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

[Bud]

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.

If you do have an electrolysis/galvantic corrosion problem in the engine room it might only show up on the aluminum because it makes the best anode compared to the other metals in the engine room. In addition to bonding You should put zinc anodes on the new tanks to cover your ass. If the zincs are getting used up then you know you have issues. In the meantime your hard work of installing new tanks is not being affected. I concur with all other posts that recommend ways to electrically insulate the tank from mounts and hull ect.

Bud

 

[lwarden]

DriDek is an excellent suggestion if you have the vertical room.

 

[Comodave]

Steve D has a good article on his website on how to do the install.

 

[Bmarler]

If the rubber strips were just to lift the tank for circulation, have the fab shop weld on aluminum flat bars the same dimension as the rubber. Weld all sides so no water can get between the layers. Come in from the ends a half inch or so so the corner seams can breathe. It would take a mighty long time to burn through that much thickness.
Bond the tanks to the ships bonding system, but I Can’t imagine an anode would help in free air. Anyone hear different?

 

[Dougcole]

If the rubber strips were just to lift the tank for circulation, have the fab shop weld on aluminum flat bars the same dimension as the rubber. Weld all sides so no water can get between the layers. Come in from the ends a half inch or so so the corner seams can breathe. It would take a mighty long time to burn through that much thickness.
Bond the tanks to the ships bonding system, but I Can’t imagine an anode would help in free air. Anyone hear different?

I like this suggestion a lot. I have about 2 1/2" open space above the tank, so I could get away with 1/4" to 5/16" thick bars. I'm going to talk to T N T tomorrow about this and see what they think.

Thanks.

Doug

 

[DDW]

With 2 1/2 inch above the tank you have all kinds of options. I'd do the hat sections (only need to be say 1/2" vertical dimension) - better than straps because it isn't necessary to weld all around. But I'd also have them put in a small sump at the low point, it only needs to be 1" deep and perhaps 6" square. Then make the hat sections 1 1/4 to space it. I disagree that water will wander all over the bottom, if you have that sump. By random motion even on a flat bottom, it will eventually fall into the sump and it won't come out again without extreme provocation or sucking it from the top. Gravity is one of the most dependable things on Earth. Some kinds of gunk are neutral buoyancy in diesel and just have to be filtered. But water always ends up on the bottom.

 

[Dougcole]

With 2 1/2 inch above the tank you have all kinds of options. I'd do the hat sections (only need to be say 1/2" vertical dimension) - better than straps because it isn't necessary to weld all around. But I'd also have them put in a small sump at the low point, it only needs to be 1" deep and perhaps 6" square. Then make the hat sections 1 1/4 to space it. I disagree that water will wander all over the bottom, if you have that sump. By random motion even on a flat bottom, it will eventually fall into the sump and it won't come out again without extreme provocation or sucking it from the top. Gravity is one of the most dependable things on Earth. Some kinds of gunk are neutral buoyancy in diesel and just have to be filtered. But water always ends up on the bottom.

I kind of misquoted on the 2 1/2". There is about that from the top of the tank to the bottom of the cabin sole, but that doesn't take into account the height of the fitting for the deck fill, the vent fitting, the tab for the bonding system or the fitting for the return line. I can't push it up much more than maybe 1/2" and still get to all of those.

Also, the tank BARELY fits down into the ER and then I have to roll it onto its side, slide it back into place and then roll it upright. It would be an unmitigated disaster if I got the tank built and it didn't fit into the ER.

I talked to the tank fabricator this morning, and the tank is almost done. He is adding the aluminum pads to keep it off the shelf so I won't have to use rubber pads, but I can't make any other changes at this point.

I like planning boat projects out and executing them to the best of my somewhat limited abilities, so even though losing two tanks in three months has sucked overall, I take comfort in the fact that I have improved the boat. The old tanks were thin and poorly installed, yet still lasted 20 years, so these should go much longer. I doubt we will own the boat anywhere near 20 more years, so her next owner will be getting a nice upgrade.

I do appreciate you help though. I will post pics when I get the tank in.

Doug

 

[Dougcole]

Got the tank in today, it was a bit of a bear wrestling. I made up a wood form in the exact shape of the new tank to verify that it would fit, but there is a big difference between slipping a 10lb form in and wrestling with a tank. My brother, buddy and one of the yard guys helped me with that part, we used appliance lifting straps (As seen on TV!) to help move the tank around once it was in the ER.
Now I need to tie up some loose ends like rerouting the 2", stiff, emergency bilge pump hose and the crossover line. Since this new tank is smaller, the bilge hose ended up exactly in front of the main fuel pickup, leaving no room to install it, so we had to cut the hose.

While messing with the bilge hose, I discovered that the float switch has kicked the bucket, so another job added to my list.

Pretty much the same issue with the crossover line, the pickup on the new tank is about 6" further forward than the old tank's setup, so now the crossover hose won't reach it. I'm making a longer one with two elbows in it.

Hopefully, the yard will drop the motor in tomorrow morning, that is the plan. After that, we need to hook it up and realign the motor and we should be close to done.

The pics below show one of the pads and brackets on the tank bottom. There is an air gap of about 3/8" under this tank. I'm pretty happy about that.

 

[DDW]

If not totally ignored, that tank ought to last a long time. I am a bit alarmed at how close the engine appears to be..... access good on that side?

 

[CharlieO.]

Nicely done, much quicker second time around!

 

[Dougcole]

If not totally ignored, that tank ought to last a long time. I am a bit alarmed at how close the engine appears to be..... access good on that side?

I think it's not too bad for a 40' twin engine boat, but everything is retaliative, I guess. For common maintenance items the stb side oil filter is a little tough to reach (I've been planning on adding a remote oil filter, but it seems to stay near the bottom of the to do list). Probably the worst job is replacing the impeller on the port side, but some of that is a function of the way the motor is set up by Yanmar, there isn't much space behind the pump to get the impeller out. Other than that it's all pretty straight forward.

On Mainship 400s with twins there is a LOT of space between the motors, which is where I spend most of my time. In many ways, because of how the ER hatches are set up, on this boat the twin has better access than the single does.

From a performance standpoint, the twin is a much, much better boat than the single, and twins maintain their value in the 400 better than the singles. At least in my opinion it is well worth it, though others would disagree.

 

[rgano]

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.

This scenario is why I made a very simple modification to the return-to-port tank only for my single engine. Both my tanks are fitted with identical return ports, but only the port tank was connected to the return fuel line. All I had to buy was a brass bard to threaded connection to replace the threaded plug in the stbd tank's port, about six feet of fuel hose and a wye valve to enable return from the engine to either tank or to both. An hour-long job.