Replacing Thru-Hull fittings-Aluminium yacht

[Wideocean7]

I'm taking my boat out of the water soon and will be replacing some of her thru-hull fittings with TruDesign. This might be a really dumb question - should I expose the aluminium and bond the fitting to that (I'm using 5200)? If so, does the aluminium need to be prepped some how? Or should I apply an epoxy and/or antifoul coat before attaching the thru-hull?

Views (especially of those who have done this with aluminium hulls) would be much appreciated.

Cheers

[Searles]

Having done this on a largish alu yacht ,we removed all coatings in the immediate area ,ground clean ,3coats of high build epoxy ,then when fully cured ,used 5200 ,the fittings were marlon ,glass reinforced nylon ,20yrs ago still good ,this was for the throughout hulls above and below the water ,salt water that is .⛵️⚓️

[Manateeman]

We are removing all the Forespar Marelon seacocks from our 60’ aluminum yacht.
Forespar seacocks bolted onto 1/2 alu plates TIG welded to the hull.
Forespar handles break off.
TruDesign valves are NOT bolted to the hull. They are supported ball valves that rely ONLY on the strength of the thru hull. Do Not put plastic junk in your yacht.

Groco makes 316 Stainless adaptor plates. You can screw a Groco or any commercial 316 Stainless ball valve or a cap onto the adaptor plate without hauling the boat.
This is how we do it. First an aluminum backing plate ...3/8 to 1/2” with three holes drilled and tapped to fit the Groco and the center hole slightly larger tan the OD of the thru hull. TIG the plate to the hull. It’s not necessary to weld all the way round.
You can use Marelon thru hulls, flat inset or mushroom. They are not holding the adaptor plate to the hull so plastic is ok. Measure the length of the thru hull so you get it correct I to the adaptor plate.
Make a thin gasket to seperate the adaptor plate from your welded in hull backer.
Tef-Gel the bolts. Use nickel Teflon tape on between the adaptor and the ball valve. I use 5200 on the thru hulls but it’s your choice.
Now you will hear comments about 316 under water. Utter, Nonsense.
Chemical plants run the nastiest stuff in 316 all day...But if that bothers you, you can have the adaptor plate coated with CeraKote.
I use FelPro 1/16 gasket for water pumps and 5200. It’s messy but works.
Once you have your ball valves in place, you can add stainless motor valves.
Now, one switch and all you hull openings are closed off.
There is a video on Marine How To for glass boats but it’s very similar.
Plastic junk seacocks were the worse mistake I made in 50 years of Boatbuilding.
PM us if you need assistance or the process isn’t clear.
Captain Mark and the manatee crew of professional boatbuilders.

[Searles]

Please not I didn’t say sea cocks is said skin fittings ,the part that goes through the hull skin ,wot you or anyone fits to the skin fitting is there choise ,I was only referring to the through hulls .⛵️⚓️

[Wideocean7]

Apologies if my original post was not clear. I'm asking about the external hull prep before fitting the thru-hull (aluminium hull). I'm not focussing on what I do inside - that was discussed at length in an older post of mine. Searles - you've answered the question thanks. I heard of one yacht where they just exposed and cleaned the aluminium, etch-primed and then fitted the TruDesign thu-hull with 5200.

I'm not yet clear on what the "right" way of doing it is (which may be what Searles suggested). Be great to have more views.

[mcarthur]

All our underwater throughhulls are alu pipes welded to the hull skins. Some are long enough to be above the waterline before fittings are attached, while others are more limited height depending on use. All are curved to fit 90 degree to the hull side and then straighten upright. So I can't help with the actual throughhull part, but have you considered welding alu pipe instead of using hull-connected throughulls of any other type?

[Wideocean7]

I must admit that I have until now been committed to the TruDesign thru-hull fittings. But I have 3 weeks to go before the lift so I have enough time to consider alternatives - and I'm already having some hull welding done. So, if I opted for a standpipe, how does that work - ie does it have an external flange and a threaded top to accept a ball valve? (I'd also hope the marine welder I will be using will know this stuff). Anything else that I should be aware of? I know this is creeping away from my original post but if a standpipe is the more reliable solution, I won't need to worry about my question in my post.

Thanks again for all your suggestions.

[GordMay]

Originally Posted by Wideocean7

... So, if I opted for a standpipe, how does that work - ie does it have an external flange and a threaded top to accept a ball valve? ...

See ➥ https://www.groco.net/products/fitti...water-manifold

[LittleWing77]

I was going to say "whatever Manteeman says" and let him chime in, but I see he already has.

Not only is Mark a professional boatbuilder, he has also just replaced/ is in the process of replacing all of the thruhulls on his custom Ted Brewer designed Aluminum hull.

So Manateeman DEFinitely knows whereof he speaks!

Fair winds and good luck, WideOcean,
Warmly,
LittleWing77

[Manateeman]

Welding standpipes in an aluminum hull is one of the most challenging welding one can do in aluminum. We have four standpipes, three inches in diameter, flush to the exterior of the hull with internal backing plates and four triangle supports on each pipe. They were welded in by an expert in aluminum TIG. The hull was new and the fit of all the pieces was perfect. You also need a perfect day. This all takes a lot of time and the cost in a commercial yard would have been very high.
You cannot “push” the welding as too much heat weakens the aluminum. This means more time, more money.
Lastly, you have to face one big issue. No valve. Very difficult to stop a leak at sea. Like every decision in vessel construction, you have plus and minus.
Had the larger Groco adaptor plates been available at the time, we might have used them or fabricated something similar in 316.
I understand you are committed to plastic. I sincerely wish you have good luck with them. ..it’s your boat, your choice. It’s just not my choice.
We posted to give readers more information so they can make up their own mind.
There are no absolutes in vessel construction. Standpipes done correctly just takes a truly skilled welder and a lot of time. Welding Tips and Tricks is a great web site, not just for welders, but for anyone who wants aluminum or stainless work done on their boat. You can see what a quality weld looks like and get some idea of the complexity of welding these two metals.
Happy trails to you.
Captain Mark and his manatee crew.

[adjo]

"Welding standpipes in an aluminum hull is one of the most challenging welding one can do in aluminum. We have four standpipes, three inches in diameter, flush to the exterior of the hull with internal backing plates and four triangle supports on each pipe. They were welded in by an expert in aluminum TIG. The hull was new and the fit of all the pieces was perfect. You also need a perfect day. This all takes a lot of time and the cost in a commercial yard would have been very high.
You cannot “push” the welding as too much heat weakens the aluminum. This means more time, more money....." Quote from Manateeman

I find your explanation a bit strange, Manateeman.

Why would you TIG weld such thick aluminium?

I MIG welded all my standpipes (6mm wall thickness to 8mm and 12mm plating) with no problems whatsoever and 30+ years later they are still fine.

Quick and easy.
Normal weld prep with a flush weld on the outside and a generous fillet on the inside plus bracing on the standpipes that extend above the waterline.
In situations like this I would normally start my welds on a sacrificial tab.

TIG is slow and you would probably need (or benefit from) an argon/helium gas mix to have enough heat to overcome the conductivity of the thick aluminium parent metal. Why bother?

[Wideocean7]

Manateeman- thanks for such detailed comments. I'll need to digest all of this. I'm not totally committed to plastic (I was when I started the thread) when a standpipe appears to be the better option. I've no idea if my welder is up for the task.

Gordmay - thanks for the link. It still begs the question - is the standpipe threaded at the top to accommodate a seacock?

Re one of my original questions, if I have to go with plastic, how should the hull outside be prepped? This will have to be my back-up plan as I cannot delay doing the work.

Cheers

[GordMay]

Originally Posted by GordMay

See ➥ https://www.groco.net/products/fitti...water-manifold

“.. the threaded inlet which screws into a seacock or strainer outlet ...”

[Breaking Waves]

Originally Posted by Wideocean7

Re one of my original questions, if I have to go with plastic, how should the hull outside be prepped?

ok, the correct answer to this question is pretty simple and clear - you prep and paint it in the standard way for aluminum: clean metal, primer (various opinions here but anything from a good brand specifically for marine aluminum), and epoxy barrier coat. Don't put antifoul on it as that will basically eliminate the 5200 adhesion.

Aluminum standpipes are a pretty decent solution. Usually, they have a triangular plate welded to their top to fit the bottom of seacocks. Some are threaded to thread into seacocks but that is significantly inferior (potential corrosion in the threaded area - cause them to break off and you now have no seacock and a big hole).

Below waterline plastic - either in the thruhull or in the seacock is NOT allowed by many commercial codes because of fire hazards. In some codes they are allowed but not in any space that has an engine or hydraulic equipment (because of the possibility of hot fuel/oil fire). The stainless solution is the practical solution in that case, it is decently easy to install - you just need to make sure to insulate well from the aluminum.

[Wideocean7]

Originally Posted by Breaking Waves

ok, the correct answer to this question is pretty simple and clear - you prep and paint it in the standard way for aluminum: clean metal, primer (various opinions here but anything from a good brand specifically for marine aluminum), and epoxy barrier coat. Don't put antifoul on it as that will basically eliminate the 5200 adhesion.

Aluminum standpipes are a pretty decent solution. Usually, they have a triangular plate welded to their top to fit the bottom of seacocks. Some are threaded to thread into seacocks but that is significantly inferior (potential corrosion in the threaded area - cause them to break off and you now have no seacock and a big hole).

Below waterline plastic - either in the thruhull or in the seacock is NOT allowed by many commercial codes because of fire hazards. In some codes they are allowed but not in any space that has an engine or hydraulic equipment (because of the possibility of hot fuel/oil fire). The stainless solution is the practical solution in that case, it is decently easy to install - you just need to make sure to insulate well from the aluminum.

Thank you Breaking Waves - you've hit the nail on the head in terms of answering my questions. Much appreciated. You said that threaded standpipes are inferior (I've experienced that in threaded risers in an alu holding tank which have snapped off because of corrosion). Now it begs the question - how then do you attach the stopcock to the standpipe if it's not threaded? Right now I can only think of one way and that's a short length of pipe lapping over the standpipe and secured by 2 hose clips???

Forgive me if this is rudimentary stuff. I've sailed for 45 yrs but this is my first aluminium yacht so the learning curve is vertical.