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Post Posted: Fri May 05, 2017 12:06 am 
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1976 Aquasport 170

Installing a new fuel tank once its fabrication is complete. Having trouble figuring out how to secure the tank. Below is a cross-section sketch of the fuel coffin liner and the adjacent stringers.

I had my mind made up to weld tabs on tank and fixate these to a pvc mounting block, the block either glassed in or bonded. I planned on overdrilling the stringer, fill with epoxy, then drill the tab screws into the hardened epoxy. But the problem I forsee is the liner does not rest against the stringers. The stringers and the liner have completely different angles from their base, as you can see from my sketch. The only contact point the stringer and liner make runs the length of the liner, bottom corner.

I took a 3/32 drill bit and drilled through the liner, in a location where tabs would line up. Once I drilled through the thin liner I used the drill bit to measure the amount of space between liner and stringer. About an 1" to 1-1/4".


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Another option would be to use Moeller's hold down strap, pictured below. I like this method. Actually, it coincides with how Aquasport originally secured tanks, 2 aluminum straps. But if I were to use these straps I'm left with the same problem as described above...no solid structure for the anchor screws to bury in.



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I'm open to enclosing the tank using foam filled plastic bags. But filling the space between liner and tank with foam will only prevent the tank from moving laterally. I would still be left with figuring out how prevent the tank from elevating.

I also don't have the option to glass in a 2x4 over top of the tank, the width of tank coffin. This is because there is only 1/2" gap between the top of the fuel inlet hose and the underside of the fuel tank access panel.

Btw, I am familiar with the pascoe method. I'm actually doing the modified version, only bonding pvc to tank to elevate for air flow. I am not bonding the pvc strips to the fuel coffin floor for the reason I believe to be obvious...future extraction of tank.

Thoughts, suggestions welcomed.

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Post Posted: Fri May 05, 2017 8:47 am 
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You want as little touching the tank as posible. I'd use the modified Pascoe method of gluing starboard strips to the bottom of the tank. This elevates the tank off the bottom of the coffin to keep it out of any water and allows air circulation. I'd secure the tank with straps. There has been no issues with factory straps pulling out. If your worried just secure them lower down. Thi will put the loads on the screws in sheer. Also you can use more screws and you can use screws long enough to catch the stringer, but I wouldn't worry about it. Whatever you do don't put any kind of foam around the tank even if it is in plastic. It is ok if the alloy used to make the tank gets wet, as long as it can dry out. What you don't want is water trapped against the tank.

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Post Posted: Fri May 05, 2017 10:34 am 
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Exactly as Bruce says. Only thing we differ on is the straps. I've seen plenty of tank corrosion beneath straps. They are stainless, tank is aluminum. Dissimilar metals, and eventually galvanic corrosion/crosstalk occurs....even with rubber as a buffer.

We used FRP hollow rectangular tube above the tank, glassed in. They come in all sizes/configurations, so the height limitation you have shouldn't be a problem.

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Post Posted: Fri May 05, 2017 3:50 pm 
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Oh I agree on the dissimilar metals issue. Use aluminum straps. I insulated mine with thick clear vinyl.

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Post Posted: Fri May 05, 2017 9:05 pm 
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wing time wrote:
I'd secure the tank with straps. There has been no issues with factory straps pulling out. If your worried just secure them lower down


You might of skipped over what the issue is with installing straps, detailed in my previous post. See below:

Quote:
Another option would be to use Moeller's hold down strap. I like this method. Actually, it coincides with how Aquasport originally secured tanks, 2 aluminum straps. But if I were to use these straps I'm left with the same problem as using welded tank tabs...no solid structure for the anchor screws to bury in


What Im probably going to do is have my tank fabricator weld 4 tabs with a 1/2" setback from being flush with side of tank. The 1/2" of space between liner and tank, plus the 1/2" tab setback will allow me to 5200 bond 1" thick mounting blocks to the liner. I'll then drill in 1" head screws into the block, 2 per tab. This will be plenty of security.

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Post Posted: Sat May 06, 2017 12:01 am 
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Jason,

Go to the Pascoe site and re-read the tank install article in detail. Pay particular attention to upper chocks, tie-downs, etc....he maintains they are not needed. 5200 to the base and fuel weight keeps it in place.

I personally don't agree. But why waste money on costly and unnecessary welding when proven solutions have been offered.

Your method will work. But compare the tank tab costs you suggest ..then your opportunity cost (personal labor rate) to install the spacers, fasteners, etc., etc.... to the two simpler methods presented. Don't waste time and money... any of the three choices should be serviceable for thirty to forty years if properly installed.

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Post Posted: Sat May 06, 2017 12:24 am 
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I didn't miss that part. The walls of the coffin while thin are more than enough to hold the tank in place.

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Post Posted: Sun May 07, 2017 1:16 pm 
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gran398 wrote:
....5200 to the base and fuel weight keeps it in place..


And you don't think the 5200 bond between pvc strips and base of coffin will pose a problem when the tank requires extraction?? I do.

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Post Posted: Sun May 07, 2017 11:22 pm 
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First off, as stated, I disagree with Pascoe on upper stability/ movement/chocks. Particularly in a heavy seaway as is the norm here, Cape Fear, Atlantic Ocean. You want the tank secured/ chocked down on the top. As per photo.

Second....don't know how long you'll keep this boat. But if you'll follow our thinking, that is, go with proven success that is not our thoughts but those of others before...you'll be pleased. The tank will never hold water on the top, sides, or bottom, moisture will always dry. May be forty years before extraction.

So third. Given a correct and proper install, why the worry on the 5200? You won't deal with it, nor any owner for a LONG time to come. And when they do... it is a simple mechanical bond. A come-along from above will pull the tank inside of two minutes, leaving the strips intack. No damage to the boat or tank. It's not kryptonite. It's adhesive.

Another thought. You are here for help and advice. The old guys around here are just that: old :mrgreen:
Everything you'll deal with regarding a boat...someone here already has. Take advantage of their experience. Will save you time and money. And thank them for their post.

Last thing we'll do is give you or any member bad advice. Go with the strips below, 5200, chock her down, and you're through.

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Post Posted: Sun May 07, 2017 11:47 pm 
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gran398 wrote:
First off, as stated, I disagree with Pascoe on upper stability/ movement/chocks. Particularly in a heavy seaway as is the norm here, Cape Fear, Atlantic Ocean. You want the tank secured/ chocked down on the top. As per photo.

Second....don't know how long you'll keep this boat. But if you'll follow our thinking, that is, go with proven success that is not our thoughts but those of others before...you'll be pleased. The tank will never hold water on the top or sides, moisture will always dry. May be forty years before extraction.

So third. Given a correct and proper install, why the worry on the 5200? You won't deal with it, not any owner for a LONG time to come. And when they do... it is a simple mechanical bond. A knife blade and a crowbar intelligently used will pull everything inside of two minutes, no damage to the boat or tank. It's not kryptonite. It's adhesive.

Another thought. You are here for help and advice. The old guys around here are just that: old :mrgreen:
Everything you'll deal with regarding a boat...someone here already has. Take advantage of their experience. Will save you time and money. And thank them for their post.

Last thing we'll do is give you or any member bad advice. Go with the strips below, 5200, chock her down, and you're through.



I won't have this boat when it needs a new fuel tank, so the extraction won't be my problem. My reluctance to bond pvc strips to coffin is entirely because I'd feel bad for the next owner. Sounds stupid, just the nice guy in me. I'll most likely do without tabs, bond tank to coffin, chocks on sides. Done. Appreciate the suggestions.

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Post Posted: Mon May 08, 2017 2:26 pm 
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Don't bond the PVC strips to the bottom of the coffin. The Pascoe method is written based on larger boats without a tank coffin. Only bond them to the bottom of the tank. The reason is two fold, one to keep them located in place, two the keep water from being trapped between the strips and the surface of the tank causing crevice corrosion. The weight of the tank will hold itself in place pretty good. The straps are just there for rough water etc. I can tell you my 200XF left the water several times and my tank held in the coffin with straps alone stayed in place just fine.

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