Strange discoloration on aluminium hull

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tonish123
Posts: 1
Joined: Wed Jun 29, 2022 12:17 pm
1

Strange discoloration on aluminium hull

#1

Post by tonish123 »

Hey! Just wanted ask a question from people with a lot more experience, what is this discoloration on my boat. Should I be worried? Will there soon be holes?
​​​​​This discoloration was already there when I bought the boat, it just looked little bit more shiny/polished. Those spots are located below waterline in the first half of the boat. If you look closely you can even see them on a big picture. The surface of those spots are smooth, no bumps or hollows. Boat has been in the water total for around 15 hours and every time I finish fishing I pull it out from the water. Now I noticed color of those spots have changed little bit darker and spottier. Boat is equipped with magnesium anode which has oxidised little bit. I will change it to aluminium.
Imagehttps://photos.app.goo.gl/fVjSq3JtLj587JhD6
Photos can be seen on the link above.
 
kmorin
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Joined: Mon Aug 18, 2008 1:37 am
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Location: Kenai, Alaska

Re: Strange discoloration on aluminium hull

#2

Post by kmorin »

tonish123,
this remark is based on the 3rd photo which appears to be 'vertical up'? that is; taken from the ground under the hull looking upward?
From this photo I'd say the boat was formerly on bunks on the trailer instead of rollers that appear in the current photos- and I'm basing that guess on the rough outline shape of a rectangle that approximates the area of discoloration in that image. Or the boat may have been on blocks that were covered with some soaked clothe or board?    Nothing more- just a guess.

That is the "type", kind, manner of discoloration that is the precursor to bunk rot or poultice corrosion (crevice corrosion).  I have no other reason to suggest this than the association of the discoloration's roughly rectangular outline.

The first image doesn't zoom well enough to be of any use inspecting the discoloration and the 2nd image seems like a tarp, or other covering might have been wrapped around the skiff and held water standing against the hull- rain water is slightly acidic (ph of 5.5 to 5.7) so if that water were held to the hull by a wrap/tarp/plastic canvass ? it could begin to produce discoloration as shown.

Just guessing, please don't rely on guesses from thousands of miles away , made on the basis of low rez. photo images online!!  (Why not?) Take some warm vinegar and a scotchbrite (tm) pad and see if you can't just scrub those spots off?  I suspect they'd come off without much effort and underneath will be nice bright, clean aluminum that will then oxidize back to an even gray to match the surrounding hull panels??

By the way, it's good practice to align the rollers that support the hull exactly on framing inside the hull- whether transverse or longs.  The majority of repairs I've seen come from highway travel damage caused by using heavy duty straps to pull the hull down onto the trailer's rollers. If they're not aligned to framing elements inside- a hull panel can be 'flexed' inward to a high degree- then when moved at high way speeds those over-flexed panels are cracked around their edges due to experiencing point loading far higher than could ever be found on the water- unless you hit a piling or very sharp rock. (Even if hit most panels just get one impact but on a trailer every trip could be thousands of deflections- causing cracking just at the toe of an inside fillet next to both longs and x-verse frames)  Just mentioning this impact seen in many, otherwise very sound hulls, I've repaired in the last 40some-odd years.

cheers,
Kevin Morin
Kenai, AK
 
kmorin
Chaps
Donator '09
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Location: Seattle, WA

Re: Strange discoloration on aluminium hull

#3

Post by Chaps »

You don't say where you are so is this boat used in salt or freshwater? I have seen that type of staining associated with the use of magnesium anodes on bare boats used in salt, brackish or dirty fresh water with the location of the staining typically showing on hull metal locations that are in close proximity to magnesium anodes that are attached to the transom (which probably applies in the instance of the corrosion shown located near the black roller in your last photo).

A quote from a good source: "Magnesium is the most active metal on the galvanic scale. It can be used in freshwater, but care must be exercised. Magnesium can over-protect aluminum hulls or outdrives in salt or brackish water or even polluted freshwater, causing the paint to be lifted with resulting corrosion. Even a few hours of immersion can cause severe damage."

Similar issues can also arise on bare aluminum such as what can be seen on your boat.
1987 24' LaConner pilothouse workboat, 225 Suzuki
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bbwilson81
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Joined: Thu Jul 07, 2022 2:20 pm
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Re: Strange discoloration on aluminium hull

#4

Post by bbwilson81 »

I have recently had a similar issue with my boat as well! Recently upgraded and purchased last month, second time on the salt for 3night\4days and it came out looking burnt. Has anyone seen this type of discoloration? Is there anything I should be worried about? Wasn't going to bottom paint but now it looks like it is a most.
 
 
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Chaps
Donator '09
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Re: Strange discoloration on aluminium hull

#5

Post by Chaps »

No harm done, just the nature of bare aluminum that didn't have the mill scale removed. I'll bottom paint it for you! (click on the link below my pic)
1987 24' LaConner pilothouse workboat, 225 Suzuki
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bbwilson81
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Re: Strange discoloration on aluminium hull

#6

Post by bbwilson81 »

Thanks Chaps! Sent you a PM regarding bottom painting... don't have a Facebook account.
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