Through hull fittings on Aluminium sailboat

[Wideocean7]

Hi All

I hope that everyone is well in this potty world we currently live in. It's occurred to me for many years how fortunate we are to have found a pastime/career/lifestyle such as sailing. The amount one can learn is limitless, unlike many other sports (or careers/pastimes). I for one am amazed at how little I know about so many aspects of a sailboat, particularly those that venture far afield. Anyway, enough of the naval gazing.

Having now bought the aluminium cutter of my dreams, I am putting together a maintenance plan for when she's next slipped. I've read a lot about through-hulls and stop cocks and would much appreciate your hands on comments.

There's a real mish-mash of types of through-hulls and stop-cocks. Some show a reasonable level of corrosion (not too good). Ideally, whilst making sure their integrity is 100%, I'd like similar fittings when I next slip her.

I've attached (or at least I hope I have) some photos of the questionable fittings associated with the head waste and holding tank. Do you have any suggestions on what I should do with them?

Attachment 1. shows a heavily taped marelon fitting (black water discharge). Maybe a cracked or poorly matched fitting.
Attachment 2. S/S stop cock for macerator discharge.
Attachment 3. Corroding valve on discharge elbow.
Attachment 4. Multi-metal corrosion on raw water inlet.

There are also 2 skin fittings with plywood backing plates. I don't have photos but my surveyor advises that the plates be replaced (as the ply may cause paultice corrosion) . I don't yet know with what, given it's an aluminium hull. Suggestions here also very welcome.

Thanks very much! Wideocean7/Andy





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Raw water inlet.JPG Views: 160 Size: 119.6 KB ID: 225276" style="margin: 2px" />

[Michael Cobbe]

Use nylon fittings https://shop.tnorrismarine.co.uk/pro...SABEgIAnfD_BwE

[Sherpa17]

Perhaps G10 backing plates put down with West System Gflex would help isolate the fittings from the hull should you be limited to metallic thru hulls. Gflex is formulated for aluminum and G10 flat stock is non -conductive and easy to cut and shape.

[double u]

if you are aware of intricacies of corrosion as "poultice cor." you are already way ahead.
On the french Alum. Sloop we owned for 10 years & rtw-ed twice all through-hulls were Polypropylene ball-valves (French) screwed directly onto tubes that were welded to the hull. One area with zero corrosion or maintenance probs.

[GordMay]

Poultice corrosion can occur when stagnent (oxygen-free) water is trapped against aluminum, such as under (porous) wood backers.
In this case, the wet wood backing plate is the poultice, and aluminum hydroxide (showing white powder-like deposits/blooms) would be the result.
Bare aluminum must be kept clean and dry, and free of any wicking (absorbent) materials.

[Wideocean7]

Thanks All. I had been lead to believe that an internal “plate” is necessary to spread the load. Is this right? If so, the fittings suggested by Michael wouldn’t do the trick (or would they?). Also. would any backing plate need to be through-bolted as I hate the idea of yet more holes in the hull. Finally, why would G10 plated suggested by Sherpa17 be any better in circumventing poultice corrosion that ply?

Thanks again. Andy.

[Benz]

^^G10 is non-porous, so it wouldn't hold water against the hull. You would of course pot it with Pro-set so there was no gap.
But the best solution has already been mentioned: welded-on tubes with threads at the end on which you put a seacock. As long as you have a good welder, I don't know of a better way to do through hulls in aluminum. I have the same system in my boat, only with fiberglass tubes glassed to the fiberglass hull. The tubes go above the waterline before the ball valve happens, so a failure there can't sink the boat.

[Sherpa17]

randex.fr supplies CNB which makes aluminum boats.

Originally Posted by double u

if you are aware of intricacies of corrosion as "poultice cor." you are already way ahead.
On the french Alum. Sloop we owned for 10 years & rtw-ed twice all through-hulls were Polypropylene ball-valves (French) screwed directly onto tubes that were welded to the hull. One area with zero corrosion or maintenance probs.

[double u]

yep, these ballvalves. boat was Pouvreau built

[double u]

chapeau, Sherpa17, btw!

[Matt Johnson]

Most of our thru hulls were welded on pipe nipples as shown, but I did have two that required Marlon thru hulls - engine intake and transducer. For those we used FRP green board (cheaper than G10) and West System G-flex to adhere the backing plate to the freshly abraded aluminum hull. This works extremely well.

All seacocks and hose barbs were Marlon until at least above the waterline.

If you are going through the trouble of welding on new pipe nipples as thru hulls, see if you can extend those to above the waterline to make stand pipes. You are then free to worry about other things sinking the boat besides a failed rubber hose Keep in mind that stand pipes usually require additional vertical support besides just the circumference weld and should still have a valve on top.

Matt

[Stewie12]

Remember the table of metals, which is very important on an aluminium boat. I would go with the nylon or anything else that is not metal unless it is monel coated.
Same when you screw a metal fitting to the superstructure. Insúltate it and use plastic to insulate the screws.

[Glassdog]

Matt Johnson always has good technical ideas and engineering help. I’m curious to know where his sources his FRP green board and what the actual product name is, cheers

[bobgarrett]

Trudesign thru hulls.

[northwestsailor]

Our two Joubert designed Meta built aluminum hulls have all stand pipes for all water entering or exiting the boat. This is good in almost all cases except where you are expecting to freeze in for the winter then care must be taken using one of several methods.