A metalurgical question.

[Charlie Jones s/v Tehani]

My boat is outboard powered and the engine lives in a well full time

Takes a wrench and about 30 minutes to get it out, so it stays in the water always. Used to take two people, but I built a harness and can now hoist it using the main sheet, by myself. But still---

The antifouling available for aluminum outdrives today is pretty much worthless. Used to use TBT but that's not available any longer.

Question is this. What if next time I yank it out, I coat the leg and prop with a heavy coating of Coal Tar epoxy ( the stuff used to protect iron oil platforms and buoys) and then apply a regular copper based bottom paint (which DOES work well).

Do you think the epoxy would isolate the aluminum enough form the copper, to stave off electrolysis?? Or should I continue to spend several days twice a year to scrub, sand and respray with the crap available?

 

[RichH]

The exhaust will be under water and will thus be flooding all the internal passages up to the level of the water .... do you plan to coat these internals too? ....
Once thats completed will you be 'spark testing' the coating to insure no water migration/permeation (and of course with its galvanic action) through the coating?
How about the metal shaft seal OD carriers that are in contact with the base metal?


For 'hard' growth have you tried 'Prop Speed', some of the Vinyl Coatings, or similar "Barnacle Barriers'?

 

[woodster]

does your ob have a sacrificial anode if it does keep it up to date and the next time you pull the ob cleane it to bare metal and spray heavily with zinc chromate primer and then and epoxy based primer..and then a hard bottom paint not copper based and plan on at least every two years repaint it with non copper based paint

 

[Hello Below]

Interlux Trilux 33 works well on saildrives. Should do the job on o/b also.

 

[woodster]

The exhaust will be under water and will thus be flooding all the internal passages up to the level of the water .... do you plan to coat these internals too? ....
Once thats completed will you be 'spark testing' the coating to insure no water migration/permeation (and of course with its galvanic action) through the coating?
How about the metal shaft seal OD carriers that are in contact with the base metal?


For 'hard' growth have to tried 'Prop Speed', some of the Vinyl Coatings, or similar "Barnacle Barriers'?

i will also say everything Rich has said is good to know aswell

 

[Charlie Jones s/v Tehani]

Interlux Trilux 33 works well on saildrives. Should do the job on o/b also.

Trilux 33 lasts about 3 months, then it just quits

 

[Charlie Jones s/v Tehani]

Thanks for the ideas. I'll look into them

 

[Allan12210]

The exhaust will be under water and will thus be flooding all the internal passages up to the level of the water .... do you plan to coat these internals too? ....
Once thats completed will you be 'spark testing' the coating to insure no water migration/permeation (and of course with its galvanic action) through the coating?
How about the metal shaft seal OD carriers that are in contact with the base metal?


For 'hard' growth have to tried 'Prop Speed', some of the Vinyl Coatings, or similar "Barnacle Barriers'?

Just thinking out loud, if the copper paint was applied directly to the exterior of the aluminum and the seawater contacted the internals as well as the copper paint, then electrons would flow between the two materials in a circuit. If a thick non conductive barrier coating separated the two, then wouldn't that galvanic action be interrupted? The internals don't need to be coated, but you have to be really sure that there is a continuous barrier coat where the bottom paint can't touch the aluminum at all (especially near the edge of the barrier coat for various penetrations of holes, screws, etc.). Don't let the bottom paint touch a bolted on zinc as the bolt would provide a conductive path through the aluminum. Try it on one of the fins or skeg in a little spot and see what happens. Let the barrier coat extend beyond the bottom paint a little in case there is some surface migration of electrons. You might also try it on an aluminum panel hung from a cleat on the dock to see what happens. Every couple of months wipe the paint down with a piece of short nap carpet.

 

[damauney]

From a metallurgical perspective Zinc Chromate is the best initial coat for subsequent coatings on Aluminum alloys and has been for a long time. The best explanation I have seen as to why is:

"For use on aluminum surfaces above or below the waterline, this primer chemically etches into aluminum and forms a corrosion-resistant surface. Helps to create an even, bondable surface for top coat adhesion. "

Hopes this helps.

Good winds
DaveM

 

[RichH]

From a metallurgical perspective Zinc Chromate is the best initial coat for subsequent coatings on Aluminum alloys and has been for a long time. The best explanation I have seen as to why is:

"For use on aluminum surfaces above or below the waterline, this primer chemically etches into aluminum and forms a corrosion-resistant surface. Helps to create an even, bondable surface for top coat adhesion. "

Hopes this helps.

Good winds
DaveM

Excellent recommendation by Dave M !!!!!!!! such is a 'chem. process industry' standard for aluminum.

 

[caguy]

My Luger 30 also has an outboard in an engine well. It is mounted on a slide track that with a 3:1 purchase block system can be easily lifted out of the water.
Here are some old pictures of the mount.

 

[Charlie Jones s/v Tehani]

Excellent recommendation by Dave M !!!!!!!! such is a 'chem. process industry' standard for aluminum.

And I've used Zinc Chromate on other parts of the housing with good success. Thanks for the memory jog.

Rich H- Also some very good points in your earlier response, about fouling on the internal parts. Have to do some more study on it. Meanwhile, looks like hoist, scrub, repaint and drop back in every 4 - 5 months seems like a plan til I figure it out