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Author Topic: School me on fuel tank coffins.  (Read 671 times)

March 17, 2018, 04:06:54 AM
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theFunsmith

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School me on fuel tank coffins.
« on: March 17, 2018, 04:06:54 AM »
One rebuild topic I am having a little bit of a hard time fully grasping is the concept of a fuel tank coffin. Is it for fuel tank impact/puncture protection? or is the coffin to isolate spillage in the event of a leak? I feel like I can do a much better job of fuel tank installation if I have a better idea of what a good fuel tank coffin is supposed to achieve.

March 17, 2018, 09:07:22 AM
Reply #1

mshugg

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Re: School me on fuel tank coffins.
« Reply #1 on: March 17, 2018, 09:07:22 AM »
I remember some early Aquasport marketing that claimed the coffin isolated the tank from the bilge and contained any fuel spills.  The problem was that any water that made it into the coffin, as inevitably it did, stayed there.  Corrosion and tank failure were the result. 

Then, Aquasport and other builders started putting drains in the coffin, better but sometimes the drains clogged.  That’s the condition mine was in when in with salt water trapped inside the coffin.  So, when I built my tank coffin, I put in 3 larger drains.

If you look over the build and tank replacement threads, you can get a lot of ideas.  They seem to fall into three main camps. 

1.  Some replicate the original tank install useing straps.  Many caution against useing neoprene, or any carbon containing material to pad the straps.  Some coat the tank with coal tar epoxy, primer & paint or powder coating. 

2.   Some use the Pascal method or a modified Pascal.  The theory is that Aluminum develops a protective corrosion, so by eliminating anything that can trap water in contact with the tank, your tank will last.  This is what I did.

3.   Another common procedure is to coat the tank, usually coal tar epoxy, and foam it in.  A drain needs to be incorporated below the foam to allow water from forward to move to the bilge.  Many use epoxy and a layer of glass to further isolate the tank.

Polyethylene tanks are a different mater.  I think straps or chocks are the go to method for securing them.  Poly doesn’t corrode, but it expands when filled and your bilge will smell like gas.  Perhaps the biggest problem to poly is it’s produced in a limited range of sizes, shapes and configurations, so it doesn’t work for everyone.  Rick took a cool approach to securing his poly tank in his 170.

March 17, 2018, 12:53:03 PM
Reply #2

theFunsmith

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Re: School me on fuel tank coffins.
« Reply #2 on: March 17, 2018, 12:53:03 PM »
I think I can deal with some fuel smell to never have to worry about corrosion. I would like to build things to last as long as absolutely possible without having to tear into the hull again. As for tank expansion, I am gathering that it is a one-time thing. If so, would it be a horrible idea to put some gas in it, allow it to fully expand, and then foam it in?

Edited to add: I went back to look at Rick's thread. I like that install a lot. I think I may be doing something very, very much like that now. Thank you.

March 17, 2018, 01:52:21 PM
Reply #3

mshugg

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Re: School me on fuel tank coffins.
« Reply #3 on: March 17, 2018, 01:52:21 PM »
Yeah, Rick’s tank mount was slick. 

I think that all manufacturers of poly tanks consider foam a no-no.  Plus, do you really want to start  a heat producing chemical reaction in contact with something made out of chemicals, and introduce gasoline into the equation?

March 17, 2018, 03:23:24 PM
Reply #4

theFunsmith

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Re: School me on fuel tank coffins.
« Reply #4 on: March 17, 2018, 03:23:24 PM »
do you really want to start  a heat producing chemical reaction in contact with something made out of chemicals, and introduce gasoline into the equation?

:embar:

March 18, 2018, 07:48:55 AM
Reply #5

RickK

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Re: School me on fuel tank coffins.
« Reply #5 on: March 18, 2018, 07:48:55 AM »
Yeah the poly tanks grow in all directions 1% when they are initially exposed to gasoline.  Make sure you leave that amount of room, or more, all around and however you secure it, make sure it still allows for that expansion.
Rick
1971 "170" with 115 Johnson (It's usable but not 100% finished)

1992 230 Explorer with 250 Yamaha

March 21, 2018, 06:00:41 PM
Reply #6

boatnamesue

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Re: School me on fuel tank coffins.
« Reply #6 on: March 21, 2018, 06:00:41 PM »
One of the mentioned purposes of a coffin to prevent fuel spillage into the bilge is erroneous.  The is because the coffin has aft vent holes to allow water to escape into the bilge.  If water can escape, so can fuel.  I know this because it was how I discovered I had a leaking fuel tank....fuel was dumping out of the transom plug hole when my boat was parked on the declined ramp prior to launching into water.   

---------------
Jason
1976 AS 170
1998 S115TLRW

March 22, 2018, 01:00:02 PM
Reply #7

wingtime

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Re: School me on fuel tank coffins.
« Reply #7 on: March 22, 2018, 01:00:02 PM »
I believe the fuel tank coffin served more than one purpose.  Separating the tank from the rest of the damp and nasty bilge is probably #1 reason for the design.  Keeping fuel fumes from building up in the bilge is another possible benefit for the design.  Remember the original  and early Aquasport designs had virtually NO bilge access or even bilge pumps.  The idea was for the self bailing deck was to keep all the water out of the bilge so there was no need to get in there.  The fuel coffin was a logical progression of that design.  Easy fuel tank service and install is another benefit.  In a perfect world the bilge would stay totally dry and so would the coffin.  In reality over time water finds it's way into both spaces.  I personally think it is a great feature and I wish they still built them that way.... well still built Aquasports too.
1998 Explorer w/ Etec 250


1987 170 w/ Evinrude 90

March 23, 2018, 04:49:14 PM
Reply #8

GoneFission

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Re: School me on fuel tank coffins.
« Reply #8 on: March 23, 2018, 04:49:14 PM »
The original early under-deck Aquasport fuel tank coffin/tub did not have drain holes or vents, so it allowed an underdeck gas tank without having a bilge blower or other bilge ventilation  The idea was that any gas would be contained within the coffin where where were no electrical connections or any chance for a spark.  The fuel tank coffin would fill up with water from condensation or leakage from the overhead cover.  Many owners drilled holes in the back of the tub/coffin for drainage.  Aquasport also put vents in the sides of boats that had drains in the fuel tank tubs.  Early CCPs did not have the side vents and had sealed fuel tank coffins/tubs.   In general, the sealed tubs did not work so well, and the ABYC (American Boat & Yacht Council) rules changed a bit to allow a below-deck tank without vents or blowers if the tank was properly isolated, sealed, and protected from leakage.   

A truly original Aquasport from the 1970s through about 1984 (especially a CCP) would probably not have a drain hole in the fuel tank coffin/tub.  But it seems the folks at Hialeah changed manufacturing processes about as often as most folks change underwear, so anything is possible. 
Cap'n John
1980 22-2 CCP
Mercury 200 Optimax 
ASPA0345M80I
"Gone Fission"
ClassicAquasport Member #209


March 23, 2018, 06:17:03 PM
Reply #9

RickK

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Re: School me on fuel tank coffins.
« Reply #9 on: March 23, 2018, 06:17:03 PM »
Until '72 the fuel tanks were inside the console - my '71 was. This kept the bilge sealed from the topside.
Rick
1971 "170" with 115 Johnson (It's usable but not 100% finished)

1992 230 Explorer with 250 Yamaha

 

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