is a link to using Marelon valves on bronze through hulls.
Mr. Hanna said he has never had a broken plastic fitting returned to them that ended up being one of their products, ever.  They manufacture their products in Orange County, California and have done so since 1983.  The Marlon threads are manufactured to marine industry standards and Mr. Hanna is proud of their customer service and end user support.  He promises that Forespar answers all inquiries.  Mr. Hanna has nothing bad to say about quality bronze through hulls and valves but does warn against the use of “household” bronze and brass components.