Here are a few points to consider regarding Marelon. For those of you who’ve “never had a problem with Marelon” please note that what I’m presenting here is not my opinion, but facts provided by Forespar, Dupont, and Forespar seacock users who have had serious problems with Forespar’s 93 series seacocks.
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. The material Forespar refers to as Marelon will 7.1% by weight water adsorption level at saturation. This is a significant saturation level.
Second, several forum members have quoted Forespar’s comparison table to make the case about Marelon and bronze. This table has been on Forespar’s website for some 8 to 10 years. This table is seriously flawed.
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.
In six months in the water, 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. Good quality PVC has a TS of 7,500psi and a FM of 400,000psi. The PVC will loose virtually no strength in water.
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. The 93 thru-hulls are non-standard in ID and OD. Forespar in their infinite wisdom created only three (3) 93 thru-hull sizes for all six 93 seacocks. On most of the units the OD is too big and the OD is too small. It appears that Forespar has come to the conclusion that their standard 849 series seacock thru-hull was not quite as robust as it should be. So they redesigned the 93 series thru-hulls in a very unique manner. The ½” and ¾” units have on OD of 1.10” and an ID of .62”. What this means is that the 1/2” unit has acceptable flow capacity, but requires an oversized hole. The ¾” unit has a ¾” flow rate with a slightly oversized hole requirement. The same is true for the 1” and 1 ¼” units and the 1 ½” and 2’ units.
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.