aluminium portholes on a steel boat
aluminium portholes on a steel boat
(OP)
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





RE: aluminium portholes on a steel boat
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).
RE: aluminium portholes on a steel boat
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
RE: aluminium portholes on a steel boat
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.
RE: aluminium portholes on a steel boat
Either way, you get holes in the boat.
Mike Halloran
Pembroke Pines, FL, USA
RE: aluminium portholes on a steel boat
RE: aluminium portholes on a steel boat
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Plymouth Tube
RE: aluminium portholes on a steel boat
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.
RE: aluminium portholes on a steel boat
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