The Ghost Tank haunting my bilges

Behold the mysterious ghost tank.

There was concern this was a refuge for brown trout.

In my steel pilothouse cutter refit, I can still find little surprises. Most, with the coming of wisdom, or at least a lower level of incompetence, are solvable, but some problems suggest a number of solutions. Those of the the ghost tank are several, but probably only one is the best.

Astoundingly, although I've clearly been badly delayed in the execution, this diagram is still largely what I want to do.

As I outlined a scant three-and-a-half years back (ain't refitting fun?),  I have an unused, approximately 40 U.S. gallon capacity keel tank directly under the engine. Formerly the original holding tank, I had no idea until last week (because unbolting the access plate requires that I fly the engine completely out of the engine bay) if it had been "put away cleanly", only that the people I bought the boat from (the second owners) had a new, HDPE tank next to the head.

I have been considering repurposing this tank as a post-Racor daytank, which would give me a sort of insurance against contamination from bad fuel going into the main tanks, and would extend our motoring range to 140 U.S gallons, or 40% more than now.

We found not the usual gasket, but a sort of rubber sheet seal. It came away cleanly.

Upon finally taking off the inspection plate, I found a few gallons of water...no smell, thank goodness, of either chemicals or of its former usage. I also found significant rust:

The pipe here is the "pump out". The plumbing needed to use this as a day tank would be smaller and forward for access.

At the time we were taking pictures, I didn't think to throw a magnet down to my wife, who was taking the shots, to determine if the tank was mild or stainless steel (SS). SS would be logical and would match the water tanks I've already pulled out in favour of smaller, easier to service and anchor down HDPE water tanks. SS would also (perhaps) suggest that the rust is superficial and could be power washed off, or treated with something like Ospho, over which could go a suitable primer and topcoat that won't foul or break away into polished diesel.. Frankly, I am not sure if that's the best course.

Going the "insert" route wouldn't mean I couldn't find a pretty close fit.

The other diesel keel tanks forward in the keel are black iron and are full of diesel, basically as a rust retardant until they are needed for use. That fuel, if even still usable, will need a dockside polish.

I suppose it's uniformly grotty.

I consider my options as follows:

1. If the tank is indeed SS, I would consider sand blasting the interior and applying a coating (if even needed). To judge by inspection of the lid and the tank top, it is pretty heavy gauge metal. There is no sign in the bilges or, thank Neptune, on the outside of the hull that there's any holes in the metal.
2. I would consider cutting off the top of the tank, cleaning and coating the interior with something like truck bed liner, and dropping in a HDPE diesel fuel tank with fittings forward (where there is access at the front of the new engine),
3. The same as 2, only with a 35-40 gallon fuel bladder. The advantage of this would be the ability to pump out the bladder of fuel and then pull it from under the engine for cleaning or service. It would be secured with bolted-down battens. A clear disadvantage could be the danger from having a working engine directly above this fuel.
4. Fill the rusty tank full of ballast to stiffen the boat and buy a load of yellow jerry cans to tie down on deck.

The last suggestion is only partly facetious. The original water tanks were on frames off the hull, and their (full) weight at the sides, plus the lack of weight in that space below the engine, made the boat almost tender, which is a ridiculous situation in a steel beast of this nature. The easiest solution would be to fill the void with shot and cement and let it harden in place. It wouldn't give me 40% more range, however.

 

Any constructive suggestions, horrible warnings or sage advice would be most welcome.

Rust never sleeps, but sometimes swims