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Old 11-04-2021, 02:29   #1
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Diesel tank of ss? Aluminium? Fibreglass?

Greetings from New Zealand.
Another bit of advice please...
I'm replacing the diesel tank in our (new to us) old Cavalier 39. I've been looking for a relevant thread without success. What is the recommended tank material? The tank will be fitted into the original spot in the keel, and I can have it made to fit the tapered profile. I would have assumed that SS would be the best, but maybe there are issues that I'm not aware of. Corrosion, or whatever... The fibreglass option would enable the tank to be the maximum size though... It would probably be about 120 litres full up. I'd also need some means of telling how much fuel is left. Any tips on this also appreciated.
Cheers!
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Old 11-04-2021, 02:35   #2
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Go for HDPE for the tank and a Tank Tender for the gauge, no electrics or moving parts and works on any shape tank.
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Old 11-04-2021, 03:27   #3
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Aluminum fuel tanks must be constructed of 5000 series alloy, specifically 5052, 5083, or 5086 (not 6061), with a specified minimum thickness of 0.090 inches.
Stainless steel, if used for fuel tank construction, must be of the 316L, or 317L alloy, and a minimum of 0.0747 inch thick.
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Old 11-04-2021, 04:07   #4
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Yes, plastic as mentioned in second post would be very good, make sure you specify a baffle.
However my first choice would be still fibreglass, as you said, to maximise volume. Make sure you pick the right resin, vinylesters, ie brand name of Derakene or similar, check the suitability for diesel.
Ask around and see how many alu and SS tanks have been leaking...... that will point you in the right direction.
From what I have seen, alu, steel and SS tanks chafe or corrode from the outside, or welds may crack/corrode due to ? dissimilar metals.
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Old 11-04-2021, 05:03   #5
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Re: Diesel tank of ss? Aluminium? Fibreglass?

If you search the recent Gone with the Wynn's videos you will find their issues with corroding alu fuel tanks. Unfortunately they are in Tonga and the only alu welder is away and con't get back due to Covid. They sealed and glassed up the old tanks and are hoping they last until they can get to NZ to order plastic ones. Pretty informative.
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Old 11-04-2021, 06:04   #6
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Re: Diesel tank of ss? Aluminium? Fibreglass?

I'll give you two very short stories about metal sailboat fuel tanks. I have owned two large sailboats.

First one was built in 1976 years old, I sold her in 2015. She spent her life in saltwater, and sailed a LOT. It had an aluminum tank, and in 2015 it was perfect--inside and out. One key here might have been it was mounted well above any possible contact with bilge water.

Second boat, now 25 years old with a 316L stainless tank. After two circumnavigations, the tank is again in perfect shape. Several hundred of these boats were built to this spec, and fuel tank problems have never been an issue. Again, mounted well above any possible bilge water contact.

Moral of these stories: IF installed properly, built of the right alloy, with good welding techniques, and given minimum care, either aluminum or stainless tanks should be in the boat LONG after you sell it.

One possible key here: both of these boats received a lot of use throughout their lives. They never sat in storage, accumulating water in their tanks due to condensation. That means lots of engine hours (both averaged about 500 engine hours a year) and lots of good clean fuel moved through the tanks.
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Old 12-04-2021, 08:49   #7
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Re: Diesel tank of ss? Aluminium? Fibreglass?

If using fiberglass check the resistance to ethanol and maybe other chemicals. Occasionally a bymix of ethanol is not avoidable - and it is very agressive to most resins and some metals.

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Old 12-04-2021, 08:52   #8
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Re: Diesel tank of ss? Aluminium? Fibreglass?

tank tender gauge mentioned earlier- you wont regret it- easy to use/install and bulletproof. my tanks are fiberglass and i dont see any possible downside to that material.
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Old 12-04-2021, 08:54   #9
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Quote:
Originally Posted by BillKny View Post
One possible key here: both of these boats received a lot of use throughout their lives. They never sat in storage, accumulating water in their tanks due to condensation. That means lots of engine hours (both averaged about 500 engine hours a year) and lots of good clean fuel moved through the tanks.
I think this part is a good point. Water in contact with the metal is going to lead to corrosion overtime regardless of if it is water inside or outside the tank.

By using the fuel regularly your fuel filter is filtering that water out of the tanks and being replaced with pure diesel.
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Old 12-04-2021, 09:50   #10
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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Originally Posted by GordMay View Post
Aluminum fuel tanks must be constructed of 5000 series alloy, specifically 5052, 5083, or 5086 (not 6061), with a specified minimum thickness of 0.090 inches.
Stainless steel, if used for fuel tank construction, must be of the 316L, or 317L alloy, and a minimum of 0.0747 inch thick.
Thanks for the info. I am about to have a 150 gal aluminum tank fabricated. If you please, what is the source of your specs?
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Old 12-04-2021, 10:14   #11
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Quote:
Originally Posted by JeremyFowler View Post
Greetings from New Zealand.
Another bit of advice please...
I'm replacing the diesel tank in our (new to us) old Cavalier 39. I've been looking for a relevant thread without success. What is the recommended tank material? The tank will be fitted into the original spot in the keel, and I can have it made to fit the tapered profile. I would have assumed that SS would be the best, but maybe there are issues that I'm not aware of. Corrosion, or whatever... The fiberglass option would enable the tank to be the maximum size though... It would probably be about 120 liters full up. I'd also need some means of telling how much fuel is left. Any tips on this also appreciated.
Cheers!
A big part of my decision to purchase my new-to-me boat was the tankage:
2 x 100 gals. (measured) s.s. Fresh Water tanks installed amidships;
2 x 75 gals. (measured) fiberglass Diesel tanks installed under the galley/nav stations;
1 x 47 gals. (calculated) fiberglass Holding tank amidships;
1 x 12 gals. (calculated) fiberglass Sump tank amidships

The boat is a 1979 construction and no tankage problems.

Most common/economical is aluminum just make sure it is elevated to always be well above the bilge or anyplace that could hold standing water underneath.

If I were to ever to replace any tank I would take steps to have have an easy accessible drain installed in the bottom for servicing and draining when/if necessary.

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Old 12-04-2021, 11:50   #12
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Any metal tank must be kept out of bilge water or any water seepage around the base. SS water tanks in the bilge were resting on the fiberglass over the ballast. They developed pin hole leaks where they rested on small rises in ballast cap surface in less than 4 years. I'm sure the same would go for aluminum. On my current boat Fiberglass water tanks are going strong but had to replace the Monel fuel tanks a few years ago because of soldered seam leaks. If you are planning on owning the boat for the next decade or two would go with other than a metal tank. They'll last a long time but even Monel doesn't go forever.
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Old 12-04-2021, 12:23   #13
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Only plastic, GRP, below the waterline. Stainless is subject to stale standing seawater with no fresh air ventilation releasing chlorine gas which attacks the welds. Aluminum is is even worse below the waterline.
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Old 12-04-2021, 13:50   #14
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Anyone advocate bladder fuel tanks?
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Old 12-04-2021, 13:50   #15
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Aluminum carry several qualities, a coast guard vessel require a very particular type of aluminum, when I used to work at Bertram Fiberglasd was very popular, diesel was not a killer and apart from old gas boats with issues they are ok, not sure who is doing custom ones.. Stainless we had issues on the welds, not so friendly to built, if you choose ac good tank manufacturer, they will prime and protect it, the welds are much better than SS, they are easy to mount, ..
Quote:
Originally Posted by JeremyFowler View Post
Greetings from New Zealand.
Another bit of advice please...
I'm replacing the diesel tank in our (new to us) old Cavalier 39. I've been looking for a relevant thread without success. What is the recommended tank material? The tank will be fitted into the original spot in the keel, and I can have it made to fit the tapered profile. I would have assumed that SS would be the best, but maybe there are issues that I'm not aware of. Corrosion, or whatever... The fibreglass option would enable the tank to be the maximum size though... It would probably be about 120 litres full up. I'd also need some means of telling how much fuel is left. Any tips on this also appreciated.
Cheers!
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