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Old 19-01-2014, 09:12   #16
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Re: Polysulfide to seal diesel tanks

There's no doubt that building a boat is different from working on or modifying a boat. Different challenges. Your solution is appropriate to your situation.

You are 100% right, I don't know if I have leaks. How do I pressure test without first installing the tank top? The tank top will be just under the sole, welded in situ, is about 15" long and 10" wide, the access port will be about 7" long and 5" wide. Pretty hard to weld 4 feet down through that opening.
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Old 19-01-2014, 09:21   #17
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Re: Polysulfide to seal diesel tanks

Thats why I was wondering if you were able to fit a tank within the space in the keel. It could be fabbed, pressure-tested, bedded with foam in the keel space, and possibly removed for servicing and inspection if needed.
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Old 19-01-2014, 09:50   #18
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Re: Polysulfide to seal diesel tanks

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Thats why I was wondering if you were able to fit a tank within the space in the keel. It could be fabbed, pressure-tested, bedded with foam in the keel space, and possibly removed for servicing and inspection if needed.
It's the keel material I'm trying to protect. It's a real PITA to install and remove the tanks. And there is never a guarantee that you are protected in between.
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Old 19-01-2014, 09:52   #19
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Re: Polysulfide to seal diesel tanks

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There's no doubt that building a boat is different from working on or modifying a boat. Different challenges. Your solution is appropriate to your situation.

You are 100% right, I don't know if I have leaks. How do I pressure test without first installing the tank top? The tank top will be just under the sole, welded in situ, is about 15" long and 10" wide, the access port will be about 7" long and 5" wide. Pretty hard to weld 4 feet down through that opening.

When we build regular fuel tanks to install in a boat, they are completely built, all welding done, most with no inspection plates.

We then pressure test them, and any leaks are welded up on the
OUTSIDE of the tank, not the inside.
A weld on the outside will hold, as where a coating only has a chance to hold if it is applied on the inside of the tank.

My question is if the inspection plate is to small to weld thru and get to all areas of the tank, how are you going to get there with your magic sealant?

It comes down to this, a weld on a steel tank is a permanent repair, a gob of sealant is a patch.

Our tanks are built in fiberglass, with many layers of fiberglass and epoxy, then the tanks were sanded and had several coats of epoxy rolled in them, then sanded again and coated with Devoe tank coating.
We are still going to pressure test them before we fill them, it is the prudent thing to do. If we find a leak, we will repair in from both the inside and the outside, as we have inspection ports in the top of the tanks.

Any welded repair on a steel tank can be done on the outside after pressure testing.
It is done all the time.
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Old 19-01-2014, 10:18   #20
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Re: Polysulfide to seal diesel tanks

Capt,

Thanks for the advice. I just don't see how that applies to my situation with integral tanks.
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Old 19-01-2014, 10:30   #21
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Re: Polysulfide to seal diesel tanks

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Capt,

Thanks for the advice. I just don't see how that applies to my situation with integral tanks.
You are over thinking the problem.
The tank is a tank is a tank, in your case it is just part of the boat.

it has a front, a back, a bottom, a top, and sides, and they are both a tank and part of the boat.

All of the seams can be accessed from the outside. The keel seams, the plating seams, and the front and back of the tank, and the lid that has not been put in place.

As long as you can access all of the seams from the outside, and I would not know why you could not, you can weld them from there.

I have put integral tanks in several boats, and be happy you are dealing with metal, as it is the easiest to fix because a good repair can be made on the outside only without having to access the inside of the tank.

Pictures would help if you could post some.
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Old 19-01-2014, 11:31   #22
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Re: Polysulfide to seal diesel tanks

[QUOTE=Guy;1444301]
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Originally Posted by Celestialsailor View Post
By the way aircraft use bladders.

Why would anyone take your advice when you say something like that? If you wanted to be even a little bit correct you might have said some small planes use bladders but yes pretty much all modern planes have wet wings and if it is made from metal it uses a polysulfide sealant. If they didn't seal them up they would leak fuel out faster than you could put it in. That said they do put it on from the inside and I've never had any luck trying to seal up a leak from the outside.
Ya...I'm sorry but I won't be loosing any sleep if you don't take any of my advice. And yes, I have seen bladder in only smaller aircraft. But...I have built 3 steel boats...2 of my own and I would not goop some sealant in seams when I could weld 5% of the area to do the same thing and have it be way more permanent. To use sealant for a cover, well that's fine but to chance an internal seal from leaking diesel into a ballasted area...why would anyone take that advice?
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Old 19-01-2014, 14:14   #23
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Re: Polysulfide to seal diesel tanks

Try this sketch. This is the compartment in the keel. The keel sheets are continuous. The dividers are just vertical plates, think baffles. The sole is 1/2".

The top is about 15" long and 10" wide. It is about 48" deep.
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Old 19-01-2014, 14:31   #24
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Using polysulfide in the hope it will stop any potential leaks from non pressure tested voids is not something i would recommend.

As another poster mentioned aircraft typically use bladders and have an inspection regime that is both arduous and time consuming.

Either the welded tank needs to be pressure tested or fitting a known integrity tank are the two practical options. Any single walled option (hull is the tank) is not a trustable design option in this instance.

If pressure testing i would use liquid (inelastic) as air in an integral tank will pose safety issues. If you're familiar with tank testing you'll know what i mean.
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Old 19-01-2014, 14:40   #25
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Re: Polysulfide to seal diesel tanks

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Using polysulfide in the hope it will stop any potential leaks from non pressure tested voids is not something i would recommend.

As another poster mentioned aircraft typically use bladders and have an inspection regime that is both arduous and time consuming.

Either the welded tank needs to be pressure tested or fitting a known integrity tank are the two practical options. Any single walled option (hull is the tank) is not a trustable design option in this instance.

If pressure testing i would use liquid (inelastic) as air in an integral tank will pose safety issues. If you're familiar with tank testing you'll know what i mean.
Yup...That was my take also. Although there are wet wings. I was grounded one time because of a small leak through a rivet on a twin engine turbo. That was fun!
To the OP...I know if you could pull it off, you would be gaining over 20 gallons in extra storage which is not too shabby. But maybe as a compromise...If the area is easily assessable, maybe just stow a couple of 5 gallon poly jugs and call it a win.
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Old 19-01-2014, 15:32   #26
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Re: Polysulfide to seal diesel tanks

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Originally Posted by leftbrainstuff View Post
Any single walled option (hull is the tank) is not a trustable design option in this instance.
Why?

The designer seems to think its a good option.
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Old 19-01-2014, 15:50   #27
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Re: Polysulfide to seal diesel tanks

Don't most all steel ships use the hull as a fuel thank? Weren't even all tankers this way until somewhat recently when double hulls became the new standard?
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Old 19-01-2014, 16:06   #28
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Re: Polysulfide to seal diesel tanks

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I have a steel sailboat. The deep keel is void, I want to use it as an integral fuel tank by welding on a cap. I'm not 100% sure the voids are completely watertight (95%) and want to run a sealant over the welded seams to be 100% sure no diesel will flow into the adjacent keel sections. Adjacent keel sections are filled with steel and cement.

I have the designers blessing on this idea. In essence I will be creating a double hull for that section of the keel.

I know they use two part polysulfides in aircraft for this application.

So I'm looking at Boatlife Calk as a sealant. It comes in three varieties
One part in a calk tube
Two part pourable
Two part heavy

Which of the three products (tube, pourable, or heavy) would be best suited to this application?
Hpeer, I had leaking fuel tank and decided to repair it with polysulfide instead of replacing the tank. I decided to use the aircraft product because of the stringent certification process that has to be met to use it on an aircraft. I used a product called Flamemaster cs 3204 class B and sealed all the seams and let cure for a week. I then pressure tested the tank and it held 5 psi for a week so I returned it to service and that was 7 years ago. Here is the link to the product:
http://www.flamemaster.com/Technical...ev%2001-07.pdf

The important part to remember is the product has to be designed to withstand the attack of sulfur and other chemicals in diesel fuel so I would stay away from any boatlife products. Just because they are both in the polysulfide family, doesn't mean they will perform equally.
I hope this helps
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Old 19-01-2014, 16:06   #29
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Re: Polysulfide to seal diesel tanks

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Using polysulfide in the hope it will stop any potential leaks from non pressure tested voids is not something i would recommend.

As another poster mentioned aircraft typically use bladders and have an inspection regime that is both arduous and time consuming.

Either the welded tank needs to be pressure tested or fitting a known integrity tank are the two practical options. Any single walled option (hull is the tank) is not a trustable design option in this instance.

If pressure testing i would use liquid (inelastic) as air in an integral tank will pose safety issues. If you're familiar with tank testing you'll know what i mean.

Nope, I'm an A&P/IA and currently manufacture one aircraft a week at Thrush aircraft Inc., most aircraft are wet wings, sealed with either B2 or Pro-Seal, both brand names. a typical aircraft wet wing section has hundreds of rivets to seal to say nothing of the wing always flexes, the inspection requirement for wet wings is fix them if they leak, a wet wing should have no problems leaking for at least 20 years and or 10,000 hour, minimum.
The fuel tank sealer he intends to use is a permanent solution that will last Lord knows how long, I don't.

As far as tank testing, your talking Hydrostatic testing, which is high pressure, that is not how to test a fuel tank, to test a fuel tank you use air measured in inches of water, like maybe a couple of feet max, only a couple of psi, max. Using air as a testing medium makes a spray bottle filled with soapy water an excellent leak finder.
Testing procedure is to use a manometer, a home one is just fine, pressurize the tank, turn the air off and wait to see if there is a drop in pressure. It's entirely normal if the test begins in the morning for the pressure to rise slowly as ambient air temp rises.

Some aircraft like my C-140 and my Maule have separate metal tanks just like say an automobile, Most smaller aircraft do not have bladders, Military aircraft due as they are easier to make self sealing
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Old 19-01-2014, 16:11   #30
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Re: Polysulfide to seal diesel tanks

The "B" class is how thick it is, B is thick, A is thin. B2 means thick with a two hour pot life B1/2 means thick with a .5 hour pot life etc. A is best used only on a flat surface where you are trying to get something to run into a crack for example, usually followed by the B product, A you can brush but it's a little thicker something between 5200 and epoxy.
You think 5200 gets all over everything? You haven't played with B2 yet
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