Aluminum Hull Rib - Bottom Paint and Corrision Control

[Fletcher500]

We just hit the 1 year mark on a new, 11.5 BullFrog, Aluminum hull Dink, with a 30HP Honda.

The waters in San Diego are warm in the Summer and generally active in regards to critters that attach themselves including coral worms. I clean the hull myself, and in order to keep it clean I now have some bare spots on the exterior AL hull. An ablative paint was originally used, which is one of the causes of the bare metal.

The hull did not have any sacrificial anodes on it, so I hung an 8 in. long Zinc off the transom directly coupled to the Aluminum frame. Did it work? I think so.

I am going to bring it to the yard in Q1, 2020 for the following SOW:

• Remove ablative, and apply a non copper based, hard paint.
• Have them drill in to the transom, and install 2 Zincs for cathodic protection. I can then get rid of the hanging zinc and replace the transom mounted ones as needed.

Questions for those who have had experience with Aluminum hulls, and/or are familiar with the chemistry involved in salt water:

1. Is there a particular brand/type of paint you would recommend?
   
2. Thoughts on my proposed Zincs to protect against galvanic corrosion? Would you do anything different?
   
3. This is my first boat with an Al hull, so any other advice is welcome.

Edit - I titled the string RIB, but is a hard shell, PUF filled.

Thanks in advance, Fletch.

 

[Russell Clifton]

You will need to choose a bottom paint made for aluminum. Not all bottom paints are. I also think you need to apply a epoxy primer first. Do a google search : prepping aluminum boat for bottom paint.

 

[C lectric]

Talk to the paint mfgrs. Interlux, Pettit and so on and get the recommendations from people who do this all the time and supply the pros.

Yes, you are likely to need to do a prime coat , you may have to do more than that. But if you dont know what truly needs to be done and the how of it you will have a failure and be redoing it in a year.

Done right the base should last many years.

 

[Bacchus]

I could not find any bottom paints that did not require multi coats of a barrier coat prior to the bottom paint.
Lots of info out there on the correct prep for their bottom paints. Just need to be diligent with prep and periodic inspection to make sure the barrier isn't compromised.

 

[Chrisjs]

Trilux 33 is a good product for aluminum hulls. Barrier coating is a good idea, though I just applied the Trilux directly onto an aluminum run-around. Read the reference below regarding anodes. Use aluminum anodes (not zinc). Even though Navalloy aluminum anodes are more active than unprotected aluminum, I would still expect them to be most effective if there is minimal exposed unprotected aluminum hull.

https://performancemetals.com/pages/how-can-an-aluminum-anode-protect-an-aluminum-boat

 

[Jon H]

I have Seahawk bottom paint on my 28' aluminum boat.

Best to sand blast hull then apply barrier coat then bottom paint.

You must use aluminum alloy anodes otherwise the anodes on your
outboard will be doing all the "work".

A zinc anode on aluminum hull in salt water does not protect at all.

 

[fstbttms]

A zinc anode on aluminum hull in salt water does not protect at all.

100% not true.

 

[stuarth44]

spent good part my life building in alu, here is what works
abrade the metal, get rid of the oxide and mill fiinish
from a plastic stray applicator, wash with fast foaming acid cleaner, hose off
while the suface is still damp, spray with a garnen plastic sprayer alodyne 1200l, from top down the metal will turn bronze, if you want to paint you could use zeca primer then high build 300 microns dft epoxy, then if you used the zeca you can use copper antifoul, if nut the antifoul with cu free, antifoul whilst the epoxy is still soft, ie you can stick a finger nail ino it, but alodyne is the key eh, been used on aircraft since the 1940, pm me if you want more info alloy yachts get a lot of probs, that does not happen if you do as i have said

 

[stuarth44]

100% not true.

yes zincs are essential, but not lots, use one on rudder, one on shaft and one on the hull

 

[kchace]

Why would any "sacrificial anodes" be required on an aluminum hull dink with no other dissimilar metals? For galvanic corrosion to occur, 2 dissimilar metals need to be electrically connected AND immersed in an electrolyte. An aluminum dink without through-hulls, shafts, shore power, etc. is only one piece of metal all by itself in the water. There is no "place" for current to flow from/to. So no galvanic corrosion can occur. No zincs required. Actually, installing a zinc connected to the hull *creates* a situation for galvanic corrosion.

Ken

 

[fstbttms]

Why would any "sacrificial anodes" be required on an aluminum hull dink with no other dissimilar metals?

Two words- “Electrolytic corrosion.”

 

[Chrisjs]

Here is more reading that certainly seems to confirm use of aluminum alloy for anodes on aluminum hulls, written by aluminum boat manufacturer. Aluminum is more active than zinc and will dissolve first. Zincs on outboards primarily protect the steel portions. Our Saildrive (continuously immersed in seawater)used an aluminum anode behind the prop to protect the shaft and prop. We lashed the Saildrive annually with Trilux 33 (versus Micron 33 for the hull) and every couple of years applied additional barrier coat before Trilux.
https://www.duroboat.com/experience/AluminumBoatsandCorrossion.htm