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Old 11-02-2019, 00:14   #1
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Thru hull material for an aluminium hull

I have a 1980 Adams 35 with a 5086 aluminium hull. I am replacing the various plastic skin fittings an their sea cocks. The three most appropriate materials seem to be plastic, stainless steel and composites. Remembering I will attached sea cocks, what material would you suggest to avoid oxidation and yet have a strong fitting. They will include engine water intake, head intake and grey water outlets and deck drains?
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Old 11-02-2019, 00:26   #2
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Re: Thru hole material for an aluminium hull

Use plastic or glass reinforced nylon (Marelon) through hulls and seacocks. Stainless or bronze will cause electrolysis and damage your aluminum hull. Plastic is plenty tough. There are a couple of brands out there. Just be sure to mount them right. A backing plate made of epoxy coated plywood or fiberglass sheet is essential.
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Old 11-02-2019, 08:47   #3
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Re: Thru hole material for an aluminium hull

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Originally Posted by Astro2 View Post
I have a 1980 Adams 35 with a 5086 aluminium hull. I am replacing the various plastic skin fittings an their sea cocks. The three most appropriate materials seem to be plastic, stainless steel and composites. Remembering I will attached sea cocks, what material would you suggest to avoid oxidation and yet have a strong fitting. They will include engine water intake, head intake and grey water outlets and deck drains?
No Metal. My Marelon, despite greasing are very hard to turn after some years, the CR-inlets, I am afraid they might break. Some smaller I changed already to Trudesign, NZ, selling worldwide. Approved for maritime use! Do not use cheap household stuff
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Old 11-02-2019, 09:02   #4
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Re: Thru hole material for an aluminium hull

Your aluminum boat doesn't have standpipes? Do not use stainless with aluminum... if there is any contact between them. Not sure what I would do. The Marelon is tough but the valves aren't and warp and are hard to turn etc. The Marelon mushroom fittings are tough as hell. When I had to replace some bad Marelon seacocks (only 6 years old) I couldn't get the Marelon mushroom out to save my soul. I literally had to use a hacksaw blade and cut/chisel them out one chunk at a time.
If there are Bronze or SS valves that will take the straight thread of a Marelon mushroom and be isolated from your hull, that may be a viable solution.
I have heard there are "upgrade" Marelon style valves That are better than the standard seacocks. Albeit you may have to use valves not base mounted seacocks.??
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Old 11-02-2019, 10:37   #5
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Re: Thru hole material for an aluminium hull

Oh crap! A vendor just replaced one in Bacchus with Marelon! Wished I had read this a month ago..
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Old 11-02-2019, 10:41   #6
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Re: Thru hole material for an aluminium hull

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Oh crap! A vendor just replaced one in Bacchus with Marelon! Wished I had read this a month ago..
Just be careful turning it. The handles/shaft break off if hard to turn. Maybe the new ones are better....?
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Old 11-02-2019, 17:08   #7
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Re: Thru hull material for an aluminium hull

Same here with the Marelon salt water inlet on my aluminum hull. Worked smoothly for years, then started to fight it closed. Thinking if you use some Tefgel or similar waterproof goo into the internal ball valve mechanism before installing, might help. I'm going to try to infuse some next haul out. The above waterline exhaust Marelon is still smooth.
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Old 11-02-2019, 18:25   #8
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Re: Thru hull material for an aluminium hull

Marelon thru-hull fittings and valves are dependable and ought to last a lot longer than the number of years most people keep a boat, "if they are opened & closed a few times a year" to keep the neck of the valve free from fatigue cracks that might result from forcing them open (more than likely from use of a cheater pipe).



I avoid use of plywood for backing plates behind thru-hull fittings, even if it's marine ply. Maybe you can get by when epoxy coating it, but I've seen water damage into marine ply with 2-part epoxy paint when exposed to water vapor condensation drips from non-insulated hulls in as little as six to eight years. For thru-hull backing plates, I like industrial strength fbg plate and use a bi-metal hole saw cut to 1/16" over the diameter of the thru-hull fitting, using a drill press if possible. There are several different compositions of commercially available fbg plate, all with specifications such as fire retardant, strength, density, etc. for comparison when ordered from McMaster-Carr with more affordable prices than most marine suppliers. If the hull surface is relatively flat, 3/8" thickness of fbg is sufficient and I have dipped it in 2-part epoxy thinned with a fair amount of acetone while mixing in order to make it a penetrant solution to prevent water absorption. This makes it essentially equivalent to various brands of penetrating epoxy. If the hull surface is not flat when laying a 4" straight edge across the hole, then I bed it with a mix of fbg particles to thicken the epoxy, using duct tape backed peripherally with caulking to dam it around the edges and a produce bag with a hole for the fitting, over the epoxy to separate it from the Marelon fittings when threading them down to just snug, when the epoxy starts to set up, beyond that of a peanut butter consistency. West System tech info on their internet site is an excellent reference for use of their thickening agents. Vaseline petroleum jelly can be used as a releasing agent on the threads to keep any loose smears of epoxy from bonding to it. As a lubricant on the threads of the Marelon fittings, I think I have read that Vaseline will not react with Marelon.



For final install, I take the fitting off the next day and use 3M 4200 or a similarly rated fixative-sealant, on both sides of the hull, (avoiding the use of 3M 5200 because it's next to impossible to remove w/o using their proprietary dissolving product. Thru-hull wrenches that grab inside the mushroom fitting are not expensive, provided it is not a bronze Groco model. Btw, instead of fbg for backing plates, some people like to use G-10 because it does not distort and expand when exposed to high humidity, but l don't think the expense is worth it.
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Old 11-02-2019, 19:13   #9
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Re: Thru hull material for an aluminium hull

1980 Joe Adams design, I am guessing it was a race boat and most likely has a faired hull. If the hull does have fairing compound, it will tend to release around a stand pipe underwater. The aluminum and fairing compound joint is exposed at the standpipe, not good. The two most problematic thru hulls I had were the engine intake and sink waste which was near ring frame. The flexing/vibration and exposed joint don’t go well together. After having to redo the fairing two seasons in a row I cut the standpipes off and used Marelon mushroom thru hulls with SS ball valves and epoxied and faired them in. Never had to redo the fairing again.
I did used plywood backing plates which had been treated with a thinned resin as Taipe suggested.
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Old 11-02-2019, 21:46   #10
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Re: Thru hull material for an aluminium hull

Thanks to all. The general consensus is Marelon but ensure I use occasionally to avoid cracking and to keep freedom of movement.

As to backing plate I was going to use 5 mm 5086 aluminium - same is as the hull. Would you recommend I not do this and consider your various ply/fiberglass materials?
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Old 11-02-2019, 23:28   #11
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Re: Thru hull material for an aluminium hull

I wouldn’t use the aluminium backing plate. There will be some hull contour and the plate will either stay flat and have a void between it and the hull or flex and put two points of pressure on the thru hull nut rather than a consistant pressure. Using something that contours to the hull shape and even pressure over the surface of the nut would be better. I used ply because that is what I had done on previous boats. Taipe’s concept/technique which moulds to the hull contour sounds interesting, but haven’t seen it done before.
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