Ocean 40+2 main fresh water tank leaking

[PNM]

When I fill the fresh water tank it takes about 70-80 gallons of water.

Then, I notice about 30 gallons of fresh clean water in the forward bilge, yet the pressure water system still provides water to the faucets and water heater.

So, I assume the tank has a leak, probably on a corner, midway up the side of the tank.

The tank is virtually inaccessible. it is below the engine room floors, centered on the semi-V hull about mid ship, behind the forward bulkhead next to the forward head and cabin. It appears to be custom designed to set in the V bottom and extend up slightly into a 1' high compartment just aft of the forward bulkhead that runs the width of the boat, used for cross yacht wiring and plumbing.

I plan to replace the sender and, while it is removed, do a fiber optic inspection. Seems like I could fill the tank and let the water leak out. This should identify the level of the leak. I would expect the leak to be in a corner.

Assuming I can find the leak, and reach the leak, I hope I can plug it using any number of sealants or epoxy and fiberglass, though I may have to make a bigger hole in the top of the tank to reach the leak.

Any suggestions would be greatly appreciated.

I have never done anything like this on a boat, though I have done a substantial amount of fiberglass work on my boats.

 

[Lepke]

If there is an inspection port or you can make one, there are tank coatings for potable water that should seal small leaks and last the life of the tank if applied properly.

I've used all of these, but they have to be ordered from a marine dealer:
https://www.international-marine.com/type/potable-water-tank-coatings
This company is in the PNW: people say it works well:
Low VOC, FDA rated epoxy coating for brewery tanks linings, potable water tanks and food processing.

 

[PNM]

Repairing a poly potable water tank leak

Thanks for your ideas.

Sounds like I will need to cut an access hole large enough for my arm and a sandblaster nozzle and a roller, brush, or airless sprayer. Am I correct?

I can do that, but I will not be sure of the shape of the tank until I do a fiber optic inspection. I hope I have a direct path to all of the corners and walls of the tank.

These coatings appear to require a contiguous coating comprising the entire inside surface of the tank, many mils thick. Am I reading the application instructions correctly, or can I just coat around the area of the leak? It will be difficult to verify that I got a good coat on all parts with my arm in the access hole rotating the sprayer up, down, and around.

Perhaps a roller would be better.

I suppose if I dyed the coating a contrasting color, I could see any thin coated or missed areas.

Any experience you have would be appreciated.

 

[Comodave]

Depending on the size of the tank it may have baffles in it.

 

[PNM]

Re baffles in water tank

Poop! If true, I can't even install a bladder!

 

[Comodave]

I don’t know if it would or wouldn’t have baffles but it is a possibility. There are companies that make metal inspection panels for fuel tanks so they might work on water tanks too. Just make sure you don’t cut the hole over a baffle…

 

[Bmarler]

You didn’t mention what the tank is made of. Can make a difference how you approach the repair.

 

[PNM]

The tank appears to be a poly material - some kind of plastic.

 

[Comodave]

If it is a poly tank then it probably doesn’t have baffles in it. But it will also be tough patching it since lots of things won’t stick to it. Can you get it out and just replace it? It may require some surgery but that is just what you may have to do. Are you sure it is leaking? If you do replace it check out Ronko (sp?) tanks. Very good quality and they have hundreds of shapes and sizes that are stock tanks. They will install fittings where you specify.

 

[PNM]

Repairing a fresh water tank

I can't figure how it would leak into the bilge over the day after it is filled except through a hole in the tank or by siphoning out the filler hose.

Siphoning on the output line would require the line to go below the level of the water in the tank, before attaching to the pump. The pump is at a level above the top of the tank, only 2 feet from the tank. The output line only goes up from the top of the tank, so, this doesn't appear to be a realistic answer.

The filler line drops down to about half tank full level, which makes it suspect, but once a small amount of water siphoned backwards through the filler line, there would be air at the top of the tank where the filler line is attached, stopping any additional siphoning.

If the filler line attached to an input pipe that extends from the top of the tank to the middle of the tank, it could enable a siphon through the input hose. With the input hose laying in a compartment at about the mid level of the tank, it might siphon 1/2 of the water out of the tank.

I do fill the tank until the filler hose is full, making a reverse siphon possible, were there an input pipe extending to the middle of the tank. Is this a realistic possibility. I would probably attach an input hose to a nipple on top of the plastic tank vs. attaching it to a pipe protruding to the middle of the tank.

Would there be a good reason to have an input pipe extending down into the center of the tank?

I will take a hard look at the input hose.

Am I missing something.

 

[Bmarler]

Older Poly tanks can develop cracks in the corners, or where the fittings attach. If you can locate the crack, you may have some luck using a piece of stainless or aluminum sheet metal glued and screwed in place with 5200 as a sealant.
Those tanks without baffles really take a pounding when partially filled and the boat takes big waves.