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.

10 comments for “The Marelon vs Bronze Thruhull Debate

  1. July 10, 2008 at 9:54 pm

    Interesting re: tapered-valve through-hulls vs. ball valve. In my copy of Good Old Boat, Don Casey says “The best seacocks, by a wide margin, are ball valves.” Thoughts?

    As far as Marelon goes, the only decent argument against them is from a friend of mine who said “I sail close to rocks”. From the inside, all seacocks/throughhulls are required to accept a lot of force (500lb come to mind but don’t quote me) at the point furthest from the hull…so Marelon, Bronze, or otherwise, if ABYC approved, can handle quite a beating.

  2. rl
    July 10, 2008 at 11:08 pm

    The argument for tapered valves, that they can easily be rebuilt, over ball valves, is quite a minor justification IMHO.

    “sailing close to rocks” would be a good reason for bronze I suppose! But a flush mount thru hull should take care of that!

    Besides in all of the search for marelon, I have not heard of a thru hull failing because it broke from side impact.

    thanks for the comment

  3. July 11, 2008 at 1:25 am

    True, rebuilding is a good argument; the argument against was that they’re prone to problems and require significant regular maintenance (rebuilding), whereas ball valves seem to require only lubrication and last as long even without rebuilding. Not much personal experience on this, just curiosity. On my boat, the tapered stuff needs to be taken care of every 2-3 years at least, but are 20 years old; the ball valves seem to be trouble-free but are only 4 years old.

    Also curious ( I haven’t read the standards), what does ABYC say in support of tapered-plugs? I thought the recent standards didn’t specify ball or taper, and didn’t even require a flange, but _did_ require a static 500lb test.

    On Marelon, I agree again…next through-hull I replace will be marelon.

  4. rl
    July 11, 2008 at 1:38 am


    My experiences are similar to yours. Our W/C flanged tapered plug seacocks on our 28 footer only get lubrication every 2-3 years. Last year, we rebuilt them for the first time in 25 years, which included removal. Removal was the hard part, rebuilding was easy.

    I just looked up the ABYC specs and you are correct. Flanged or threaded ball valves doesnt matter, as long as it withstands the 500 lb force:

    “27.7.1 A seacock shall be securely mounted so that the system will withstand a 500 pound static force applied for 30 seconds to the inboard end of its connecting fitting, at any point in its most vulnerable direction, without the system failing to perform as intended.”

    Thanks for clarifying my error.

    I think I got the “no ball-valve-threaded-onto-thruhull” from the few surveyors we have talked to. They seemed to emphasize flanged seacocks where possible. Perhaps it is because “one less joint” to fail?

    My memory is cloudy on that one obviously.

    Oh, there could be one more issue with flanged bronze seacocks. When we rebuilt ours, you could not interchange parts between assemblies. Ones plug and housing would be machined slightly different than the other. You cant get parts for these anymore, and if you found a used one you wanted for parts, they might not fit.

  5. July 11, 2008 at 9:50 pm

    P.S. on your “flanged” comment on the last paragraph…did you mean flanged or tapered? I can see how either can be worn down to not be interchangeable. Flange bolts etc. can not line up, and tapered plugs can be lapped to their own parts so if you mix them they would leak.

  6. July 11, 2008 at 9:50 pm

    I actually don’t have the abyc code handy so I’m more relying on your resources than trying to point out any errors 🙂
    First, tapered vs. ball valves: from what I’ve heard (I’ve never actually done this) these require taking appart, lapping, and lubricating every several years. Mine get jammed and leak, but again, they’re 20 years old.

    Second, flanges: I’ve seen both, and although it’s possible to convince me otherwise, the flanged kind just look stronger. I’ve installed both, worked with both…and I would not install a non-flanged fitting of the kind I’ve seen used. I think the flange definition was there on the ’77 standard and went away in ’88, but not sure of the date on that.

  7. rl
    July 11, 2008 at 9:55 pm

    I guess I was meaning the flanged AND tapered plug seacocks as for poor interchangeability. At least the ones we have.

    I am with you on sticking with flanged bases. In the case of marelon, the cost is not that much more.

    I could see using thru-hull-threaded-on-ball-valves ABOVE the waterline. Maybe for drains or something. But not below WL.

  8. July 11, 2008 at 10:06 pm

    Agreed…but even then, in sailboats the standard is above “maximum heeled waterline” I believe, so if you’re going to go to the effort to install a valve, why not spend the extra few bucks?

  9. July 3, 2012 at 12:52 am

    I'm thinking about writing an article about seacock and thru-hull materials. I happened to see your blog and saw that you were using Forespars Marelon seacocks.

    I wondered if you'd answer a few questions for me?

    1. The ABYC non-metal thru-hull/seacock materials standard is Tensile Strength (TS) minimum 10,900 psi and flexural modulus (FM) minimum 500,000 pst. PVC has a TS of 7,500 psi and FM of 375,000 psi. Would you consider using PVC?

    2. If no, would you consider using a material that would degrade on a boat to values less than 7,000 psi (TS) and 250,000 (FM)?

    3. If no to 3, why?

  10. July 3, 2012 at 1:00 am

    Jed, am no materials expert, but here is my $.02.

    I would not consider PVC, just cause it seems more brittle (non scientific).

    In marelon, ALL of the seacock bits are the same material. Handle, hose barb, etc. So, if a PVC seacock had a PVC handle and hose barb, I would most certainly be worried about those breaking under stress. I would want those reinforced. Hence marelon (a glass reinforced material).

    So if you make a seacock out of PVC, what will the handle, and hose barbs be made out of? Does that make it more or less to produce over marelon?

    An interesting idea.

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