Fuel Tank: Removal without damage to Foam; Test PSI

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
rnln
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Fuel Tank: Removal without damage to Foam; Test PSI

Postby rnln » Mon Nov 08, 2021 10:32 am

[In regard to a Boston Whaler 22-foot classic hull and its fuel tank, separated from a discussion on another topic--Moderator]

Q1: If I lift the [aluminum OEM Florida Marine Tanks fuel] tank [out of the fuel tank cavity on a 1983 REVENGE 22], can the fuel tank be easily put back in place without damaging the foam surround?

Q2: I plan to pressure test the fuel tank: how many PSI should I test with?

jimh
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby jimh » Mon Nov 08, 2021 10:40 am

rnln wrote:Q1: If I lift the fuel tank, can I easily put fuel tank back in place without damaging the foam surround?
I doubt the foam would remain intact. You probably will have to saw the foam away from the tank on the sides of the tank. The foam on the bottom of the tank will probably have to be broken away when you lift out the fuel tank. Usually when a tank is removed, the foam is removed and not replaced when the tank (or a new tank) is installed.

rnln wrote:Q2: I plan to pressure test the fuel tank: how many PSI should I test with?
Use very low pressure, probably only 3-PSI at most. Consider the total force exerted on the interior surface of the tank. Calculate that area and multiply by the PSI to find the total force.

For example: a 77-gallon fuel tank can be filled with 77-gallons of gasoline. Gasoline has a volumetric density of 231-cubic-inches per gallon. Thus the volume of the tank must be at least 17,787-cubic-inches.

Since no information is given about the surface area of the fuel tank, the surface area will be deduced from the volume of the tank. For this purpose the fuel tank shape is assumed to be a cube (to simplify calculations), and thus each side of cube would have a length of 17787^0.33 = 26.1-inches, and a surface area of 26.1^2 = 681-square inches. There are six sides to a cube so the total surface area is then 6 × 681 = 4,087-square-inches. Applying a uniform pressure of 3-lbs-per-square-inch will then exert a total force on the the tank of 4,087-square-inches × 3-lbs-per-square-inch = 12,262-lbs.

Other complex fuel tanks shapes may have a different ratio of volume-to-air; calculation of surface area can be done if the shapes and surface areas of each facet of the tank shape are known.

In some instances, the OEM label on the fuel tank will give advice on the maximum pressure to be used in testing. If the OEM label is no longer legible, contact the OEM manufacturer (FMT) for advice. At one other source of advice on the web cautions against using no more than 3-PSI.

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Phil T
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby Phil T » Mon Nov 08, 2021 11:12 am

Removal of the tank is not required. Take the boat to the vendor location.

There is a significant chance you may damage the tank during foam and tank removal.
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dtmackey
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby dtmackey » Mon Nov 08, 2021 2:03 pm

Test the fuel tank in place at the recommended 3-PSI or less. If you decide to remove the tank, there's the chance the foam adhered to the tank and will act like glue, holding it in place. Some people use a Sawzall with a 12-inch blade, others use a hand saw. Another quick way [to cut foam] is a pressure washer with a straight tip. The water stream cuts right through the foam and makes quick work of it.

Pressure testing a tank in place may tell you it's not leaking but doesn't let you know if corroded on the bottom or sides and going to fail two to three years later. Removal allows you to inspect the tank surface to know if it will last long term. If you see some form of corrosion on the tank, you can also take measures to extend the tank life if the corrosion isn't deep by treating and coating the tank.

D-

rnln
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby rnln » Mon Nov 08, 2021 2:56 pm

Thanks everyone for valuable opinions.
It is sad that I have to get rid of those foam on an unsinkable boat. I probably get a camera to see the inside and pressure test first and go from there.
Thanks again.

jimh
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby jimh » Mon Nov 08, 2021 3:14 pm

rnln wrote:It is sad that I have to get rid of those foam on an unsinkable boat.

The reserve buoyancy of the hull with the cockpit is flooded is listed by Boston Whaler as the SWAMPED CAPACITY in lbs. For the 22-foot hull as a REVENGE, the swamped capacity is 5,000-lbs.

By removing a few cubic feet in foam volume in the fuel tank cavity, you will allow more water to be held in a swamped hull. Water weight per cubic foot is about 62.4-lbs.

Assuming the foam you removed was completely dry and unsaturated, and you left air space in its place equivalent to 3-cubic feet, the swamped capacity would then reduce by 187-lbs. If you do get in a swamped situation and need to get back those 187-lbs of reserve buoyancy, just throw some big guy overboard with a PFD and tell him to tread water for a while until the rescue boat arrives.

Vance's Revenge
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby Vance's Revenge » Mon Nov 08, 2021 4:01 pm

rnln wrote:Thanks everyone for valuable opinions.
It is sad that I have to get rid of those foam on an unsinkable boat. I probably get a camera to see the inside and pressure test first and go from there.
Thanks again.


I have to say I highly recommend that you remove and inspect the tank. Even fresh water in that foam will cause corrosion to eat away at it over the years. If it is not corroded through you may be able to clean the outside of the tank and protect it from further damage. Your already there; its not a fun job but pretty easily done and if you take your time you should not hurt the tank. I used a pry bar and shop vac chipping away at it but like the idea of the pressure washer if it works.

The peace of mind of knowing the tank isn't leaking fuel into the foam and water from the foam into the tank is worth it.

Vance's Revenge
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby Vance's Revenge » Mon Nov 08, 2021 4:24 pm

jimh wrote:
rnln wrote:It is sad that I have to get rid of those foam on an unsinkable boat.

The reserve buoyancy of the hull with the cockpit is flooded is listed by Boston Whaler as the SWAMPED CAPACITY in lbs. For the 22-foot hull as a REVENGE, the swamped capacity is 5,000-lbs.

By removing a few cubic feet in foam volume in the fuel tank cavity, you will allow more water to be held in a swamped hull. Water weight per cubic foot is about 62.4-lbs.

Assuming the foam you removed was completely dry and unsaturated, and you left air space in its place equivalent to 3-cubic feet, the swamped capacity would then reduce by 187-lbs. If you do get in a swamped situation and need to get back those 187-lbs of reserve buoyancy, just throw some big guy overboard with a PFD and tell him to tread water for a while until the rescue boat arrives.


Jim is obviously great with numbers and I like his humor with the "big guy" overboard!

When I replaced my fuel tank I didn't foam in the new tank and went to extremes changing the original under deck drain design to prevent water from gaining access to the fuel tank compartment. I was successful but it was a lot of work. First thing to remember is not foaming the tank in creates a big area around the tank to hold water that you cannot get out if you don't make those changes correctly. First change is simple by adding a 1" ID drain tube and plug installed at the bottom between the fuel tank cavity and the stern live well. Without this drain tube there will be no way to get water out of the tank compartment. I chose to use fiberglass tubes because they are easier to install and never will corrode and rot away.

Since I did my build I have seen where guys have foamed in the tank and then put a thin layer of fiberglass over the entire tank area level with the top of the tank and foam so the original drain design can be utilized without water getting to the tank and foam. This would be fairly easy and pretty inexpensive to do.

If I were to start over I think I would do the fiberglass over the top or another very easy solution would be to put in the tank, trim the foam level to the top of the tank and then coat then entire area with waterproof epoxy bilge paint. The main reason the foam absorbs the water so easily is the factory trimmed the foam along the tank which opens the cells to begin taking on water. The epoxy bilge paint would prevent that from happening.

dtmackey
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby dtmackey » Tue Nov 09, 2021 4:38 pm

The most common coating for tank preservation is coal tar epoxy. In fact, some tank builders offer that as part of tank fabrication. Bare aluminum will not last forever and depending on the environmnet and how wet the tank cavity becomes, corrosion will take place. What every you do, NEVER use rubber strips or neoprene to pad a bare tank. The carbon in these materials will accellerate the corrosion process. Many people think that using this helps the tank from wear. If you can find equivalent materials that are carbonb free they you should be fine.

I've seen tanks fail in under 15 years that were factory installed and replaced one in a Mako center console boat. I find they typically corrode on the bottom.

Some opt to spray their tanks with bed-liner material from a pickup truck.

D-

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andygere
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Re: Fuel Tank: Removal without damage to Foam; Test PSI

Postby andygere » Fri Nov 12, 2021 1:32 am

If you are going through the trouble to remove the fuel tank deck cover on a 1983 22-foot hull, you should just plan on replacing the tank. [A 38-year-old aluminum fuel tank] is long past its service life.

There is no way to remove the tank without cutting away the foam. The tank is literally bonded into the tank cavity with the foam.

Along the sides of the tank, you are likely to find many deep pits in the aluminum where water was held next to the tank by the saturated foam. These may well pass a 3-PSI pressure test, but they will leak soon, and that can be a dangerous mess.
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