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Old 12-04-2021, 14:36   #16
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Take a lesson from the failure points of the old tanks, and do not duplicate those errors!
If the tops are rotten, build some angle into the new tops so they’ll shed water, and for gods sake, seal up the source of the water intrusion.
Allow air space all around, and cushion the supporting surfaces, or better yet, support them on tabs so they don’t sit on their bottoms.
Provide a low point sump, this can totally eliminate corrosion from the inside.
Access ports are much easier to install when fabricating the tanks, so do it now instead of later after some other disaster.
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Old 12-04-2021, 14:37   #17
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Fibreglass with a decent sized inspection cover on the top so you can reach every corner inside to clean. Screw cap filler over the deepest bit and use a wooden dowel to check the level. Calibrate the wood with cuts on the side.
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Old 12-04-2021, 14:58   #18
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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Originally Posted by bailsout View Post
Anyone advocate bladder fuel tanks?
My Vetus bladder tanks (2 x 200litres) have been good for 35 years. Much to my surprise!

A big advantage is that they contain very little air when nearly empty and so there is correspondingly little condensation
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Old 12-04-2021, 15:00   #19
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Re: Diesel tank of ss? Aluminium? Fibreglass?

I built my own 25gal (~100L) tank out of fiberglass... Pretty simple. $250ish with all fittings and sending unit. Make sure the resin is impervious to fuel.


I installed a sending unit so I could have a gauge in the cockpit. A dip stick is probably the most simple & cheap way to check it.
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Old 12-04-2021, 15:10   #20
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Re: Diesel tank of ss? Aluminium? Fibreglass?

If you decide on using stainless (and I would) make sure it's 316L, not plain 316. The "L" suffix is for the 316 plate that welds more securely.

And +1 for having inspection/cleaning access built in.
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Old 12-04-2021, 15:20   #21
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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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.
This is really the trick of it. I’ve seen many tanks leak.
Example: Aluminum tank, just contacting a couple of stainless steel keel bolts under water in the bilge. The boat was evidently fine for some 30 odd years until the sink sprung a pinhole leak. The bilge pump kept up, but for the whole winter (who knows?) the boat had salt water in the bilge with the aluminum tank and the electrolysis from dissimilar metals in the presence of an electrolyte destroyed those parts of the tank in contact with the bolts.

I won’t bore you with the others, mostly dissimilar metals far apart on the galvanic scale, salt air, condensation, and other factors working in tandem. Regardless of how you do it, if you do it right and mind your p’s & q’s it should last a long while.

Additional morals: close your seacocks, mind your metals, and as you’re doing - select appropriate materials.
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Old 12-04-2021, 19:57   #22
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Re: Diesel tank of ss? Aluminium? Fibreglass?

I am looking into Fiberglass tanks now. old black iron tanks got rusty and have to be replaced. I am looking hard at integrated tanks for the extra capacity, however i just seen a youtube that came out the other day where they cut the top off their old metal tanks and made the new fiberglass tanks inside the old ones. its pretty informative and a good plan for someone looking into doing this.

you really need to look at the resin you use to make sure its low sulfur diesel safe. diesel has started to get blended with ethanol and biodiesel, which are not good for several resins. its expected that higher percentage ethanol and biodiesel blends are in the future, so i highly recommend planning for this and making sure you use the correct materials.

as a possible option you can use aviation fuel tank sealants. they use an entire wing section that has been riveted together and roll or spray the sealant to cover the entire tank surface. they also used it as a bedding compound for sensors and fittings which appears to be something we should implement as well. I talked to one of the manufactures a couple years ago, but didn't ask about biodiesel/solvent ethanol compatibility. JP5/Jet fuel is similar to diesel and is also starting to get blended fuels or in the works for blended fuel, so i would expect that aviation sealants are a place to keep your eye on.
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Old 12-04-2021, 20:02   #23
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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I am looking into Fiberglass tanks now. old black iron tanks got rusty and have to be replaced. I am looking hard at integrated tanks for the extra capacity, however i just seen a youtube that came out the other day where they cut the top off their old metal tanks and made the new fiberglass tanks inside the old ones. its pretty informative and a good plan for someone looking into doing this.

you really need to look at the resin you use to make sure its low sulfur diesel safe. diesel has started to get blended with ethanol and biodiesel, which are not good for several resins. its expected that higher percentage ethanol and biodiesel blends are in the future, so i highly recommend planning for this and making sure you use the correct materials.

as a possible option you can use aviation fuel tank sealants. they use an entire wing section that has been riveted together and roll or spray the sealant to cover the entire tank surface. they also used it as a bedding compound for sensors and fittings which appears to be something we should implement as well. I talked to one of the manufactures a couple years ago, but didn't ask about biodiesel/solvent ethanol compatibility. JP5/Jet fuel is similar to diesel and similar to marine diesel is starting to get blended fuels, so i would expect that aviation sealants are a place to keep your eye on.
This is really really interesting to me! Thanks for sharing!

First boat I owned, which I bought practically falling to pieces for less than a paycheck had leaky fuel tanks - and we just glassed the outside of them. To my knowledge it’s still holding today. There was no access to remove them or get at the top without cutting it into small pieces for the trip out of the boat, so we skipped that bit and went the easiest route we could.
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Old 12-04-2021, 20:13   #24
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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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.

Capt. Claus - ocean tramp of the eighties
This is 100% irrelevant to the discussion at hand, since commercial diesel fuel in not mixed with ethanol.
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Old 12-04-2021, 20:57   #25
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Re: Diesel tank of ss? Aluminium? Fibreglass?

If you make a new tank out of any metal, use a tank coating approved for diesel and the tank will last the life of the boat.
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Old 12-04-2021, 22:39   #26
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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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!
HDPE - a decent plastic worker can weld one up for you if you need a custom one. A faulty one will split, other than that they have no issues. Most production boats use them these days.
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Old 13-04-2021, 02:33   #27
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Re: Diesel tank of ss? Aluminium? Fibreglass?

If a one-off, 316 stainless is probably the most effective. You'll find mor competent fabricators in most engineering estates.
I'd prefer glass fibre, but that requires a specialist, which are few and far between.
The cheapest is a standard moulded plastic tank, off-shelf in Vetus range.
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Old 13-04-2021, 02:48   #28
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Re: Diesel tank of ss? Aluminium? Fibreglass?

Article by Stece D'Antonio on tank design. In short, fiberglass is the best. But you do have to use the correct resin (which one escapes me)

http://stevedmarineconsulting.com/wp...TankDesign.pdf
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Old 13-04-2021, 03:16   #29
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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Originally Posted by chasfgr View Post
If a one-off, 316 stainless is probably the most effective. You'll find mor competent fabricators in most engineering estates.
I'd prefer glass fibre, but that requires a specialist, which are few and far between.
The cheapest is a standard moulded plastic tank, off-shelf in Vetus range.

I would think building a fiberglass tank would be easiest for a do it your selfer with no welding skills.
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Old 13-04-2021, 04:18   #30
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Re: Diesel tank of ss? Aluminium? Fibreglass?

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Originally Posted by chasfgr View Post
If a one-off, 316 stainless is probably the most effective...
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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