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Old 06-02-2019, 02:55 PM   #1
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Leaking Diesel Tank

My 36 year old Allweather has a 112 gallon fuel tank which is a lot for a boat that burns 1/2 to 3/4 gallons an hour. The tank is the floor of the main cabin, You are walking on the steel top. I am not sure what kind of steel it is but it is magnetic and fairly thick as there is no flex. The rest of the tank is fiberglass. I recently fill up the tank to within a couple inches of the top, I had never filled it that much before. The boat is on a mooring and sometimes is rolling quite a bit in certain conditions. Today I noticed that there are several places around the perimeter of the top of the tank that are leaking slightly. The fuel in the rough conditions is sloshing up around the edges. There are no leaks anywhere else. The amount is very small but in several places. There is a raised edge all around the perimeter that joins the steel tank top to surrounding plywood. The lid must be fastened somehow to the stringers and sealed somehow. It would be fairly straight forward to grind down that ridge and redo it but the new bond to the steel might be a problem.

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Old 06-02-2019, 04:19 PM   #2
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Greetings and I wish you weren't dealing with leaking diesel! I looked at your other photo you have showing the fuel tank layout

Trawler Forum - ak-guy's Album: Allweather - Picture

Its certainly an oddity to have the tank top part of the floor like that!

I'm sure you've seen the drawings on the allweather website - (it looks like your boat is there?)

Allweather Boats History

Click image for larger version

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That drawing is pretty clear, it shows the wood stringers, radius filler, stringer tabbing, fiberglass layer and the dang sheet of steel on the top! Assuming yours was built according to that plan, it doesn't show fasteners, but it also doesn't show any floor detail. From your previous photo in your album, it looks like there is some sort of a furring strip around the top edge of that steel plate, I'm wondering if you have some mechanical fasteners under there, or if there was previous leakage and a PO oversealed it with that furring strip. You might reach out to other allweather owners to see if you can find someone else who has dealt with a leak, or at least compare floors to get a better idea of what you're dealing with.

Based on your thought its a steel plate on the stringers, and the construction drawing from the allweather site, I'd say you're looking at draining your tank, removing that plate, and recreating your tank top from scratch - just to make sure you have a bullet proof and safe tank.

If it were my boat, I'd look at a solution that has a separate floor above the tank top, my intuition says don't use a sheet of 10g steel as a floor!

Hopefully some fuel tank gurus stop by and give some more authoritative advice on that particular tank design. Its odd that it doesn't seem bulletproof when the rest of the construction of the boat seems aimed at just that!

Good luck!
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Old 06-02-2019, 04:55 PM   #3
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fractalphreak

Wow, thanks for the drawing. I have been to the Allweather site many times but didn't remember that image. I am thinking I will pump down the fuel level in the tank now and do a repair this winter. The lower fuel level shouldn't be a problem. I have liked the difference the extra ballast has made.

It has been suggested to me to use vinyl ester resin if I do a repair. The critical factor would be adhesion to the metal so also considering West system G/Flex.
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Old 06-03-2019, 06:14 AM   #4
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I would use Epoxy resin and thin cloth to allow the resin to flex and stat adhered.
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Old 06-03-2019, 10:05 AM   #5
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My dos centavos

I'd suggest pulling everything out and have a poly tank made to fit. Integral tanks are great, until they're not. Remediation of a failing system vs replacement becomes the choice. In this day, double hull for integral tanks is close to a mandate in the event of a strike or grounding.
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Old 06-05-2019, 09:02 PM   #6
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My dos centavos

I'd suggest pulling everything out and have a poly tank made to fit. Integral tanks are great, until they're not. Remediation of a failing system vs replacement becomes the choice. In this day, double hull for integral tanks is close to a mandate in the event of a strike or grounding.
You are probably right. I am thinking of cutting a rectangle out of the top of the tank big enough to fit a poly tank inside. The tank is 112 gallons and my engine burns less than 1 gph so a smaller tank would be fine by me. The top is 10 gauge steel. What would you use to cut 15' in 10 ga steel? I have circular saw, sawzall, jigsaw, right angle grinder. Any particular blades? Removing the tank top would require a lot of grinding of fiberglass and putty and probably heat once down to metal.
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Old 06-05-2019, 09:08 PM   #7
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I second a smaller tank in the space the old tank is in now. With your fuel burn you could certainly get by with less fuel.
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Old 06-05-2019, 10:52 PM   #8
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Originally Posted by ak-guy View Post
The top is 10 gauge steel. What would you use to cut 15' in 10 ga steel? I have circular saw, sawzall, jigsaw, right angle grinder. Any particular blades? Removing the tank top would require a lot of grinding of fiberglass and putty and probably heat once down to metal.
I would use a nibbler if you can come up with one. I don't know Gustavus is going to have a United Rental though! Is there someplace/body that does sheet metal work? Somebody local might have one... The right size nibblers will cut 10 g easy, no flame, spark, etc. Drain tank, drill a hole large enough to start nibbler head, go to town. They are like a punch that punches out little rectangles of metal.

Not the greatest video, but you get the idea....
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Old 06-08-2019, 02:54 AM   #9
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Originally Posted by fractalphreak View Post
I would use a nibbler if you can come up with one. I don't know Gustavus is going to have a United Rental though! Is there someplace/body that does sheet metal work? Somebody local might have one... The right size nibblers will cut 10 g easy, no flame, spark, etc. Drain tank, drill a hole large enough to start nibbler head, go to town. They are like a punch that punches out little rectangles of metal.

Not the greatest video, but you get the idea....
I've not heard of a Nibbler before and the YouTube clip certainly made it look easy. You recommend it over using a sawsall with the appropriate blades if cutting out a steel tank?
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Old 06-08-2019, 04:25 AM   #10
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I've not heard of a Nibbler before and the YouTube clip certainly made it look easy. You recommend it over using a sawsall with the appropriate blades if cutting out a steel tank?
I would, especially if its a large tank like many of us have on our trawlers. I haven't personally used one to cut out a tank, I've used them to fabricate odd shaped sheet metal pieces in a past life. If I were going to cut a large tank up in place I would go the nibbler route first. Because of the way they cut, there are no sparks, they literally punch the metal out and go through it like butter. Because of the size of the tool and the fact you have to have the head on both sides of the cut, you'd have to cut the tank into more pieces to access it all and make the best use of the tool, but because of how fast it goes it still wouldn't be that big of a deal...now if you wanted to save it intact to make a pattern...


It would be a cost to do the project, I don't have one, and they are $4-500 new for a model to cut 10 gauge like ak-guy has. It may not be worth it for him if he can't find one to rent or borrow in his borough because by the time you count up the cost of a day and some blades for a sawsall -vs- buying a pricey tool....


When I did a recent project to add a couple access plates to my own tanks I did use a jigsaw, just cutting out two 8 inch holes. I have a jigsaw with a variable speed and slowed it all the way down and let the blade cut on its own, I did kink one blade when I didn't turn the tool evenly due to the crowding. My tanks are 1/8", just shy of 10 g. I wouldn't want to try to cut up a 100+ gal steel tank with a jigsaw or sawsall if I didn't have to. I'm not too worried about the fuel/spark aspect with those tools, but the time....still might have to use a sawsall some, depending on access, corners and such.
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Old 06-09-2019, 06:46 AM   #11
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There may be countersunk screws holding the top down to the wood stringers. Sanding away the coating around the tank top perimeter should show signs of them.
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