How serious is this corrosion?

A Place to Show and Tell
larrytalley
Posts: 5
Joined: Wed Oct 21, 2020 9:33 pm

How serious is this corrosion?

#1

Post by larrytalley »

I am considering buying a used aluminum boat. It is 46' long and was built in 2006. I have the materials sheet and know the alloy and thickness of hull plating and etc. The hull was 5086 alloy and the bottom plates were 0.313".

The boat has been on the hard for about 3 years although I've been told they put it in the water and run it around some once per year.

Before that it was a work boat and saw several hundred hours/year.

The bottom shows signs of corrosion. I didn't see evidence above the water line or on the inside of the hull.

I've put photos in a Google Photos album at https://photos.app.goo.gl/DaRWiybrNuHUZAsi9

Can you identify the most likely cause of this corrosion?
Can you suggest an approach to determining how serious this is?
What investment might be required to solve the problem?

Thanks very much for information.

Larry

User avatar
gandrfab
Posts: 524
Joined: Fri Mar 28, 2008 12:33 pm
Location: Edgewater Fl

Re: How serious is this corrosion?

#2

Post by gandrfab »

Is it pitted, rough, chalky, flaking metal away?

kmorin
Donator 08, 09, 10, 11, 12, 13, 14 15, 16 17, 18, 19, 20
Posts: 1560
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: How serious is this corrosion?

#3

Post by kmorin »

Larry, the word serious when applied to corrosion usually means 'deep'? So your question is "How Deep is this corrosion?" and the answer is 'deeper the worse it is'. Shallow or less than 10% of the matl thickness isn't serious but 1/2 or more is "serious."

The wheels look like they're bronze (IF) so? they're the cause of what we see in the stern photos. Brass/bronze/copper and aluminum are essentially a battery.

Clean the paint and surface muck off... measure the depth and that's how "serious" your corrosion is.

Cheers,
Kevin Morin
Kenai, AK
kmorin

larrytalley
Posts: 5
Joined: Wed Oct 21, 2020 9:33 pm

Re: How serious is this corrosion?

#4

Post by larrytalley »

There is definitely pitting, this isn't just the paint/coating layers.

I picked at it with finger tips and there is surface pitting but nothing comes off on my fingers or seems to move around, nothing feels like it can be flaked off or scraped off. (Well, there might be places where I could flake off some paint by getting at an edge, although I didn't actually find any of those places.) I didn't find any place where the pitting appeared to have depth as much as a 1/16 inch. I think there is pitting with depth of 1/32". I didn't measure with any tools but I'm experienced with woodworking and I can tell the difference between 1/16 and 1/32.

But there is something going on, or perhaps was something going on in the past.

Larry

kmorin
Donator 08, 09, 10, 11, 12, 13, 14 15, 16 17, 18, 19, 20
Posts: 1560
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: How serious is this corrosion?

#5

Post by kmorin »

Larry,
with a sanding disk, like you'd use on wood, and half face APR, using both dust and VOC cartridges or filters, sand a big area flat using a 100 or 120 grit paper. The paper often loads up badly with paint removal so having a rubber "belt cleaning" stick handy is good practice.

Then lay a straight edge over any area and it will help you guage thickness of pits? I realize your eye may be well calibrated already but this exercise is considered the 'simplest' way to be sure.

What is going on is the different metals underwater. In the photos I'm pretty sure I see SS, and Bronze/Brass and the hull is aluminum. Not even sure if the bottom paint was correctly installed? or if it was primed correctly or if the paint is the right (inert) type for aluminum?

So the something that is going on is: every time the battery is put in to a conductive liquid.... galvanic corrosion begins again, with aluminum being the least resistant to 'giving up molecules'!!

Also there's a patch forward on the topsides that may have been the result of some other cause?? But I'm referring to the stern too images on the link above.

it will all sand blast, sand, buff off the stern and by replacing the wheels with passivated SS, and passivating all the remaining SS parts, and making sure the stern plates of the transom are correctly recoated w the right bottom paint- correctly: you will reduce the galvanic corrosion to a maintainable level.

It seemed from your last post you aren't aware that bronze wheels on an aluminum boat are not acceptable practice? Looking at the photos, it seems to me that's what's the main cause of the corrosion shown? Not certain in my opinion from thousands of miles away, but whatever is the problem- it seems clearly centralized around the props' vicinity!

Cheers,
Kevin Morin
Kenai, AK
kmorin

kmorin
Donator 08, 09, 10, 11, 12, 13, 14 15, 16 17, 18, 19, 20
Posts: 1560
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: How serious is this corrosion?

#6

Post by kmorin »

Larry,
as I posted above, it occurred to me that you'd asked about costs/estimates of costs in your original post?

I can only suggest that you contact a local bottom coating service? or a yard where the boat is close by who does this type of work. To do it correctly the bottom should be blasted to white metal and then each of the subsequent films applied. Several product lines out there for aluminum bottom paint with slightly different treatments in sequence.

In SC Alaska where I'm located this work costs more than 1,000.00$ /ft of water line and can be more than 1,500.00$ / ft if the most expensive paint films are used. This may be indicative of your area? OR, the local service base may do the work for 1/2 Ak'an rates? I have no idea.

Further, if you're going to be using the boat full time- I'd suggest new wheels but these 'go-fast' wheels and steering gear are known to be pretty costly, so not sure how to advise you because your use may not be the same as the high speed/sprint boat as was built into the running gear you've shown.

Looks like there have been at least two bottom coatings, but maybe there's a primer coat showing through the lighter blue? Whether its two coats or just one primer and top coat?
The really long term solution is to blast to white metal and start over, while changing out the stern's bi-metal batteries or tending them w special coatings on the aluminum near by?

Cheers,
Kevin Morin
Kenai, AK
kmorin

larrytalley
Posts: 5
Joined: Wed Oct 21, 2020 9:33 pm

Re: How serious is this corrosion?

#7

Post by larrytalley »

I very much appreciate all the information.

The Sea Fury SF 30 drives which are installed on this boat are cast Silicon Bronze (C 87500) drive bodies with S/S 316 Steering Arms and Nibral (AB2) rudders and propellers. These parts are an integrated system and I think I need to figure out how to make them work with the aluminum hull.

I'll correspond with Sea Fury and see what they suggest.

Larry

kmorin
Donator 08, 09, 10, 11, 12, 13, 14 15, 16 17, 18, 19, 20
Posts: 1560
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: How serious is this corrosion?

#8

Post by kmorin »

Larry,
good luck with your boat and the galvanics involved with the drives to hull differential!

These guys (mfg's) must have dealt with it before? But by the looks of the stern of that landing craft? I'd say there are some serious challenges ahead?

cheers,
Kevin Morin
Kenai, AK
kmorin

Chaps
Donator '09
Posts: 2180
Joined: Sun Jan 13, 2008 12:19 am
Location: Seattle, WA

Re: How serious is this corrosion?

#9

Post by Chaps »

I bottom paint lots of aluminum boats here in the Seattle area. Unless the pitting is quite deep (per Kevin) that is a manageable situation. Primarily, it appears that the boat has never been fitted with proper sacrificial anodes. The one anode I see on the transom looks like it is made out of zinc which isn't up to the task of protecting the entirety of that boat.

The entire bottom including all the transom bits (except props & shafts and other SS stuff) needs to be sandblasted clean as suggested above to "white" metal. Then apply one coat of epoxy barrier coat primer that is thinned enough to soak into the pitted areas as well as the rough sandblasted hull plating. Now fill your pitted areas with an epoxy fairing compound and sand off excess once cured. Apply another coat of epoxy primer over the pitted areas then two more coats over everything to end up with about a 10 mil (dry) epoxy primer build-up. Apply your first coat of anti-foul paint over the last coat of primer all on the same day. Apply two more coats of anti-foul at your leisure.

Install a large aluminum hull anode on the transom on bare, clean mounts. Install aluminum prop shaft anodes, install aluminum pancake anodes to the rudders on bare, clean locations. You'll be good to go IMO. That job should run about $10-12k plus anodes at a full service boat yard that has blasting capabilities,

The landing craft pictured here was rough looking like yours, they can be brought back.
IMG_7602.jpg
IMG_7602.jpg (92.31 KiB) Viewed 1049 times
1987 24' LaConner pilothouse workboat, 225 Suzuki
Image
please view and like: https://www.facebook.com/bottompainting/

JonH
Posts: 84
Joined: Fri Jun 10, 2016 12:20 pm
Location: Houston / Ketchikan

Re: How serious is this corrosion?

#10

Post by JonH »

Have you scheduled a sea trial?

Reason I ask is those propellers are of the "cleaver" type which are used in surface piercing (party submerged)
high performance applications. They are mounted high enough to be partly out of the water when on plane.

They are typically very expensive to replace, they are hard to repairs due to the very thin leading edges.

I would inquire as to what a new prop would cost in addition to the sea trial.

Jon

larrytalley
Posts: 5
Joined: Wed Oct 21, 2020 9:33 pm

Re: How serious is this corrosion?

#11

Post by larrytalley »

Good point, I've been told that the props are about $15k each.

I haven't sea trial'ed yet but I have talked to an independent skipper who has driven the boat a lot under a variety of conditions. He impressed me as reliable and unbiased. He says the boat is fast. Also he notes that the behavior is a little weird when docking and etc., but it basically behaves like a boat.

larrytalley
Posts: 5
Joined: Wed Oct 21, 2020 9:33 pm

Re: How serious is this corrosion?

#12

Post by larrytalley »

Thanks all, I sure do value all the knowledgeable advice.

kmorin
Donator 08, 09, 10, 11, 12, 13, 14 15, 16 17, 18, 19, 20
Posts: 1560
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: How serious is this corrosion?

#13

Post by kmorin »

Larry, this is not really on point to your post, or the subsequent discussion of the boat's corrosion issues.

What will you be doing with a landing craft fitted with racing running gear? Are you planning to begin "free-lance 'farma-suit-itcle' imports"? I mean; why does a landing craft need to go 50plus knots?

Not saying there's no reason, just that the reason does seem a remaining question?

15k$ for a wheel? You better be importing for the narc0's to afford one ding on those beauties!! I can't even contemplate what those cast bronze stern housings are worth??

cheers,
Kevin Morin
Kenai, AK
kmorin

User avatar
gandrfab
Posts: 524
Joined: Fri Mar 28, 2008 12:33 pm
Location: Edgewater Fl

Re: How serious is this corrosion?

#14

Post by gandrfab »

I'd like to see how the exhaust is routed on the inside.

!!1.PNG
!!1.PNG (764.69 KiB) Viewed 985 times

Post Reply Previous topicNext topic