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

[slowgoesit]

Since both of your tanks appeared to have failed from the inside out, I wonder if the application of a tank sealer to the inside of the tank, such as is regularly used on aluminum aircraft "wet wing" fuel tanks would have prevented the damage, or at least slowed it down enough to extend the life of the tank.
These coatings are used on wet wings with aluminum skin that is typically 0.038" to 0.050" thick.
By comparison, 3/16" in decimal thickness is 0.1875".
So your 3/16" fuel tank starts out almost FIVE TIMES thicker than the typical wet wing skin, yet still only lasts about the same number of years as a wet wing before resealing is required. (Typical wet wings go for 20 to 40 years before requiring resealing . . . )

So, assuming the sealer lasts only the amount of time it does on aircraft, it would STILL be at least 20 years before the 0.187" aluminum (3/16") would BEGIN to be attacked by whatever is eating through your tanks . . .

It is definitely something I would consider . . .

 

[heysteve]

Perhaps those of us with aluminum tanks over 20 years old could form a club. We could get baseball caps embroidered with "Living on Borrowed Time" and establish a support number to call when things go south. Maybe buy a good bottle of booze to be given to the last owner with original tanks.

Also, if you haven't already done it, check the O-rings on your fill caps. An easy place for water to enter.

 

[rslifkin]

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.

Ports on the bottom is fine for a diesel tank. It's only disallowed for gas.

 

[lwarden]

Doug, maybe it’s just me but I think there was standing water in that corner and the holes came from the outside in. There seems to be a wedge shaped area of a telltale contact with salt water. Is there any metallic component that was in the vicinity of that pocket of water?

 

[Mac2]

Just a thought for down the road: drill the 2” fill cap and add a pickup tube that goes all the way down to the problem area. A cheap fuel polishing setup would make it easier for you to maintain this area along with future dirty fuel issues. The return port on the opposite end of the tank could be addressed now as well. I did this on my last boat. The access cover was easy as I could remove to drill, but the hole I drilled in the opposite end of the tank didn’t have an access cover, so I had to be careful to limit the shavings going into the tank. I used a walbro pump, racor fuel filter and a timer. Thanks for posting your tribulations. It will help many others down the road.

 

[Dougcole]

Doug, maybe it’s just me but I think there was standing water in that corner and the holes came from the outside in. There seems to be a wedge shaped area of a telltale contact with salt water. Is there any metallic component that was in the vicinity of that pocket of water?

No, there wasn't any water sitting in that area on either tank. The ER on my boat is very dry, I wash it out with fresh water and soap about once a year, but other than that the only water that gets in is the tiny amount that leaks out when I change impellers.

It's pretty clear that the leak on the aft end of the tank corroded through from the inside, nothing touches the end, and it has never been exposed to salt water in the 13 years I have owned the boat.

The leak on the bottom may have corroded from the outside in, it's hard to tell.

 

[DDW]

It is quite possible it corroded from both sides. If it is sitting on something, it is unlikely ever to dry at that point. If it were my tank, I'd make sure there was a low point by tilting the bottom slightly and adding supports (of welded on aluminum) where necessary. Doesn't need much tilt. If you can get an air gap underneath of any amount, so much better. Then put the port at the bottom corner and make sure it is flush on the inside - no dam to retain a puddle. This will do two important things: heavy stuff like muck and water will go out through the system and get trapped by the filters. Diesel injection systems (especially common rail, but even mechanical) turn over more fuel than they use, effectively polishing the fuel while you drive. But only if the bad stuff can get out the outlet. Second thing, is that if you are explicitly flushing the tank to polish the fuel, it is far easier to persuade the stuff you are trying to flush to go downhill, than uphill.

 

[Dougcole]

It is quite possible it corroded from both sides. If it is sitting on something, it is unlikely ever to dry at that point. If it were my tank, I'd make sure there was a low point by tilting the bottom slightly and adding supports (of welded on aluminum) where necessary. Doesn't need much tilt. If you can get an air gap underneath of any amount, so much better. Then put the port at the bottom corner and make sure it is flush on the inside - no dam to retain a puddle. This will do two important things: heavy stuff like muck and water will go out through the system and get trapped by the filters. Diesel injection systems (especially common rail, but even mechanical) turn over more fuel than they use, effectively polishing the fuel while you drive. But only if the bad stuff can get out the outlet. Second thing, is that if you are explicitly flushing the tank to polish the fuel, it is far easier to persuade the stuff you are trying to flush to go downhill, than uphill.

That's pretty much my plan with this tank, at least as much as possible. The inboard aft corner of the tank (where they leaked) is the lowest spot.

I don't have room under the tank for a sump, unfortunately. What do you think about having them install a pickup tube int the top of the tank, right over that bottom corner that goes as close to the bottom as possible? I could put a plug in the pickup, and just attach a pump to it to suck the water/crud out periodically using a transfer pump? I was going to put the cap there, but it would be easier to have a pickup already in place. I still want the cap, I'd just move it further forward.

 

[Dougcole]

An interesting clue I discoverer, that I think deserves its own post.

I was reading the label on the old tank, and it lists the thickness as .125 (1/8"). My new tank is listed at .190 (3/16"), so these old tanks are 1/3 thinner than the new ones. I'm not saying that is the sole cause of this issue, but it sure didn't help.

I'd be interested to know if this is the case with other Mainship 400s, I bet it is.

Doug

 

[Comodave]

An interesting clue I discoverer, that I think deserves its own post.

I was reading the label on the old tank, and it lists the thickness as .125 (1/8"). My new tank is listed at .190 (3/16"), so these old tanks are 1/3 thinner than the new ones. I'm not saying that is the sole cause of this issue, but it sure didn't help.

I'd be interested to know if this is the case with other Mainship 400s, I bet it is.

Doug

Absolutely thicker tanks will last longer. And the cost differential isn’t that much. I would have the new tanks built out of 1/4”.

 

[lwarden]

This area inside the red triangle shows what looks like corrosion and contact with some sort of corrosive agent. I had this same issue on my Shamrock and it was a limber hole that was set up a bit and left a small pool that the tank sat in. It had holes just like this.

 

[DDW]

I could put a plug in the pickup, and just attach a pump to it to suck the water/crud out periodically using a transfer pump? I was going to put the cap there, but it would be easier to have a pickup already in place. I still want the cap, I'd just move it further forward.

Having some way to get a suck tube to the lowest point in the tank is worth it, if you are diligent about using it.

This area inside the red triangle shows what looks like corrosion and contact with some sort of corrosive agent. I had this same issue on my Shamrock and it was a limber hole that was set up a bit and left a small pool that the tank sat in. It had holes just like this.

I had a sailboat tank like that years ato. Odd shape, low corner sat in a little recess in the stringers and frames, and was constantly wet with salt water - no limber. I pulled it for another reason, and saw it was half corroded through. I just cut that little triangle off and it welded off, also drilled a limber hole. When I put the tank back, I set it on DriDek, allowed me to raise it up about 1/4, which allowed hosing periodically and drying. DriDek top surface is tiny little cylinders (but a lot of them) so a very short distance to drying air.

 

[jleonard]

You could use a simple outboard engine squeezeball to do a periodic sludge/ water removal. Real estate asy to set up.

 

[DDW]

Many of us have a suction pump on board for oil changes. That also works to suck the bottom of the tank.

 

[Dougcole]

I have a 12v diesel transfer pump, I will use it suction out the tank.

 

[Comodave]

You could use a simple outboard engine squeezeball to do a periodic sludge/ water removal. Real estate asy to set up.

That seems the way to go since the amount of water should be small. You could leave the bulb permanently installed with a shutoff valve. Then it would be very quick and easy to pump out the water, if any.

 

[DDW]

If you have a transfer pump, you can plumb in an inexpensive filter and polish the fuel yourself. I've built just such a rig. Wait until you are less than half fuel, then use the pump to pump (and filter) the fuel from one tank to the other until it is empty. Then transfer it back again to now empty (and clean) tank. If the tank is pretty dirty, when there is still a gallon or two in it, run the pump output to a rigid pipe that you can direct around the tank to flush the remaining crud towards the pickup in the same tank.

 

[Dougcole]

If you have a transfer pump, you can plumb in an inexpensive filter and polish the fuel yourself. I've built just such a rig. Wait until you are less than half fuel, then use the pump to pump (and filter) the fuel from one tank to the other until it is empty. Then transfer it back again to now empty (and clean) tank. If the tank is pretty dirty, when there is still a gallon or two in it, run the pump output to a rigid pipe that you can direct around the tank to flush the remaining crud towards the pickup in the same tank.

I like this idea a lot. I've seen plenty of portable polishing systems, it would be easy enough to rig something up. I am having a fill cap installed on this tank, so I could run it from the pickup tube through the filter and back into the fill cap.

 

[Dougcole]

Dropped the remains of the old tank off yesterday at the fabricator ( T N T in Fort Pierce) nice people and very knowledgeable. They built my port tank as well.

I showed him the leaks. His opinion is that they were caused by some sort of electrolysis where there was very slight humidity where the tank was touching the rubber pad. My tanks are bonded and I am not having any other stray current issues on the boat (knock on wood) but he thinks maybe it could be a weak connection to the bonding grid, or just from a tiny bit of moisture in that spot. He agreed that the tank does not, and has not, sat in any sort of puddle nor is there any evidence of any salt anywhere on the tank.

Needless to say, I will check the bonding system very carefully before install.

He also thought that the water doesn't sit in that lowest corner in the rear of the tank. He said the tank "sloshes around and the water gets trapped in different places, not all concentrated in one spot."

Lastly, he looked at the old tank and said "Jesus, this is super thin and they made a lot of mistakes, this construction really sucks."

Not sure I 100% agree with everything he said, but putting it up for informational purposes.

 

[rslifkin]

Thin, poorly built tanks certainly don't help lifespan.

As far as moisture, in some weather the tanks will develop condensation on the outsides. Cool spring weather followed by a hot, humid day (while the tanks are still cold) will usually do it. So how the tank is mounted and whether that condensation can dry effectively matters. If parts of the tank don't breathe well and stay wet for a week, that's certainly not helping anything.

 

[Dougcole]

Thin, poorly built tanks certainly don't help lifespan.

As far as moisture, in some weather the tanks will develop condensation on the outsides. Cool spring weather followed by a hot, humid day (while the tanks are still cold) will usually do it. So how the tank is mounted and whether that condensation can dry effectively matters. If parts of the tank don't breathe well and stay wet for a week, that's certainly not helping anything.

Yes, I have always heard about that theory as to how tanks get condensation on the inside, which is why many people recommend keeping your tanks full. Others say otherwise and that it is best to turn the fuel regularly, rather than storing a large quantity of fuel for an extended time.

I understand the concept, it makes a lot of sense, but I've never seen my tanks get wet on the outside. Seems more likely that they got damp underneath from me cleaning the ER and because there is not much space under them they didn't dry.

 

[rslifkin]

Yes, I have always heard about that theory as to how tanks get condensation on the inside, which is why many people recommend keeping your tanks full. Others say otherwise and that it is best to turn the fuel regularly, rather than storing a large quantity of fuel for an extended time.

I understand the concept, it makes a lot of sense, but I've never seen my tanks get wet on the outside. Seems more likely that they got damp underneath from me cleaning the ER and because there is not much space under them they didn't dry.

I don't see mine wet on the outside often, but in the spring it happens occasionally. When we get a sudden 70 degree and humid day after a while of being in the 40s, the tank temperature ends up below the dew point, so the tanks are dripping wet.

 

[Mac2]

I like this idea a lot. I've seen plenty of portable polishing systems, it would be easy enough to rig something up. I am having a fill cap installed on this tank, so I could run it from the pickup tube through the filter and back into the fill cap.

rpelton is selling a really nice fuel polishing system in the general classified for $500. You can’t go wrong at that price.

 

[Dougcole]

I don't see mine wet on the outside often, but in the spring it happens occasionally. When we get a sudden 70 degree and humid day after a while of being in the 40s, the tank temperature ends up below the dew point, so the tanks are dripping wet.

That never happens in South Florida where our boat lives.

Doug

 

[Blissboat]

That never happens in South Florida where our boat lives.

Doug

Doug, somehow I had the notion that you and Morgan are over in the panhandle, near St. Marks, FL. Did that used to be the case? Or am I getting you mixed up with a different T.F.er with a Mainship?

I regret that with the rollout of the new TF platform, avatars no longer show where a member is located unless they go back and edit their profiles, which few have done. Location adds helpful context to any post, IMO.

I was feeling sorry for you schlepping your tank all the way from near Tallahassee to Ft. Pierce, but apparently my sympathy can be reserved for other things . . .

 

[Dougcole]

Doug, somehow I had the notion that you and Morgan are over in the panhandle, near St. Marks, FL. Did that used to be the case? Or am I getting you mixed up with a different T.F.er with a Mainship?

I regret that with the rollout of the new TF platform, avatars no longer show where a member is located unless they go back and edit their profiles, which few have done. Location adds helpful context to any post, IMO.

I was feeling sorry for you schlepping your tank all the way from near Tallahassee to Ft. Pierce, but apparently my sympathy can be reserved for other things . . .

Good memory.

Yes, I live near St. Marks, and we kept the boat in Carrabelle for a few years, also our old boat before that. We have a flats skiff that we use here in Wakulla but found we primarily use the big boat on our extended Bahamas trips, so we moved the boat to Stuart in 2017 or so. I also work down is South Florida a lot, so I use it as a home base when I am down there.

 

[boomerang]

Doug,I just have so speak up and say that I can't imagine the gut-punch you experienced after tank # 2 developed a leak but what the heck are you gonna do other than keep pushing on. Yours is certainly one of the most painful threads on here. I'd be embarrassed to post any of the complaints I have about the ongoing repairs on our boat...they'd pale in comparison to your issue.

 

[DDW]

This is why I think having the tank sit on rubber strips, or anything else, is a bad idea. That spot isn't going to dry easily if ever, and will collect salt - everything on a boat does - and the concentration of salt increases with time. Once salty it won't dry ever. So you want the points of contact very small (shortest possible path to air) or sacrificial (the hat sections I mentioned). The first can be accomplished with DriDek, each point is no more than 1/8" diameter so the path to nearest air is 1/16, and it will dry. Also it leaves air space underneath, allowing you to hose it out (to decrease the salt concentration) periodically. The material seems unaffected by diesel.

 

[Dougcole]

This is why I think having the tank sit on rubber strips, or anything else, is a bad idea. That spot isn't going to dry easily if ever, and will collect salt - everything on a boat does - and the concentration of salt increases with time. Once salty it won't dry ever. So you want the points of contact very small (shortest possible path to air) or sacrificial (the hat sections I mentioned). The first can be accomplished with DriDek, each point is no more than 1/8" diameter so the path to nearest air is 1/16, and it will dry. Also it leaves air space underneath, allowing you to hose it out (to decrease the salt concentration) periodically. The material seems unaffected by diesel.

I don't have the vertical space to mount the tank on brackets, but I could possibly use dri deck in place of the rubber strips. I'm familiar with Dri Deck, I've used it to line lockers on my smaller boats.

Is that what you are suggesting?

Did you use strips of dri deck or did you line the entire tank bottom?

Interestingly, T N T suggested I use rubber strips, but have them run lengthwise and none across the tank so the water could run out fore and aft.

 

[DDW]

I used DriDek under the whole tank when I did it. You could probably do strips, you'd want them to be wide enough for reasonable bearing. You can stand on one foot on it, so that gives a good estimate of bearing. It seems to me that rubber strips lengthwise has just as much chance of being bad as athwartships, pretty hard to predict trim all the time and could change depending on load etc. On the sailboat, I did the hat sections, but the water tanks are FRP (as is the hull) and I still put DriDek under them. Nothing to corrode, but it lets it dry for sure, which is always better for housekeeping.