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Old 12-04-2015, 06:49   #1
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Through Hulls: Plastic vs Metal

What is the consensus; I have an older Columbia with plastic through hulls. Should I replace them with steel or is plastic ok? I am changing the valves that are frozen. Pretty sure that its a good thing to shut them off when leaving the boat but thats probably another topic.
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Old 12-04-2015, 07:01   #2
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Re: Through hulls: plastic vs metal

Like you I have an "older" boat. I IMHO, I would not go cheap on the through hulls. Install quality bronze fittings. A few buck more, but there is no other fitting that can sink a boat quicker than a through hull or sea cock.
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Old 12-04-2015, 07:06   #3
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Re: Through hulls: plastic vs metal

Always best to close the ones you are not using! This thread will provide a lot of traffic akin to guns on boat or batteries, since everyone has their own theory. Plastic has advantages in terms of corrosion and its tendency not to be blown out of the hull in a lightning strike-but, if I had to replace my through hulls I think I would stick to bronze. I just can't get my mind around plastic throughs (even though they are in my PLASTIC boat).


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Old 13-04-2015, 11:45   #4
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Re: Through hulls: plastic vs metal

I would not use "plastic" below the waterline but some "Marelon" fittings and valves are approved for this use.


I have bronze below the waterline and Marelon above. Works for me.
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Old 13-04-2015, 11:53   #5
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Re: Through hulls: plastic vs metal

I want bronze underwater, but I believe all mine above water are plastic.
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Old 13-04-2015, 13:29   #6
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Re: Through hulls: plastic vs metal

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Originally Posted by a64pilot View Post
I want bronze underwater, but I believe all mine above water are plastic.
Same here. And all the bronze ones are grounded to a strap that runs all the way around the inside of both hulls.
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Old 13-04-2015, 14:42   #7
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Re: Through hulls: plastic vs metal

I have bronze all around. None of them are grounded. So when I took a direct hit by lightening two years ago I didnt worry about having a thruhull get blown out. And none did.
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Old 13-04-2015, 20:12   #8
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Re: Through Hulls: Plastic vs Metal

All of mine are bronze.


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Old 13-04-2015, 21:00   #9
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Re: Through Hulls: Plastic vs Metal

The only safe choice below the water line is a high quality bronze seacock with bronze thru hull (not a gate valve and not brass) or a Forespar Series 93 OEM Marlon seacock and the special thru-hull that comes with it.

The Forespar Series 93 is the only plastic seacock that I'd use below the waterline (and do instead of bronze). Forespar makes other seacocks that have had problems but these are an entirely different design.

While top quality bronze is stronger than Marlon when new, there are far too many stories of poorly maintained corroded bronze sea cocks that break or freeze when needed.

There are lots of threads about seacocks on this board. I'd do some searching.
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Old 13-04-2015, 21:39   #10
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Re: Through Hulls: Plastic vs Metal

Our boat uses the type 93 Forespar seacocks, we sleep soundly aboard her..
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Old 13-04-2015, 23:23   #11
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Re: Through Hulls: Plastic vs Metal

We use the Forespar Marlon fittings. I had a bronze valve fail in my hands - in dry dock as I was doing "maintenance" on it. It was a gate valve, a bad design, though from a top quality manufacturer from the 70s, so it lasted 40 years. As we gradually work on the boat, we are replacing the old bronze ones with Marlon. It's a tough choice though, bronze is appealing in many ways, but what convinced me is the corrosion issue. Especially if the through hulls are bonded together, then you provide a current path for electrolytic corrosion if you stay in a "hot" marina.

Marlon needs regular lubrication (mineral oil, etc) and regular operation, at least every few months they should be opened/closed. And they need to be installed correctly with a mounting plate, bolts, etc. It's non-trivial.
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Old 14-04-2015, 02:09   #12
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Re: Through Hulls: Plastic vs Metal

Stainless steel?
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Old 14-04-2015, 08:52   #13
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Re: Through Hulls: Plastic vs Metal

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Stainless steel?
No...
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Old 14-04-2015, 09:44   #14
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Re: Through Hulls: Plastic vs Metal

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No...
Least not on the underwater part, now if above the water line and we are talking about the discharge ports that are normally plastic, then yes the SS ones are pretty
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Old 14-04-2015, 10:00   #15
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Re: Through Hulls: Plastic vs Metal

Look for seacock failure testing and proper installation videos:
Compass Marine How To Articles Photo Gallery by Compass Marine How To at pbase.com



Marelon testing
https://www.google.com/webhp?sourcei...marelon%20test
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