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Wednesday, July 09, 2008

The Marelon vs Bronze Thruhull Debate

Search the Internet on this topic, and you will find all sorts of arguments.

For those not in the know...

Traditionally, ocean going boats have used bronze for their underwater metal fittings: thru hulls and seacocks. There are a couple downsides to bronze underwater fittings. First is price. Cost of the metals (especially copper) has gone up over the years. Second, is controlling corrosion. The saltwater ocean is basically a big tub of electrolyte. Insert two pieces of metal close to one another and you essentially have a weak battery (anode and cathode). Small currents will flow between them causing the metal to corrode over time. That may not be EXACTLY how it happens, but that is the general idea. The last thing you want is the metals to be slowly eaten away as that is a good way to sink the boat when the fitting ultimately fails. This is why boats are fitted with "bonding systems" that introduce a sacrificial zinc. The bronze will not corrode until the zinc has corroded away.

Enter Marelon. Been around for about ten years. ABYC certified and approved for underwater use. Basically, a composite of special resin and glass. Advantages: no metal therefore wont corrode, cheaper materials, and lighter!

Downsides? well that is the debate. Most often cited is the "sideways strength". That is if the thru hull pipe stem (on the inside of the boat) is side impacted hard enough, it can break off at the surface of the hull, letting water in.

A valid concern. What is not often included in these arguments is what "type" of thru hull/valve are we talking about. Seems it is often assumed it is just a thru-hull with a backing nut followed by a ball valve threaded, followed by a hose barb. What is often NOT mentioned is that Forespar, the exclusive maker/distributor of Marelon plumbing fittings, does make a "proper" flanged marelon seacock/seavalve. The difference in a "proper" seacock is the backing nut and ball valve are one and the flange, which bolts against the hull, provides more strength and resistance to side impacts. Effectively, the flange and ball valve housing provide more sturdiness compared to the straight thin walled pipe nipple on the end of a plain thru hull.

Why am I bringing this up? Well, we are starting the thru-hull/seacock installations, starting with the cockpit drains. There will be four of them, 1 1/2" diameter each. We have decided to go with the marelon seacocks, for the positive reasons cited above. As they will be proper thru-bolted seackcocks, I do not believe there will be much side-impact breakage risk. Regardless, I will try to position them in "safe" areas of the engine room as best as possible.

It also should be noted that, ABYC does NOT recommend ball-valved-threaded-on thru-hulls under the waterline, bronze or otherwise. Though many boat manufacturers do this simply because proper good quality bronze seacocks are getting hard to find. It used to be Wilcox/Crittenden and Groco made the "standard" tapered plug seacock. But no more. Seems the few seacock makers, including Forespar, are basing them on ball valves, which is considered less desirable because they are not easy to rebuild. The one maker of tapered plug bronze seacocks I know of is Spartan Marine.

In the end, they could always be swapped out for bronze if there is some reason to change.

Anyway, enough rambling. I am sure some of you out there have some opinions on this, feel free to post a comment.
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