I’ll give it a try.
1. Tensile strength (TS) and flexural modulus (FM), nylon and H2O.
Marelon is simply Dupont Zytel nylon 70G13L-BK. It’s Dupont’s 6/6 nylon resin with carbon black for coloring and 13% glass fibers for added strength. The following Dupont Zytel website will provide you with the data. The two most important items are Tensile Strength and Flexural Modulus. The categories are:
DAM – dry as molded (taken shortly after molding)
50%RH - 50% Relative Humidity
100%RH – 100% Relative Humidity
If you look at the numbers from the Forespar ISO/ABYC and comparison tables I cited in my first post, you’ll see all of them on the Dupont data table.
Nylon is a great product, it is tough, cheap
, versatile, and easy to mold
. But, nylon is hydroscopic, it absorbs moisture, as it takes in moisture (up to 10% of its weight) it degrades in both TS and FM. However, it gains in elongation. The elongation factor it what makes nylon so good as anchor rode
for two reasons:
1. In water it elongates, stretches.
2. The water that’s absorbed in the nylon acts as a lubricant against internal and external friction. Even in the air nylon will stabilize at the humidity level of the air. However, in salt water
it tends to stiffen. The reason I believe this is the case is that not all of the salt
flush out and stays within the nylon resin. I’m not a plastics engineer
so I’m happy to be corrected on this issue.
Depending on a variety of factors (material thickness, glass content, temperature, etc.) full saturation, like below the waterline on a boat, will occur within 6 months or less. However, the rate of saturation varies over time with the fastest rates occurring early on then slowing down over time. The fact is that by the time a nylon thru-hull or seacock is installed on your boat it will most likely be at the 50% value or greater. Also, remember that if you take a nylon fitting out of the water, it will seek equilibrium of the ambient humidity.
What about elongation? Elongation it helpful with regard to a static load test, but has little relevance with regard to impact and shear. And impact, according to all the experts is far more critical than static load.
2. “On most of the units the OD is too big and the OD is to small.”
Sorry, I meant to say the “ID is too small.” Almost all seacocks and ball valves follow the ASTM standards for thread sizes and other basic dimensions (ID and OD). So if you have a ¾” seacock you should be able to use any ¾” standard threaded thru-hull fitting. That thru-hull fitting should have an ID of about .750” and an OD of about 1.050.” The Forespar 93 series, in order to get around the problem Main Sail exposes in test
on the over the counter 849 seacock, Forespar simply thickened the thru-hull walls by:
a. Increasing the OD
b. Increasing the ID
Compare the ID’s on the 93 series ¾” video
and the 849 video above.
So the Forespar ¾’ 93 series thru-hull instead of requiring a hull
opening of 1.050” it requires an opening of 1.100”. This is not a major difference, however, to gain the thickness they need they decrease the ID from .750” to .62.”
On the ½” seacock/ thru-hull system the opposite is the case. The standard OD for ½’ is about .850,” Forespar ½” thru-hull is 1.100.” However, the ID standard is about .500” where the Forespar is .620.” The same thru-hull is used for both seacocks.
This follows through-out the 93 series. The 1” and 1 ¼” share the same thru-hull, and the 1 ½” and 2 “ share the same thru-hull. The ½”, 1” and 1 ½” 93 series seacocks have higher ID’s than standard, therefore better flow rates. The ¾”. 1 ¼” and 2” have significantly lower flow rates. And they all require larger hull hole sizes.
If your genset requires a ¾” seacock and you install a ¾” 93 you could overheat the engine
I’ll add this to the mix.
a. A 16 lb alternator
dropped from 4ft (impact) onto a 1 ½” Forespar fitting will destroy the fitting when a 500lb static load will not.
b. In the Main Sail video on the 93 Series, I would conclude that the fitting did not meet the ABYC/UL standard.
c. What is it you want for a thru-hull and seacock standard?
As I said before, there’s a lot more to this than I’ve mentioned here.