Cleaning fuel tanks

[sepeteus]

I have a concern about previous owner keeping all three fuel tanks empty. Boat has been moored in Gold Coast QLD last 2 years and the climate is not exposed to huge temperature changes. I have to go on hard stand and do quite a bit of maintenance and thinking about cleaning up the tanks. Questions:
1. Is it likely to have growth of algae on the walls of tanks

2. How can I clean the tanks properly? Do I have to remove them? Is there somebody come and do this professionally when on hard stand?

3. Some people say not to worry at all, just drain old fuel out of drain plug and fill up with fresh fuel as there is proper Separ change over primary filter system fitted. True????

BTW the yacht is Nautor Swan 57rs. I think it would be quite easy to pull the tanks out as Swans are pretty much "lego assembly" as far as whats under floor boards. We are planning long distance cruising and really annoys me that previous owner was such a tight ass not maintaining full tanks. Everything else is in pretty good order.

Thanks

[DeepFrz]

Cleaning your tanks should be a regular maintenance item, at least every 5 yrs. or more often if you have indications of dirty fuel. You don't say how old the boat is and what the tanks are made of but if it is 20 yr old boat or so then the tanks will at least need a very thorough inspection and cleaning if not replacement.

[sepeteus]

The boat is 15y old. Tanks are made of stainless. I don't know but assume that only way to do 100% job is removing the tanks for peace of mind, right???


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[BriaF]

This is how I clean my tanks.
They are made of epoxy fibreglass, with good access ports.

1/ pump out remaining fuel through filters to polish fuel.
2/ remove dregs through filters
3/wash with water & detergent to remove residue scrubbing the inside with
A brush
4/ use spray bottle with straight bleach spray the whole of interior & scrub
5/ use spray bottle with water only to wash bleach off be sure to get the whole
Interior to remove all traces of bleach
6/ leave open to dry completely
7/ seal access port & refill
8/ change filters

Using bleach will kill ALL the bugs
If your tanks are any kind of metal be sure you thoroughly remove all
Traces of bleach.
Short term bleach will not harm metals but long term it may eat the metal
This works well for me for the past 15yrs

[slowpoker]

I'll tell you what I know from recent experience:
Bought a 35 year old boat always in humid Florida, very neglected, 55 gallon mild steel diesel tank, well supported with no damp contact points anywhere, 1/2 tank of fuel was watery and black, engine not running for months.
Was of course, concerned as you, about the tank. Tank was too large to take out from engine compartment and had no way to clean or inspect.
Decided to empty the watery black fuel, unstrap it, cut 2 large access holes on the top and deal with whatever I would find before cutting it in pieces and settle for 2 smaller new tanks and devise a way to support them.
Inside, there was only a 1/8 cup of real nasty, thicker soup made up of black algae and some fine sand at the bottom not able to be picked up by the suction tube.
The fuel screen was still clear, meaning to say.... after 35 years of the worst conditions, if tank would have been just drained and refilled, it would still be an average tank, except if stirred up by lumpy seas, that 1/8 cup of crud would clog up a few filter changes.
What I did find was that the inside steel had been painted ( not your case with a stainless tank ) by the manufacturer 35 years back, with a gray coating that by now, the lower 1/4 of the tank was soft and the gray would scrape off easily. Also there was about 6 square inches of surface, with small shallow pitting where most of the water would always puddle under the pick up tube.
Well, I scraped, sanded, degreased, etched and re-coated the interior with a special coating, put in a new fuel level sender, made 2 aluminum bolt on inspection covers with fuel proof gaskets and insulated the mounting bolts to avoid corrosion, painted the outside and re-strapped the tank. As new now !.
Check the electrical ground connection from the tank to the fuel fill inlet and change the 'o'ring at the fill often. Look closely where the tank rests on a support and see if it would ever be damp, watch for tiny cracks or the rusty color of crevice corrosion. Same applies to an aluminum tank, they also corrode when resting on a damp surface.
In your case, It might be good to first attach a rag onto a stiff wire and try to swab the bottom of the tank through the fuel fill if possible, after fully emptying the tank and look at the rag to judge the condition of the sludge.
A boating friend found an old rag inside his fuel tank!.
Hope all is well with yours,as to avoid a very tedious job. Good luck.