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Old 10-09-2009, 17:26   #1
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Converting Integral Fiberglass Water Tank to Diesel

I have a 1978 Cheoy Lee 38' Offshore.

Currently the tankage consists of a 30gal integral diesel tank and a 100 gal integral fiberglass water tank.

I am considering adding additional steel tankage for water. I would like to use the 100gal integral tank for diesel.

Are there any issues with using fiberglass tanks for diesel that I should know about?

Thanks for any info.
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Old 10-09-2009, 17:37   #2
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Your Fiberglass might not be compatible with fuel.

There might be a permeability issue

I wouldn't do it
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Old 11-09-2009, 03:19   #3
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Diesel fuel tanks can be integral with a fibreglass hull, provided the core material (if applicable) is compatible with diesel (many are not), and will not permit fuel to migrate (many will). Due to the difficulty of ascertaining these properties of an old construction, I cannot generally recommend such a conversion.

See another current discussion at
Converting Water Tank to Diesel
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Old 11-09-2009, 07:15   #4
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Thanks

I was the one who asked the question on the other post as well.

My hull has no coring.

How do i find out if it is compatible with diesel. Is this an issue of resin used? The resin used in this tank construction is polyester.

What are the issues I need to be aware of?

Thanks.
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Old 03-04-2010, 15:45   #5
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I recently purchased 1979 39' Cheoy Lee which has the integral tanks. The previous owner had a serious issue with the fuel tank. It leaked into the hull and seeped out the bottom of the keel. The glass was ground down and a number of layers of glass were reapplied.
The salesman told me the tank was replaced with a new stainless steel tank. Unfortunately he was mistaken and the tank was just repaired somehow with glass and it has never been filled since. I really don't want to fill it because if it leaked once, why won't it leak again. I am trying to figure out to put some kind of a new tank in, any suggestions?
I'm not happy about drinking the water from that tank either. I would like to find new tanks for both.
Thanks for any help.
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Old 03-04-2010, 17:44   #6
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Integral Tank Issues

Did you get an offshore 39? Your tank construction is probably pretty similar to mine, i have an 1979 Offshore 38 Richards Design.

As far as I know, my diesel tank has not had any leaking issues, although there is a spot on near the top that looks like it has been patched with glass. The solution I decided on to deal with this issue is to keep and use the original tanks as-is but also to install additional tankage. I'm installing a 40 gal water tank in stainless steel and an additional 35 gal diesel tank in aluminum.

This will put me at 140gal water and 70 gal diesel total. I think i will get to avoid drinking out of the fiberglass water tank for the most part. 40 gal should be able to provide the primary drinking water and with a watermaker it should be easy to keep that topped up. The integral glass tank water could be used for laundry, showers, misc as well as backup if/when the watermaker fails. I needed the extra tankage anyway as I hope to do some long distance cruising

Thats my plan anyway. The way my fuel tank was created (built down deep in the keel) it would be possible to cut the top out and create a template to have an aluminum tank made, (though this would not be a fun task) i would imagine loosing a fair amount of capacity though, and having the tank down in the keel, i would imagine it would be easy to get water trapped along side of it possibly causing corrosion.
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Old 03-04-2010, 18:42   #7
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Yes, the offshore ketch version. Sounds like we may have similar boats. Where are you going to put the new tanks? Keep in mind if your current fuel tank does start leaking it's a real mess. Hear is a link to a company that has a lot of tanks.
http://www.moellermarine.com/oem/fuel_tanks/
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Old 03-04-2010, 19:15   #8
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additional tanks.

I'm putting the water tank inside the icebox (im not going to be using it). I'm putting the fuel tank against the hull under the nav station. It eats up a bit of the foot room but i'm willing to make that sacrifice. This balances the weight out pretty well i think.

So how did your fuel tank leak? it was leaking into the keel? These keels are concrete with iron punchings i gather... i imagine the concrete would allow diesel to seep through. The diesel was then coming out through the keel? There must have been a hole through the glass encapsulating the keel right?

i guess you could patch that up from the outside but you would still have some diesel in the keel, that should be no big deal, right?

I imagine if you sealed the tank well from the inside wherever the crack was you would be OK. My tank is really hard to work on though, the access hatch is not very big.
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Old 04-04-2010, 10:58   #9
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I don't know the details on the leak, it was with the previous owner. All I know is what the salesman told me and that the tank is completely empty and I can see fresh fiberglass on the bottom of the tank from the access. Outside I see lots of glass work now painted along the bottom of the keel and 12" up the sides. I'm not going to chance it again.
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Old 13-04-2010, 10:31   #10
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water to diesel tank

Quote:
Originally Posted by sv.Crake View Post
I was the one who asked the question on the other post as well.

My hull has no coring.

How do i find out if it is compatible with diesel. Is this an issue of resin used? The resin used in this tank construction is polyester.

What are the issues I need to be aware of?

Thanks.
The ONLY safe way to change a water tank in GRP to a fuel Tank is to fit a diesel liner. Regards Pete 50year boat builder that has done the job many many times
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Old 19-04-2011, 08:46   #11
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Re: Converting Integral Fiberglass Water Tank to Diesel

If you have access to the interior of the tank you could coat the interior multiple times with an epoxy. Check the West System library for more information.
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