Aux. fuel tank

Our twin Perkins powered 78' Gulf Star has a main fuel tank holding approx. 500 gal. and an aux. tank that is inaccessible under the aft cabin that holds a reported additional 200 gal. As we plan to cruise to and through the Sea of Cortez this winter might like to make use of the aux. tank if possible. The clues are: I know the fill inlet location, the pump location, pump switch and from the engine room can just barely see what I think is the forward end of the tank. It seems to be made of metal. The pump seems to work. The previous owner (8 years) said he had never used the tank and really knew nothing about it. I fear it is likely filled with gunk, ruined fuel and/or rust. I'm considering disconnection the fuel line from the aux. tank to the main, pouring maybe 6 gal. of diesel into the aux, starting the pump and seeing what comes out of the line and into a bucket. Good idea? Better ways to go?
Any suggestions appreciated.

aft tank on 44 gulf star

I know its an old post, the aft tank is extra water tank possible 150gals. 2nd Filler cap on port deck should say "water" It should be fiberglass and some owners turn them into holding tanks, since the boat didn't come with them.

Thank you for your reply. In this case some prior owner we learned had turned ours into a fuel tank. 150 gals. of free fuel! But $400 to polish same.

The price seems high just to polish 150 gals. Does that included tank scrubbing? I suspect probably not with that limited access.

A couple of years ago, I spent about $1K to cut into the tanks and install 2 8- or 10-inch SS inspection plates, scrub 2 tanks of 125 Gal each and polish about 150 gal of fuel split between the two tanks. It was a 2-day job by a local professional 2-man team with mobile capability.

Before you flush the tank and polish the fuel, maybe a tank pressure (or vacuum) test would give you an idea of tank integrity. If it leaks, there's no point in prepping it for use.

Tanks

Ok, aft tank on yours was fuel tank, then where did they install holding tank, since there were none when built. Also what size and location.
I just bought a GS44 so I am learning fast! To my knowledge they were built with two Lectrasans. I haven't decided what to do with my aft tank, don't know whats more valuable, fuel,,water, or mega holding tank.

Greetings,
Mr. SZ. "...whats more valuable..." Given your location I would think THIS would be most valuable.

First question - can you describe the tank as best you know.

Unless you are 100% knowing of the tank's current and going forward condition, recommend you use it for water only, but iffy even for that purpose as water quality could be forever contaminated. A possibly compromised tank under the aft cabin sounds like a bad thing - to find out later when trying to get rid of $$it or diesel liquid and smells.

Why not bite the bullet and pull up the flooring and see what you really have. Fix it, clean it, replace it, or whatever and then use it accordingly. I've known of several difficult to access tanks needing this more detailed and permanent approach, even here on TF.

Unless FRP where corrosion cannot easily occur, abandoned metal tanks are that way for a reason. Best to find out why.

Aft tank

Internet service limited, slow to respond. All tanks are fiberglass on the GS, I know the water lines are still hooked up. With 250gals under the galley, I guess most people really had a need for more. I will check it out , its on my d list! I still need a holding tank.