Your question was:
I'm looking for the pros and cons of Marelon polymer composite versus Groco Bronze hardware
for replacement. Here's a few:
1) The breaking strength of Bronze is over double that of Marelon.
2) Marelon doesn't corrode.
First, Forespar calls Marelon a “polymer composite” and a 21st century material. The fact is Marelon is simply a Forespar trade
name for Dupont Zytel 70G13L – BK 13% glass-filled carbon blacked 6/6 nylon (that’s not carbon fiber). The nylon 6/6 resin used in Marelon has been around for over 70 years. And the 70G13L – BK can be purchased by anyone. It’s an off the shelf material, nothing exotic.
It has a tensile strength (TS) of 17,000psi and a flexural modulus (FM) of 750,000psi. Both these values are measured “Dry as Molded” (DAM). The DAM measurement is important to remember, especially with nylon. Nylon is great in rode
because in water
it looses strength and elongates.
Second, you state, “The breaking strength of Bronze is over double that of Marelon.” I’m assuming that you’re getting your information from the Forespar’s Marelon Comparison table. This table has been on Forespar’s website for some 8 to 10 years, and in all that time it’s been in error, or so they claim.
Look at the following Forespar sites:
Besides the obvious mPa “error”, it should be psi, Forespar has provided you with two different values for Marelon. There’s a significant difference in values. Why? Dupont Zytel 70 series comes in several flavors, the 70G13L (13% glass) has a TS of 17,000psi and a FM of 750,000psi (DAM). The 70G33L (33% glass) has a TS of 27,000psi and a FM of 1,300,000psi. The 33% glass material makes a better comparison to bronze than the 13% glass don’t you think.
Third, “Marelon doesn't corrode.” This is an absolutely correct and totally useless statement by Forespar or anyone else. Metals corrode, plastics degrade, sadly we simply give Marelon a pass on corrosion
and ignore degradation. Put your boat in the water
with a properly installed bronze seacock and a Marelon seacock for 6 months what will happen, a 60% overall loss in TS and FM values.
The bronze seacock will still have a TS of 35,000psi and a FM of 15,000,000psi. The Marelon seacock and thru-hull will have a TS of 7,500psi and a TS of 250,000psi. This is a average 60% loss of strength. These figures are directly from the Dupont data tables. These Dupont/Marelon values are lower than good grade PVC.
has done an excellent job testing these seacocks and putting some substance to this long running discussion. Even he will admit that his testing is less than perfect, however, they are a good representation of the facts. I wish Mainsail
would explain about why he decided to do his tests, it’s all about impact.
The ABYC in H-27 and corresponding UL 1121 Seacocks and thru-hulls standards have a static load test but no test for impact. Yet impact is the issue of concern of all the marine
experts, as Mainsail will attest.
I’ve done some impact testing on marine fittings and Forespar does not fair well at all. A 15 lb alternator
or a 150 lb person hitting a thru-hull can do many more times the damage than a 500 lb hanging weight.
When it comes to the fact that some people have never had a problem with Marelon seacocks and thru-hulls is nice anecdotal information, but it’s not something you want to base your decision on. And Forespar seacocks have had a lot of problems that relate both to design and materials. Most people know about the problems with the Forespar 849 seacocks and the stem/handle breaking off. Forespar relegates that to a maintenance
problem, that might be part of the issue, the real issue, I believe is a material and design problem. As for the 93 series they have two basic issues.
1. They thru-hulls are non-standard in ID and OD. On most of the units the OD is too big and the OD is too small.
2. The redesign creates a serious hazard. I’ll just say read the following:
CRITICAL UPGRADES - DO THESE OR ELSE!!!
Catastrophic seacock failure! - Page 3 - Catalina 36 International Association Forums
There’s a lot more to be said. But I’ll stop here for now.