Leaky Holding Tank

In Two Deep

In Two Deep

Member
Aug 23, 2007
237
Long Island Sound
Boat Info
2001 340 Sundancer
Engines
8.1 Mercruiser
Well my worst fears have come true. I suspected my the holding tank for the head was leaking, and have now confirmed this. I cleaned the entire bilge, empty the tank, filled it half way with dyed water, let it sit for a day, and sure enough, I can see the trail of water coming from the tank and into the bilge. I have a 2001 340 Sundancer with 8.1's. The bilge is tight to say the least. Replacing the tank is probably going to be a major undertaking. So I have to decide to replace or repair. It appears the leak is in the back of the tank which sits very close to the hull with less than an inch clearance. I can not see the where it is coming from. Would need to try to lift the tank and see if I can even locate the crack.

So has anyone had this experience or have any suggestions? I was thinking of buying "Flex Seal" and giving that a shot. You know - the stuff they use to float the row boat with the screen door as the bottom. It is a spray product so location exactly where the leak is may not be as critical. But, I have read mixed reviews about this stuff. Others suggested JB Weld, but that would involve actually getting to source of the leak.

Any ideas would be greatly appreciated.
 
I honestly think you need to look at replacing it. Successful repairs on this type of plastic is tough because the crack will just spread. Now with that said, it may be easier to install 2 holding tanks that are smaller versus the one large one. Can you take an share some pictures?


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I had the same problem and wound up replacing the tank last year. I could not find one single product on the market that would adhere well enough to the polyethelene plastic. Even the spray coating just rolled off. That's why the fittings are all heat welded to the tank. I measure the tank dimensions, the exact locations of fittings and diameters and went online to tankdepot. They made a tank that was an exact replacement and for an extra $25 I got the double thick wall version. With the spin weld fittings installed the tank (25 gal) cost me about $250 delivered. I verified the order with them on the phone and rep told me I could get the one with the Sea Ray label for twice the price but it would be the same tank. Install wasn't too bad.
 
The removal of the old tank and the install of the new seems to be the biggest challenge. Because the bilge is so tight, my mechanic feels we will need to pull the engine in order remove and replace. He dissent even feel there is enough room to cut the old one out.View attachment 49422
 
We definitely need some more pictures but you may be able to remove that exhaust and get to it. Does the deck come off?


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These boats were never designed to be serviced, hence why you don't see a maintenance manual for them.

You may want to deactivate the tank and find another place (if you have one) to put in a new tank.
 
So true CaptRonn They build the boat around all the systems, but never think about how someone is getting in there to service anything. It is so tight, I don't think there is any other place for the holding tank. Oh the joys of boat ownership.
 
The holding tank bottom corners each have a "mold release" screw - at least they did on my '03 410. These rust and may be associated with your leak. My 410 had the problem from one corner. I was able to lift the tank an inch at each corner, put JB Weld Marine epoxy over each screw, and it has held for 5 years. There is a thread here about it.
 
Look at my thread I started 4 years ago... "black water holding tank leaking " Those dometic tanks have hardware running thru the tank for mounting and they rust from the bottom. I am going on 4 years and it's still holding. I used JB weld plastic epoxy on all four corners. Most told me it would not work and would require plastic welding, but it worked. Getting the tank out is next to impossible. Others have tried this method and have not been as lucky. To remove I am pretty sure you have to take manifolds and risers off. I would give the epoxy a try and see if it holds. When are you due to replace manifolds? Perhaps replace tank then
 
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I had a water tank fail on our 28 footer. Cut old tank up in place and removed. A new one the same size would not fit unless I removed an engine I did as indicated replaced it with two smaller ones. Just make sure you put the fittings in the correct place. I had a fitting placed at the top of both so they vented as separate tanks. Make sure the bottom connections will fit between the two tanks. Spend a few hours at a Building Supply store playing with the fittings to make sure you place the connections on the tanks in the correct places and they are the correct size.
 
I had same problem. Removed both motors, replaced tank and used opportunity to replace bilge pumps, water heater, all new vacuflush plumbing, etc. etc.


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With that type of plastic I have had some sucess sealing cracks with hot melt glue. Drill the end of the crack, V the crack as much as possible, rough up the surface, lay in a wide heavy bead of glue. Ive even saved batteries this way believe it or not.
 
I had the exact same issue with a 2002 340. As many have commented on here, and on the other referenced thread, the issue is the embedded bolts in the plastic tank and corroding and cracking the tank. I tried JB weld twice as suggested, and it failed. I eventually replaced the tank with a direct replacement 28 gallon tank with the pump, etc. To do this it was not necessary to pull the engine! If you remove the mufflers and exhaust on that side, you can get a new tank in to the boat. To save money, I had cut up the old tank and removed it my self and then took the new tank and boat to my marina to have them pull the boat and remove the muffler system and put the new tank into position. I then reconnected all the plumbing and put the tank in place myself.
Rob
 
Well after 6 years of JB weld plastic epoxy the dreaded leak returned. I was able to takr the tank out of the boat by removing the batteries and muffler along with exhaust via riser to muffler. Once out of the boat I welded the tank using a polyethylene welder, Melting additional poly material to each of the corners with the corroding hardware worked like a charm and saved $1200 on buying a new tank. The new tank has the same problem from what I read, so why not fix the one I had. Attached are some pictures.
 

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Well we just purchased a 380 sea ray 2000 and I noticed a small leak in the holding tank. I found a lot of info on this site so I figured I would share my research and results. The holding tank is made of polyethylene which is very difficult to patch. I found the leak on the starboard corner of the tank so i loosed the tank and was able to slide the tank so it overhung the shelf it was on. With a mirror I could see the round circle where it was dripping. The common place where others on this site mentioned there was medal on each corner that rust and then leaks. I used marine tex, flex seal which is one of the only products that I found that will adhere to polyethylene, but make sure you flame treat the area that you want to patch (read the instructions). Marine tex can be used underwater so I mixed the 2 part epoxy and placed it on a plastic bag and applied it under the tank and then slid the tank back on the shelf so the weight of that tank would apply the pressure. So far so good,and my boat no longer smells!
 
West Systems makes a plastic specific product called G-Flex that adheres very well with proper surface prep.
 
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Tom Tyr said:
Well we just purchased a 380 sea ray 2000 and I noticed a small leak in the holding tank. I found a lot of info on this site so I figured I would share my research and results. The holding tank is made of polyethylene which is very difficult to patch. I found the leak on the starboard corner of the tank so i loosed the tank and was able to slide the tank so it overhung the shelf it was on. With a mirror I could see the round circle where it was dripping. The common place where others on this site mentioned there was medal on each corner that rust and then leaks. I used marine tex, flex seal which is one of the only products that I found that will adhere to polyethylene, but make sure you flame treat the area that you want to patch (read the instructions). Marine tex can be used underwater so I mixed the 2 part epoxy and placed it on a plastic bag and applied it under the tank and then slid the tank back on the shelf so the weight of that tank would apply the pressure. So far so good,and my boat no longer smells!
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