Fuel Tank Replacement (Mainship 400) Here We Go Again.

[Dougcole]

Three weeks after I finished replacing my port tank, my starboard tank started leaking. Gut punch.

Not sure how many of you followed my thread from April about replacing the leaking port side fuel tank on our 2005 Mainship 400. It was not an easy or inexpensive project, my boat was in the yard for almost a month, and I did about 60% of the work myself. If you did not see it, here is a link:

Mainship 400 Fuel Tank replacement project

OK, this one is not a fun project, not fun at all. Port side tank started leaking in 2018, had an access plate cut in the side and had the tank repaired from the inside, cost about $1,500. It held for 6 1/2 years, so not bad, but it started leaking again in the same spot on the aft bottom a...

www.trawlerforum.com

The yard manager (good guy, very experienced) and I debated on changing out the starboard tank at the same time. It would have saved a bit of trouble, and a little expense long term, but not as much as you may think. In his considered opinion, the starboard tank had a decent chance to last quite a few years longer and since my budget was stretched to its absolute limit, we decided not to change it. Furthermore, The starboard tank did not leak a single drop the entire time the boat was in the yard.

I agreed with him. We were both wrong.

In what may be the worst luck ever, I got the boat back in the slip, all cleaned up, put back together, running perfectly and less than a month later a hole about the size of a dime opened up in the starboard tank, it leaked so badly that I had to have an emergency pump out done ($$$) and ended up with 3 to 4 inches of fuel in my bilge.

So now I am back in the yard again, the boat is chaos again, the starboard engine is out, the floor is torn up again, deja vu all over again.

This time I decided to cut the tank into pieces and will put a slightly smaller tank back in (I'll lose about 20 gallons from a 156 gallon tank). My generator pulls from the port side, so I don't need as much capacity on the stb tank and cutting saved a little bot of work.

The yard pulled the motor yesterday about 7:30 AM, they found three bad motor mounts. My brother and a buddy came over after that and we worked steady until about 6PM. Got the tank cut and loose in the ER. Pulled up the floor pieces, which were already cut from the last tank removal, and got the tank out today before lunch. Spent 4 hours cleaning after that.

I will post pics as I go along if anybody in interested in seeing the progress.

I'm tired.

Doug

 

[Todd R]

Ugh! So sorry for your troubles. Good luck with a speedy repair. You need to be out enjoying your boat.

 

[Dougcole]

Here’s a few pics of the progress so far.

 

[rslifkin]

Out of curiosity, how does Mainship mount the tanks in that boat? Are they just sitting on a flat surface?

 

[Dougcole]

Out of curiosity, how does Mainship mount the tanks in that boat? Are they just sitting on a flat surface?

There are 4 or 5 hard rubber strips under the tanks, about 1/4” thick. On the port side there is a flange on the front of the tank and another flange on the aft inboard side. Those flanges are screwed to a platform.
I don’t think they put enough strips under the tanks, which MAY have contributed to the leak. I put 16 under the port side to keep the tank off of the fiberglass.
On the starboard side there is a flange on the fwd inboard side but not one on the aft end. On the aft end they put a flange on the top across the back of the tank and through bolted it to the engine room bulkhead. Very difficult to see as it is buried behind a wire chase with a big bundle of wires in it. Almost impossible to reach. It took us almost 2 hours to get those two bolts loose.
I’m going to have the tank company put flanges on the inboard ends where they can be reached. That’s what we did on the port side and it worked well.

 

[lwarden]

Sorry you are having to go through this again Doug. I believe some rubber loaded with carbon can react with aluminum when salt water is introduced which will cause corrosion. Might want to look into this and reconsider what the strips are made of or put a barrier coat of epoxy in that location.

 

[DDW]

Anywhere the aluminum touches a surface, salt will collect and accelerate corrosion. I've seen this many times, the tank corrodes from the outside in.

In my opinion the best solution to this is to weld an inverted hat section to tank, which is the only thing contacting the hull. You may need 4 of them on a long tank. The hat section raises the tank about 1/2 - 3/4 inch from the surface, allowing air underneath to dry it, and allowing hosing it out periodically (if you have or suspect water is leaking on/underneath). The hat section itself may corrode for the same reasons the tank did - but it is not keeping fuel in. This is a very inexpensive addition to a tank, either new or used.

 

[Bmarler]

Your previous experience with the other tank is evident by the speed you got this one out. When you’re done you’ll be the go-to guy for mainship fuel tank replacement.

 

[Dougcole]

Anywhere the aluminum touches a surface, salt will collect and accelerate corrosion. I've seen this many times, the tank corrodes from the outside in.

In my opinion the best solution to this is to weld an inverted hat section to tank, which is the only thing contacting the hull. You may need 4 of them on a long tank. The hat section raises the tank about 1/2 - 3/4 inch from the surface, allowing air underneath to dry it, and allowing hosing it out periodically (if you have or suspect water is leaking on/underneath). The hat section itself may corrode for the same reasons the tank did - but it is not keeping fuel in. This is a very inexpensive addition to a tank, either new or used.

The flanges hold the tank up partly, but unfortunately there is very little overhead space, so we can’t lift it much. It’s a good suggestion though. There is no salt on the old tank, it corroded from the inside in the lowest spot. Exact same spot that the port tank leaked.

 

[Dougcole]

Sorry you are having to go through this again Doug. I believe some rubber loaded with carbon can react with aluminum when salt water is introduced which will cause corrosion. Might want to look into this and reconsider what the strips are made of or put a barrier coat of epoxy in that location.

Yes, the rubber strips come from the yard who gets them from the tank company. They are made for this purpose.

 

[Dougcole]

Your previous experience with the other tank is evident by the speed you got this one out. When you’re done you’ll be the go-to guy for mainship fuel tank replacement.

Haha. Just what I always wanted to be!

But yes, doing it the second time is easier in some ways and having the floor already cut makes a big difference.

If they haven’t had it already, all mainship 400s are going to have this same issue within the next five or six years, literally every single one. Hopefully someone will see my posts and that will make it easier for them. That’s why I am posting.

 

[rslifkin]

If the tanks are corroding from the inside out, I'd expect longevity will depend on how much moisture, etc. has made it into the tanks over time. But that's not particularly a Mainship specific issue, as it doesn't appear that the tank installation has contributed to outside-in corrosion.

 

[DDW]

The flanges hold the tank up partly, but unfortunately there is very little overhead space, so we can’t lift it much. It’s a good suggestion though. There is no salt on the old tank, it corroded from the inside in the lowest spot. Exact same spot that the port tank leaked.

Must have had water in it. Bugs too maybe - an aluminum tank won't corrode through from diesel for sure.

 

[Dougcole]

Must have had water in it. Bugs too maybe - an aluminum tank won't corrode through from diesel for sure.

It’s possible, it’s really hard to say exactly what caused it. The spot they both failed is in the very lowest corner, below the level of the main pickup. It’s logical that any water in the tank would have sat there.
I had this discussion with the woman who runs the company that is making my tanks (T and T marine in Ft. Pierce) and she said it is just an age thing. According to her, 20 years is about all you can expect out of a 3/16 aluminum tank.

 

[DDW]

I don't agree - it should last indefinitely if properly built and kept away from corrosives. If you have a diesel injection system than can handle it, it may not be a bad idea to put some additive in that disperses water. Often not a good idea with common rail diesels as they can be very intolerant of even a little water in the fuel.

The other thing I did on my tanks was to make sure there was a lowest point, and added a small 2" deep sump at that location. Directly above it is an 1 1/4 plug fitting, allowing periodic sucking out of the sump. Most of the crud and nearly all of the water end up there.

Also, if they will do it, is to insist that they weld the fillet inside the tank, at least at the bottom. Typically they weld a bead on the outside, the backside of an aluminum weld generally looks pretty gnarly and is unlikely to be fully penetrated, resulting in a very rough corner and probably a crack between the plates where the weld did not penetrate through. Perfect landscape to trap water and junk and corrode. The welded side on the other hand is relatively smooth.

 

[Dougcole]

I don't agree - it should last indefinitely if properly built and kept away from corrosives. If you have a diesel injection system than can handle it, it may not be a bad idea to put some additive in that disperses water. Often not a good idea with common rail diesels as they can be very intolerant of even a little water in the fuel.

The other thing I did on my tanks was to make sure there was a lowest point, and added a small 2" deep sump at that location. Directly above it is an 1 1/4 plug fitting, allowing periodic sucking out of the sump. Most of the crud and nearly all of the water end up there.

Also, if they will do it, is to insist that they weld the fillet inside the tank, at least at the bottom. Typically they weld a bead on the outside, the backside of an aluminum weld generally looks pretty gnarly and is unlikely to be fully penetrated, resulting in a very rough corner and probably a crack between the plates where the weld did not penetrate through. Perfect landscape to trap water and junk and corrode. The welded side on the other hand is relatively smooth.

I've had the boat for 13 years now, and have put quite a few hours on it. I've used various additives consistently over that time, mostly Valve Tech Bio Guard, but also Biobor. Is that what you mean by a water dispersing additive?

I have never clogged a single filter since I have owned the boat. I change my Racors at 200 hour intervals, I used to do them at 100 hours, but they were never dirty, so I pushed it to 200. They still look great, even at the longer intervals. The inside of the tank, which I cut open so I can see it clearly, is mostly clean. A little gunk in the low spots, maybe two handfuls for the entire tank. There wasn't much water in either one.

All 20 year old tanks get some water in them, it's inevitable. I think the issue with these is that what gets in sits in the lowest corner just below the pickup and does not get pushed through the racors. There is no way to get that final few ounces out. A low sump with a drain on it would be great, but there really isn't room to do that on my boat.

I will ask the tank company about the welds on the inside and also ask them to push the pickup as low as they can get it. Thanks for your suggestions.

Doug

 

[DDW]

There are some additives that essentially dissolve the water into the diesel fuel. It's still there, just made more soluble and dispersed through it, so that it gets pumped through the filters and engine. Old style mechanical injection it just goes through and gets burned in the engine.

It is common to have the pickups a couple of inches off the bottom of the tank, so anything in there that can settle (crud and water) sits there and builds up for years. With my little sumps, the pickups when right to the bottom of the tank proper, just not into the sumps which hold about a pint or so. So when you suck the tank dry it really is, except for that pint. With a flat bottomed tank this is not possible. The tanks I am talking about were custom built for my sailboat. My trawler has flat bottomed aluminum tanks, and rather than top pickup tubes, it has exit fittings welded near the bottom. You can't get a threaded fitting welded in tangent to the bottom, so it results in the same thing - an inch or so of fuel (and other stuff) that never really gets turned over. The only recourse then is a tank cleaning which can be done with fairly cheap equipment, if you have good access covers. But there again, most don't, or have sections of tanks hidden behind baffles. Unfortunately, most builders - even the high end ones - don't think about the guy stuck with the boat 10 or 20 years on. Again on the sailboat (which was custom built) I insisted on the tanks being installed after all the cabinetry was in place. I know they can be removed without any destruction because they were put in that way. On the trawler, they were put in before the deck went on, same situation as yours.

 

[Comodave]

I would have the new tanks made out of thicker aluminum. That way it will take longer to corrode through.

 

[Blissboat]

Painful. So sorry you are having to go though this, Doug. I have always believed that nothing beats aluminum for fuel tanks, so now it's dispiriting to consider your supplier's opinion that 3/16 aluminum tanks age out at twenty years. But, your experience seems to bear her out.

As to retrofitting tanks, DDW wrote (above) that, "Unfortunately, most builders - even the high end ones - don't think about the guy stuck with the boat 10 or 20 years on," and man that's the truth.

 

[rslifkin]

Painful. So sorry you are having to go though this, Doug. I have always believed that nothing beats aluminum for fuel tanks, so now it's dispiriting to consider your supplier's opinion that 3/16 aluminum tanks age out at twenty years. But, your experience seems to bear her out.

As to retrofitting tanks, DDW wrote (above) that, "Unfortunately, most builders - even the high end ones - don't think about the guy stuck with the boat 10 or 20 years on," and man that's the truth.

Aluminum is a good material for fuel tanks, but if they're not kept dry enough they will eventually corrode. How long it takes will depend on thickness and the grade of aluminum used.

My fuel tanks are nearly 40 years old and made of .190" thick aluminum. Can't remember the grade off hand. Mine hold gas, not diesel. But when I had the tanks moved a few years ago to replace a water tank they looked great on the outside, and from what I could see inside when I replaced the level senders, they looked good in there too. And I've yet to hear any other other of the same boat report an issue with the fuel tanks (which unlike the original water and waste tanks are actually very well installed).

 

[hollywood8118]

It may be a flex/hard point issue but I have always wondered why aluminum tank mfgs don't add a 1/4"+ thick angle to the bottom to sidewall transitions of tanks where they contact the hull. I would think the additional metal would add additional strength and years of additional corrosion protection. It seems most fail due to corrosion from water at the places they touch fiberglass/wood and even plastic stripping, not just in "free space".
Hollywood

 

[Heap]

"3/16 aluminum tanks age out at twenty years"
Thanks. Now I feel like I'm whistling past the graveyard.
Every spring when I start up after the winter, I douse my engines with Spray 9, then perform a fresh water washdown in the engine room and engines from the deck to the bilge. I run a large fan in the engine room afterwards to promote drying. This flushes under the fuel tanks and results in clean engines and a clean bilge so I can spot leaks easily. I also put Lucas Oil or Power Service in the tanks every fill up. Finally, I treat with AJX about every year and a half no matter what my filters look like.
Hopefully this will keep me whistling a couple years more.

 

[DDW]

The tanks on my sailboat are 3/16 aluminum, now 17 years old. They look perfectly new, inside and out. But also were built as I mentioned, and the access cover allows inspection of every corner. I would expect properly built and maintained aluminum tanks to last longer than any owner.

 

[Dougcole]

Here are some pics of what is left of the tank. The fitting next to my hand (I realize my fingernails are dirty, I’ve been changing a diesel tank) is the pickup, it is in the aft, bottom, inboard corner of the tanks. It’s 1 to 2 inches off the bottom of the tank.

Pic two is the inside of the tank, but it is in the forward end, so I don’t know how much it tells us. It is quite clean though, zero gunk. A lot of the “grime” in that part is the shavings from cutting the tank.

Pick three shows the leaks. Very close to the pickup, the small one is on the bottom of the tank, the big one is on the end. The big one clearly corroded from the inside. I can’t say for certain what caused the small one on the bottom. There is another spot an inch or so from the small leak that was corroding from the outside, but hadn’t popped through yet.

 

[Dougcole]

Here are picks of the mounting system. Picks one and two show the flange on the front end of the tank (it was not screwed down, thank God).

The aft end has no flanges at all, the tank sat on a rubber strip there.

Those flanges lift one side of the tank a tiny bit, perhaps 1/8”. So the front of the tank MAY have been held off the fiberglass by a hair.

I’ll let y’all draw your own conclusions.

Im putting flanges on both ends and the inboard corners.

 

[Dougcole]

"3/16 aluminum tanks age out at twenty years"
Thanks. Now I feel like I'm whistling past the graveyard.
Every spring when I start up after the winter, I douse my engines with Spray 9, then perform a fresh water washdown in the engine room and engines from the deck to the bilge. I run a large fan in the engine room afterwards to promote drying. This flushes under the fuel tanks and results in clean engines and a clean bilge so I can spot leaks easily. I also put Lucas Oil or Power Service in the tanks every fill up. Finally, I treat with AJX about every year and a half no matter what my filters look like.
Hopefully this will keep me whistling a couple years more.

Heck, your tanks have already lasted a year longer than mine, since your boat is an '04 and mine is an '05. We have followed the same basic maintenance schedule/practices, but maybe Mainship installed your tanks a little different or maybe you just got better fuel over the years.
I hope they hold out for at least as long as you own the boat. If not, I'll be here to consult.

Doug

 

[jleonard]

What if you had another tjreaded fitting installed next to the one for the pickup. Plug it then you could use a squeeze ball rig to occasionally go thru that hole and suck any water out of that corner,
Just a thought

 

[Dougcole]

What if you had another tjreaded fitting installed next to the one for the pickup. Plug it then you could use a squeeze ball rig to occasionally go thru that hole and suck any water out of that corner,
Just a thought

I did something similar on the port tank, I had them install a 2" fill with a screw on cap on the top of the tank above that corner. I have a cheap harbor freight 12V transfer pump that i rigged with a dip tube that I can stick down to the bottom of the tank. I'll do the same for this tank.

My original idea was to use it for access when the tank needed to be pumped out, as there is not room on top to reach a full access plate, but it would work for water removal too.

I've already used the fill cap on the port tank. I had to get the boat from my home slip to the yard, and since I am way up a very narrow twisty river I need both motors running. The starboard motor pulls from and returns to the starboard tank, which was empty (and leaking). I was able to close the pickup on the stb tank, open the crossover valve and get fuel for both motors from the port side, but I had to run a temporary hose into that fill cap for the return. It worked perfectly.

Doug

 

[Comodave]

Any rubber strips need to be glued onto the tank completely so no moisture can get between the rubber and the tank. I would prefer to have the tank raised up some more to let air under the tank so it can dry if any water gets under it.

 

[TBill36]

My diesel tanks have ports on the bottom. Not sure if it’s ABYC approved to do it that way, but no water can accumulate in the bottom of the tanks. It’s a production boat with 1000s made that way. On another forum I hear of occasional tank issues, but many 15-40 year old boats running around with original tanks.