aluminium portholes on a steel boat

[shearwater]

Hello, I need some help with this-please bear in mind that I don't have a background in metallurgy. The problem is this....I have a small steel sailing boat and I want to fit aluminium portholes through the 4mm mild steel topsides. What bolts should I use for the job? I have a weeks holiday starting next week and would very much like to get the job done before winter sets in. Please advise

 

[pauljohn]

I would use stainless steel. With this job strength should trump any corrosion concerns. You can slow down the corrosion considerably by covering the bolts with a sealant (I usually use the same one I bed the porthole in with).

 

[MikeHalloran]

Find some steel portholes and use them.

Aluminum in contact with the steel will cause the steel to disappear in a matter of weeks, maybe less.

You _might_ get away with it if you electrically isolate the portholes from the hull with a couple mm of dielectric everywhere; bushings around the screws, alligator grommet around the hole in the steel, sheets of dielectric between the planar surfaces of the porthole and the hull, on both sides... and you'll need to goop it up with a lot of sealant to get it watertight.



Mike Halloran
Pembroke Pines, FL, USA

 

[Ussuri]

Aluminum in contact with the steel will cause the steel to disappear in a matter of weeks

Surely it is the other way round. The aluminium will act as a sacrificial anode to protect the steel. Seawater acts as the electrolyte. Depending on the size of the steel area protected (the hull) the portholes may corrode quickly.

 

[MikeHalloran]

One time I patched a steel shed roof with aluminum tape. In a week or two, the aluminum was on the shed floor, completely undamaged, and there were patch-shaped holes in the roof. Maybe it depends on the alloys.

Either way, you get holes in the boat.



Mike Halloran
Pembroke Pines, FL, USA

 

[pauljohn]

Aluminum is used on steel boats all the time. I have seen aluminum portholes, hatches, trim parts, deckhouses, decks, railings, gratings etc. on private boats and commercial ships. Also Aluminum masts and spars on steel sail boats all the time. They always suffer some corrosion but is controllable. If corrosion is rampant then it usually signifies bad grounding or charging problems. Insulate from each other and don't let seawater lay on the metals long so lots of gaskets/sealant. Beware of zinc (as in corrugated roof)! Zinc is sacrificial.

 

[EdStainless]

If the Al is an alloy designed for sea water (5xxx) then it usually works. You will need to assure that there can not be any water trapped between the two metals.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[lancer360]

Yes, aluminum is more reactive than steel and in theory would rot away first, BUT aluminum oxide is LESS reactive than steel. Therefore, when the aluminum forms its protective aluminum oxide coating, the steel turns into the anode and rusts away.

If you do decide to use aluminum on a steel hull, make sure you do as much as you can to electrically isolate the two. Also make sure the aluminum has some type of protective coating. It won't corrode as fast as the unprotected steel, put it will still corrode. I would look at anodized with a protective film over the top of that.

 

[RajeshB]

Hi Shearwater,

I see that this post may be too late for your project, but may be helpful in the future.

Bimetallic corrosion between two different metals, in an atmosphere rich in Chlorine (sea side), aluminium will deteriorate faster is a given, but, I have seen superstructures built of Aluminium on steel hulls. This practice is still followed on ships and instead of bolts the Al is generally riveted, ensuring the gaps left between the Ali and steel holes that can accumulate water are almost nil.

Another point to note is use of Gasket (3-4 mm thick Rubber/Nitrile) between the Ali and the Steel surfaces. The port hole may have to be tightened with Steel fasteners at selected points (which could be 4), so that maximum rivetting strength can be achieved. Once all other rivets are in place, the fasteners can then be replaced with rivets. Needless to mention, protective coating of paint will result in delaying the corrossion process.

Hope this helps

Rajesh