OT: Stainless stringers in an aluminum boat?

I am working on a bit of a different project...a 1961 Lone Star CruiseLiner.

This boat started out life as a 23' cabin cruiser, but back in the 90's, someone removed the roof and converted it to an open bow configuration. It's a great boat...looks a bit like some sort of old military setup. We ran it around all last summer on the Ohio River, and aside from some obvious flex (not good on an aluminum boat), it did great for us.

Anyway, from what I understand, this boat spent most of it's life running around the Chesapeake Bay. While the hull itself has zero problems, the stringers are another story. Over the years, about 60-70% of the rivets have pulled out of the transverse stringers, and many of them are cracked. The stringers also appear to have had some corrosion issues....I'm thinking they were a lower grade of aluminum than the hull.

Here's the question:
I plan to replace all the transverse stringers and add some additional longitudinal strength while I'm in there. While I can track down marine grade aluminum for this, I am not really set up for aluminum.

What I am set up for is stainless. We buy 304L in coils and have material to spare. If I used this, I could do anything I wanted with the hull structure of this boat. My concern is galvanic corrosion between the aluminum and stainless, which would be connected by rivets. I know it is possible for this to occur, but I also know that they are used together in many applications, usually with sealer between the two metals.

If I use sealer at the points of contact before setting the stringers into place, and coat the rivets before inserting them, would I really need to be concerned about galvanic corrosion between the 304L and aluminum in my fresh-water application? (on a side note, this boat is stored on a trailer, so nothing will ever see constant immersion in water)


Thanks,
Phillip

I think your ok. In my experience, galvanic reaction between these two materials in contact with one another is very slow. The sealer would not only help with contact but also provide some adhesive action as well. Weight would be my only concern, what gage SS would you use?

Seems like special rivet may be available too that would play nice with both.

Maybe same as a shim between to keep from touching.

omrc7771 said:

I think your ok. In my experience, galvanic reaction between these two materials in contact with one another is very slow. The sealer would not only help with contact but also provide some adhesive action as well. Weight would be my only concern, what gage SS would you use?

Click to expand...

To my surprise, the original formed aluminum stringers were only 16ga. I was planning to use at least 12ga if I went back with the aluminum, but if I use stainless, I will probably go with 14ga just because we have plenty of it.

As far as the weight goes, this hull is 23' long with an 8' beam...and it only weighs around 1200lbs. It could actually stand to weigh more than it does.

Any recommendation on rivets? Aluminum? SS? Other?

Thanks,
Phillip

What are you going to use to buck and set the rivets? I think SS would be the better option for the rivets.

omrc7771 said:

What are you going to use to buck and set the rivets? I think SS would be the better option for the rivets.

Click to expand...

That is another thing I am trying to decide on...closed-end (sealing) blind rivets, or solid rivets?

If I end up going with the blind rivets, I have a nice big rivet gun that makes installation a breeze. If I go with solid rivets, I have pneumatic tools to buck them.
Basically, I can go either way, though I know the solid rivets will take some time to install. There are ALOT of them.

Thanks,
Phillip

How do you seal the blind rivets ?

omrc7771 said:

I think your ok. In my experience, galvanic reaction between these two materials in contact with one another is very slow. The sealer would not only help with contact but also provide some adhesive action as well. Weight would be my only concern, what gage SS would you use?

Click to expand...

I'm just curious as to your experience. I would expect for corrosion to be reasonably bad - especially if you are exposing the two to salt water in an area where you are probably going to wear off the oxide layer of the aluminum.

I've got no direct experience myself, but like I said I would expect significant corrosion.

I have a 47' aluminum sailboat which is used in salt water. I do all the maintenance myself, and have fought battles against a variety of corrosion issues for years. There is no argument that anyone could make which would convince me to do what you propose. Stainless and aluminum in contact in water is a BAD combination. Even out of the water there are a variety of serious issues which can arise. Stainless fasteners into aluminum, for example, will suffer poultice corrosion and/or galvanic issues unless bedded with Tef-Gel or similar product. If you put stainless rivets into an aluminum hull, my expectation would be for total failure of the hull in a few years, as all the rivets corrode the surrounding aluminum, leaving un-fixable (in any easy manner) strings of holes. My experiences suggest that there is no practical, long term sealant that would isolate the metals in a below waterline application such as you propose for any extended length of time. The appropriate material to use is 5086 or 5083 aluminum, at least for offshore sailboats. This may be obtained from Aluminum Distributing in Florida. Otherwise, I suggest consulting a manufacturer of similar boats and finding out their recommendations.

My experience with boats, over 35 years, is that saving money on (the wrong) materials or doing something marginal because it is "easy" is usually a disaster in the long run. The cost of using the right materials and techniques, regardless of their convenience, is small compared to the nightmares which can result from NOT doing the job right the first time. This is the result of bitter experience. Your mileage may vary.....

Best wishes,
Michael

First: riveted boats flex. Lonestars always move around, removing the cabin made it more flexible and easy to see. Welded Al boats and riveted boats flex or break. SS will cause corrosion a lot of places, where the boat sits on the trailer for example. I have been working on AL airboats and john boats for years. JC Rivets is a good source for rivets.

Mark Leigh said:

How do you seal the blind rivets ?

Click to expand...

McMaster-Carr

About 6 years ago a in-law of mine decided to make cutouts in the top of the pontoons on his deck boat
and install bilge pumps. They had small pain in the butt leaks.
He did a really nice job installing stainless plates and access covers out of 304ss ( maybe 18 gauge.)
I don't know what alloy the pontoons were, but within 3 years they were all corroded around the flanges
and rivets....really bad!
Have no idea what he used to seal the flange to the floats.
This was in fresh water about 5 months per year.
David

A neighboring sea farm made oyster racks out of aluminum and stainless. they corroded away in 2 years. Hell of an investment lost.

They told me the aluminum was "marine grade" but didn't tell me which alloy--those are typically 50XX series.

I do not know which grade of SS they used.

They also tried a few racks using aluminum and plain old steel rebar. Those corroded just about as fast.

In both cases the aluminum was the sacrificial metal.

While I have no experience with the combination you are proposing I do have a great deal with marine fasteners made out of 304. I think the stuff is no better than galvanized carbon steel in most marine uses. I am always amazed at how it is touted as the material for saltwater use--its really not very good at resisting corrosion.

Aluminum and stainless in contact will corrode in saltwater. Worse, the aluminum is what will corrode, in this case your hull plating. You will be left with stripes of pitted, corroded aluminum under the stringers. There is no practical way to isolate them. You might get away with it in fresh water for awhile, but in salt water you will notice this immediately.

ferretlegger said:

I have a 47' aluminum sailboat which is used in salt water. I do all the maintenance myself, and have fought battles against a variety of corrosion issues for years. There is no argument that anyone could make which would convince me to do what you propose. Stainless and aluminum in contact in water is a BAD combination. Even out of the water there are a variety of serious issues which can arise. Stainless fasteners into aluminum, for example, will suffer poultice corrosion and/or galvanic issues unless bedded with Tef-Gel or similar product. If you put stainless rivets into an aluminum hull, my expectation would be for total failure of the hull in a few years, as all the rivets corrode the surrounding aluminum, leaving un-fixable (in any easy manner) strings of holes. My experiences suggest that there is no practical, long term sealant that would isolate the metals in a below waterline application such as you propose for any extended length of time. The appropriate material to use is 5086 or 5083 aluminum, at least for offshore sailboats. This may be obtained from Aluminum Distributing in Florida. Otherwise, I suggest consulting a manufacturer of similar boats and finding out their recommendations.

My experience with boats, over 35 years, is that saving money on (the wrong) materials or doing something marginal because it is "easy" is usually a disaster in the long run. The cost of using the right materials and techniques, regardless of their convenience, is small compared to the nightmares which can result from NOT doing the job right the first time. This is the result of bitter experience. Your mileage may vary.....

Best wishes,
Michael

Click to expand...

rbjscott said:

SS will cause corrosion a lot of places, where the boat sits on the trailer for example. I have been working on AL airboats and john boats for years. JC Rivets is a good source for rivets.

Click to expand...

Thank you for presenting these sides of the argument. I posted my question on the PM forum because, in reality, most people on boat forums know nothing about metals...only what they have been told about metals. I thought this would be the best place to find people who actually know what they are talking about. Glad there are some boat guys on here to offer info from an educated standpoint.

It seems like there is a great deal of conflicting information out there regarding the subject of dissimilar metals in marine applications. There are many boat builders putting aluminum on steel hulls, stainless on aluminum hulls, and everything in between. Some say this is bad, others say it's okay if done correctly.

On my particular boat (which is over 60 years old), the only place I have found any corrosion was in these aluminum stringers I am planning to replace. They have corroded, not only around the aluminum (solid) rivets, but also have a fair bit of pitting on any surface that might have been in contact with water at one time or another. The hull, which the stringers are riveted to, shows no signs of corrosion whatsoever. To my surprise, there is not even any corrosion around any existing stainless rivets, stainless brackets, or even the (painted) mild steel eyelets on the bow and stern of the boat.

I guess I'm at a bit of a loss when trying to determine what exactly happened here. Why are my aluminum stringers corroded, while other areas that should have been corroded are fine?

rbjscott said:

First: riveted boats flex. Lonestars always move around, removing the cabin made it more flexible and easy to see. Welded Al boats and riveted boats flex or break.

Click to expand...

Most of the flex in this boat was due to the stringers being cracked/haven broken free from the hull. That said, I want to stiffen it up and remove as much flex as possible.

Thanks,
Phillip

Sea Farmer said:

While I have no experience with the combination you are proposing I do have a great deal with marine fasteners made out of 304. I think the stuff is no better than galvanized carbon steel in most marine uses. I am always amazed at how it is touted as the material for saltwater use--its really not very good at resisting corrosion.

Click to expand...

One of our product lines is stainless steel swimming pool panels. These panels are in constant contact with chlorinated water and more recently, salt water. (with all the salt water systems being used these days) We have found out, over our 60-some years of manufacturing these panels, that we have to use American stainless, because the stuff coming from overseas can't be trusted. That said, to my knowledge, there has never been a corrosion issue on any of our stainless panels, with a great number of the early ones still being in use to this day.

Stainless is definitely better than galvanized carbon steel, but I would only trust it if you get the certifications on it. I would venture to say that most of the "stainless" fasteners coming in from overseas are not exactly what they say they are.

Thanks,
Phillip

David J. said:

About 6 years ago a in-law of mine decided to make cutouts in the top of the pontoons on his deck boat
and install bilge pumps. They had small pain in the butt leaks.
He did a really nice job installing stainless plates and access covers out of 304ss ( maybe 18 gauge.)
I don't know what alloy the pontoons were, but within 3 years they were all corroded around the flanges
and rivets....really bad!
Have no idea what he used to seal the flange to the floats.
This was in fresh water about 5 months per year.
David

Click to expand...

This is pretty interesting. So the flanges of the stainless were corroded...or the flanges on the aluminum?

I guess I can see where stainless/aluminum would have some major issues in constant saltwater submersion...but I'm surprised that the corrosion would be that bad on the topside of freshwater pontoons.

Thanks,
Phillip

Also, what about using magnesium sacrificial anodes on the aluminum? Would this actually prevent any corrosion from occurring?

Thanks for all the replies so far! I may seem like I'm set on using stainless on my aluminum boat...but I'm not. I'm just trying to get as much info as possible since there seems to be so much confusion about how much corrosion actually occurs. I know that SS is passive, aluminum is active, and this would mean the aluminum will always corrode "first". Guess I'm just trying to figure out if that corrosion would be an issue in 5 months, 5 years, 50 years, or 500 years.

No matter what I do, I doubt my boat is going to last another 500 years.

Thanks,
Phillip

Just a bit more info. Any dissimilar metals coming in contact with each other, will cause galvanic corrosion eventually. They will corrode even in dry climate; this is due to the electrolytic action of the 2 metals; they will corrode faster in wet climates, in contact with water or any conductive material, and when left unprotected from the elements. As an example 6061T6 in contact with 2024T3 will corrode, as one has different alloy metals than the other...albeit both aluminums.

The "softer" metal, in your case, aluminum, will become the sacrificial anode and corrode first.
The solution is to separate the 2 metals from contacting each other.
The easy method, although time consuming, is to paint the aluminum with epoxy primer and paint. There's no real advantage to painting stainless, as paint doesn't adhere well to it. For that same reason, it would be better to use stainless rivets, making sure the holes in the aluminum stringers are also protected. This would mean painting the aluminum parts before riveting, but if you intend to keep the boat for a few years, this is time well spent. Epoxy paint is flexible also; we used it on all aircraft painting.

You could then apply a coating of corrosion resistant material, like the stuff used in cars, after the installation is finished, thereby "sealing everything from the elements.This stuff is also flexible. There are other methods also available.
A Google search will give you more info.

Good luck with your project,
Herman

Kentuckydiesel said:

Also, what about using magnesium sacrificial anodes on the aluminum? Would this actually prevent any corrosion from occurring?

Thanks for all the replies so far! I may seem like I'm set on using stainless on my aluminum boat...but I'm not. I'm just trying to get as much info as possible since there seems to be so much confusion about how much corrosion actually occurs. I know that SS is passive, aluminum is active, and this would mean the aluminum will always corrode "first". Guess I'm just trying to figure out if that corrosion would be an issue in 5 months, 5 years, 50 years, or 500 years.

No matter what I do, I doubt my boat is going to last another 500 years.

Thanks,
Phillip

Click to expand...

What you are experiencing on your current aluminum is just plain corrosion. There are different aluminum alloys and different surface treatments. Aluminum naturally should corrode quite a bit. The reason that it does not is that it forms a thin oxide layer which is protective. Anything that removes that oxide layer is going to result in a lot of natural aluminum corrosion.

The reason the aluminum will always corrode first is because it is the anode (the negative side of the galvanic potential).

If you want to create a sacrificial anode you could use something like zinc or magnesium, but you will have to make sure it is hooked up to any mating surfaces between stainless and aluminum in a way that will conduct electricity well. Which is probably not really practical.

While I have no direct experience with stainless and aluminum in a marine application, I would be surprised if your boat lasted more than 5 years.

I am not an expert, but I actually think stainless steels will cause more galvanic corrosion than plain steel. At the very least there are a lot of differences in galvanic potential among the steel alloys.