>>> PeterFlynn - I'm glad to see the discussion on bronze vs Marelon through hulls. I found out today that the boat we wish to buy has Marelon through hulls. I will be very interested in the comments regarding these fittings.
Marelon is not "plastic" - it is a composite of resin and re-enforcing fibers in a way like the basic hull layup
of all FRG boats. Here is a link to the manufacturers page: Forespar: About Marelon Plumbing
From that page, here is an extract of the Marelon versus Bronze
Marelon® Comparision Chart MARELON® BRONZE
U.V. Degradation 10 10
Resistance 10 4
Abrasion Resistance 10 10
Tensil Strength** 10(27000psi) 10(35000psi)
Flexural Modulus*** 10(1300000mPa)) 10(15000000mPa)
ABYC* (Reg.) 10 10
U.L.* (Reg.) 10 10
Total Score 70 64
* MINIMUM PHYSICAL PROPERTIES REQUIRED FOR APPROVAL ARE: Tensil strength 10,900psi 75mPa. Flexural modulus 500,000psi 3480mPa
** Value by test to ASTM D638 (American Society of Testing & Materials) Marelon is a registered trademark of Forespar Products Corporation.
***Value ny test to ASTM D790 (American Society of Testing & Mate
The only difference between the two is the corrosion
resistance. With bonded bronze throughhulls/seacocks there is a problem with the tin leaching out of the bronze. This turns the bronze "pink" and is a precursor to thru-hull failure. Inadequate movement exercise and lack of lubrication will make the Seacock "freeze" up and be near to impossible to close.
Un-bonded bronze through hulls have more of a problem with leeching tin but do provide protection against lightning
strike over-heating of the metal through hull
and it's "falling out" of the hull. So it seems to be 6 to 1, half dozen to the other between bonding and not bonding.
Marelon being a composite material is inert to any leeching problems. But if not exercised and lubricated can also "freeze up" and be very difficult to close. There is a very easy fix to that and it is inherent in the construction of the Marelon Valve which is made in 2 parts
with a ball (valve) in between. The two parts
screw together to capture the ball (valve) so un-screwing the two parts of the main body a slight amount relieves the pressure on the ball part and you can then rotate it. You also can set the tension or stiffness of the ball (valve) by tightening or loosening the 2 parts of the body.
The "through-hull" is the piece of "pipe" that actually passes through the hull of the boat. This part is screwed into the "Seacock" which is the shut-off valve. Ideally seacocks should be "flanged" and bolted/screwed to the inside of the hull. Then the only stress on the through-hull piece is tension of how tight you screw it into the flanged seacock. Failure of the through-hull with a flanged mounted seacock will probably never be noticed as the seacock is securely attached to the hull. Unfortunately many boats use ordinary plumbing
ball valves or gate valves for seacocks and these are very prone to being broken by accidental side loads - like somebody stepping on them or something falling on them. Flanged seacocks attached to the hull eliminates this problem.
Through-hulls come in two major styles - one has an exterior "lip" like a nail's head
that is raised away from the surface of the boats hull. The other major style is "flush/flat" head
which is counter-sunk into the hull and does not offer any "raised" lip (much like a flat head screw). That raised exterior part is the weak part of a Marelon through-hull. If the boat "scrapes" along an underwater obstacle, the raised head can be sheared (chiseled) off the boat by the obstacle. Bronze is stronger in those cases. The fix for Marelon is easy - use the counter-sunk flat head version of the through-hull.
The major production sailboats switched many years ago to Marelon through-hulls to eliminate the "tin" problem, the bonding problem, and extra costs of installing bronze. Considering liability and warranty concerns, their switching to Marelon translates to a lot of confidence in the Marelon product.