View Full Version : diesel tank material

01-02-2008, 02:44 PM
We're having new tanks made. The 2 old tanks, made in 1931, were 170 gallons each, riveted and soldered copper. Converted from water storage to diesel fuel storage sometime after 1937 and before 1982. Probably in the 1960's.

The old tanks don't leak, hold 5 psi air pressure but they sure look like they're on their last bit of life! Plus, they have a black lining material (might have been put in when the tanks were converted from water to fuel) that probably started out gooey but is now dry and flaking away. What a hassle to get that out.

So, I'm getting bids for new custom tanks (these fit the hull shape under the cockpit seats like the old tanks did) and I'm getting bids with prices all over the place but note a preference for working with AL alloys on the part of all the vendors. I have personal issues with stainless steel in use for tanks as I've seen weld problems and fatigue problems near welds on large stainless steel tanks. So, I'm not really wanting to put in stainless tanks. I hear/read that mild steel is a better choice than AL for diesel tanks ... for example, take alook at this article: http://www.boatus.com/goodoldboat/tanksalot.asp

But, all the vendors are pushing stainless steel (316) or aluminum (5052 mostly, one bid with 5086). When I say "what about mild steel" I get the "well, it'll cost just as much as AL after you've powder coated the steel" and "it'll weigh a lot more than AL or even stainless because I'll have to use heavier mild steel than stainless steel." Well, bottom line is it just seems that no one wants to work with mild steel.

OK, from a weight perspective, I'd love to use aluminum because that's going to be close in weight to the very thin copper tanks which are original. But I keep thinking of corrosion, abrasion if things aren't just so, and, corrosion, and did I mention corrosion?

So, can someone tell me that AL is really OK (someone besides all those people trying to sell me tanks that is!). You won't be able to tell me stainless steel is OK because I've seen too many cases where it wasn't ok.

Thanks, all, looking forward to some responses.

Lew Barrett
01-02-2008, 03:03 PM
You'll be fine with aluminum. Mine look great after eight years.

01-02-2008, 03:03 PM
Since when do mild steel tanks need to be powder coated? I sense someone attempting to maximize cash flow here.

I've seen mild steel diesel tanks 60 and more years old coated with zinc chromate primer or red lead and were in perfectly good working condition.

Have your fabricator include a sump made of schedule 40 pipe or at least heavier material than the tank proper, with a petcock or valve for draining condensate. Your mild steel tanks will outlive your great grandchildren.

When you install the tanks, place Irish Flax, or heavy building paper in the way of all bearing surfaces and chocks. It wouldn't hurt to give the tanks a coat or two of good enamel over the primer as well.

The tanks in my boat, "Ozpup" are mild steel, installed in 1948 and are perfectly serviceable.


Ian McColgin
01-02-2008, 03:14 PM
Granuaile had black iron tanks, diesel and water, that were not coated with anything and were finastkine.

But for your boat aluminum or stainless are super.

01-02-2008, 03:37 PM
Oh, I don't think I was clear, my worry about corrosion and abrasion is if we use aluminum. I actually am less worried about mild steel. The only thing that is making me consider the aluminum is the extra weight of the steel. So, corrosion and 5052 aluminum anybody? aluminum would simply weigh less, thus, that would be nice. These tanks are below the cockpit seats and hanging out there on the counter timber. 2/3 of the tanks are above the water line. So, the weight is somewhat of an issue to me. Therefore, I'm trying to talk myself into aluminum rather than the mild steel. :)

Sven Heesterman
01-02-2008, 04:24 PM
Vetus has a solution that is probably the easiest. It's a flexible tank that is inflated in situ and then a uv light is turned on inside the tank causing a chemical reaction causing the material to harden.

If the tank is not too big it might be forth a look.

If you can find a round stainless tank the fatigue issue won't be as great.


The Bigfella
01-02-2008, 04:38 PM
I had similar tanks on Grantala - 2 x rivetted, plated (Nickel?) brass tanks of 100+ gallons each. I've replaced them with date-expired car 316 stainless LPG tanks. One of my reasons for this was to reduce the problem of stale fuel and another was to be able to get any tank out without ripping the deck up if I needed to.

I can run two of these tanks - one for each engine for all my normal needs. If I want to go further, just fill the outer tanks.

Each tank is about 25 US gallons - and I have six installed. The centre tanks have a rather nice fitting (not shown in this shot) for the fuel lines.


Stu Fyfe
01-02-2008, 06:59 PM
I replaced my steel tank with aluminum about ten years ago. Everything was fine until I had a voltage leak from a bilge pump wire. The aluminum tank developed pits where they came in contact with the tank supports and started to leak. I had the leaks welded and the stray current fixed. When I put the tank back in, I supported it with strips of rubber to avoid chafe and just in case there is ever another stray wire current. It's been fine ever since.

Lew Barrett
01-02-2008, 07:16 PM
Aluminum is superior to black iron in my opinion, and probably would have been more common years ago had the welding techniques and costs been more in line at the time. Don't fret and go ahead. Recently I've made the acquaintance of Starboard and think that might be good padding material, for resting/landing tanks on, if it isn't chemically reactive. Otherwise, Irish felt or the like would work.

01-02-2008, 07:20 PM
Vetus has a solution that is probably the easiest. It's a flexible tank that is inflated in situ and then a uv light is turned on inside the tank causing a chemical reaction causing the material to harden.

I searched high and low for this, sounds like a great idea.
Could you post a link to this product please?

The Bigfella
01-02-2008, 07:57 PM
When I put the tank back in, I supported it with strips of rubber

I forgot to mention that .... When I mounted my tanks, I had straps rolled to suit the tanks - I think it is about 1/4" thick 316 stainless which is about 1.5" wide. They have rubber tank strapping that is a wide U-shaped extrusion that just clips insode. Works brilliantly. Can be seen in the photo above if you look hard.

01-02-2008, 08:23 PM
They have made home heating oil tanks from carbon steel for as long as we have been alive. Highway trucks use aluminum and so do airplanes and fuel delivery trucks. Some of the work boats here on The Chesapeake use aluminum beer kegs for fuel tanks for whatever that is worth.

01-02-2008, 08:34 PM
I went with aluminum for BETTY's new tank. As Lew mentions starboard above, I thought about it or UHMW, but ultimatly went back to Irish felt. It just seemed to me like it would hold it more snug without rubbing. As opposed to slightly moving all the time against the plasticky stuff.

The tank is on Irish felt covered floors. Then after the deck was corked, more felt in between the wood brackets not shown here.



Sven Heesterman
01-03-2008, 12:25 AM
About the flexible tank: I could not find it on their site/catalogue at www.vetus.nl try contacting them directly.


01-03-2008, 08:56 AM
Fisheries (http://www.fisheriessupply.com/online/search.asp?N=0&Ntk=All&Ntt=deisel%20tank&Nty=1&D=deisel%20tank&act=A02&hideprops=1&Dx=mode+matchallpartial&Ntx=mode+matchallpartial+rel+Inactive) in seattle sells the vestus flexible tanks, however they are expensive, about the same as having one built. In my opinion aluminum should be your only choice. Provided they are sufficiantly insulated as stated above, bare and oxidized they will last. Mild steel is fine if weight isn't a concern but they will Always rust, weigh alot more, and be prone to corrosion inside from condensate. In a larger application with a much larger volume of fuel being turned over this isn't a issue, however on a small boat, especially a sailboat where the tank isn't pressed, it is. With the cost of metal through the roof you don't really get the savings you would have gotten before by using mild steel. The only drawback I've seen on aluminum tanks are the port fittings. What ever goes in needs to be properly sealed and probably left in for good. Aluminum pipe threads are not ideal but considering the pros and cons of the other options it's a small drawback. SS is too prone to tiny defects in the build, welds etc, that can lead to leaks down the road. You only want to do this once.

Soldered copper sounds pretty nice as long as the metal is sound and they don't leak???

My 2cents


01-03-2008, 02:54 PM
Dear all--thanks for the replies. I truly was stressing this (I guess I stress on everything until a decision is made)....the "easy way" for me would have been to use mild steel BUT the weight really is an issue. The old tanks were super light weight/thin copper. Don't understand how they actually lasted all these years being so light but really don't want to increase weight so far aft in the boat.

Met with the fabricator (American Tank in La Mesa), we'll be doing them in aluminum. I don't like aluminum but I don't like adding weight for steel...only way seems to be aluminum. (big sigh)... It will be two hull-conforming tanks 172 gallons each (like the old ones). There will be AL pads welded in strips on the outside of the tanks (where the frames support the tanks every 12"). 3x the baffles of the original tanks and reinforced appropriately for the shape and size of the tanks. The old ones were installed with a fir shim/frame on top of the boat's oak frames and had Irish felt between the tank and the "shim". We'll use rubber/ neoprene to isolate the tank on its pads from the frame/shim. These tanks are built in under the cockpit seats and had wood framing across the top of each tank that the seats sat on and that precluded the tanks from moving upwards when heeled. Would expect to use the same system on top of the tanks but isolate with rubber rather than the Irish felt previously used. The aft end of each tank had similar framing, the forward end is against framing that is part of a bulkhead. Lots of places for rubber/pads.

I'm forwarding the fuel line info you all suggested on to hubby who's doing the engine install and figuring out a fuel scrubber. Thanks!

I do really wish that ONE project on this boat was something that I could be thrilled with the "solution" but they all seem to be big compromises in various "lesser of evil" situations. This compromise give us light weight :D and low cost :D but brings me worry about our measures to reduce chafe and corrosion resistance :confused: .

The idea of letting the tanks oxidize and then painting makes sense but probably won't happen because once the guys have hefted those tanks in place, I won't be able to sweet talk them into lifting them out again. Plus, the schedule has hubby putting in all the fittings and everything for the fuel system immediately.

The place where it is most important to be painted would likely be the top of the tanks which is flat and could have water drip down on it (even though the cockpit seats will be in theory be watertight. I can paint the top a couple months after the tanks have been put in place and can access the outboard 1/2 of each tank for inspection after they're installed. The inboard part is directly under the seats. The hull conforming part of the tanks would be hard to get at to paint and inspection would be with mirrors. However, that whole area also is above the water line and shouldn't be sitting in water either. So, I expect raw aluminum except for where I can get at it after install. :rolleyes:

Again, thanks for all your responses and support. I really appreciate the insights alot. Will post pics when we get them.


Jim Ledger
01-03-2008, 03:19 PM
I made a welded aluminum diesel tank of 22 gallons for my catboat out of 3/16" 5052 alloy with two baffles. It sits under the cockpit seat and is partially visible. For this reason I painted it with black coal tar epoxy on the outside, which is holding up well. When I worked at an aluminum boat builders, coal tar epoxy was used for work boat decks and provided a very tough coating. Grinding with a coarse disc immediatly before painting provided sufficient "tooth" for the paint.

The inside needs no coating at all.

01-03-2008, 03:20 PM
Following the advice of my marine fabricator buddies, I painted BETTY's aluminum tank with zinc chromate primner then a couple coats of this ultra nasty two part navy surplus epoxy paint...