Need Guidance on a 30 Year Old Fuel Tank - 24'Skipjack Fisherman

peter7883

Newbie
Apr 21, 2012
29
34
San Diego
Name
Peter
Boat
Skipjack
I dug out all the foam on the back of the tank all the way to the bottom so I could get a cargo strap underneath it. I then used a 26" hand saw to cut the foam the full length of the tank down to the hull to relieve the pressure away from the tank. I then attached a cargo strap on the tank and a rope around my hardtop. I then attached a come along between the cargo strap and rope and started to put steady pressure on it. I continued to cut the foam to relieve the pressure and the hold it had on the tank (having a hardtop on my boat made it a easier). I also used a few wood wedges that I placed at the bottom back of the tank and would alternate the pressure on the come along and driving the wedges further under the tank to get it moving upward. This was more tedious than the pictures made it look. It took me a solid 10-12 hours over the course of two days to get it done, but I was working it alone. If you don't have a hardtop you can use a 4x4 the width of your boat as the anchor for the come along.
 

peter7883

Newbie
Apr 21, 2012
29
34
San Diego
Name
Peter
Boat
Skipjack
I closed out the holes in the stringers with foam wrapped fiberglass cloth and encapsulated them and the area with resin. I also slid the fishbox forward and removed the remaining foam around the fish box. Now I'll remount the fishbox without foam in preparation for the new tank coming in another week or so.

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Albiebac

Member
Jul 23, 2004
288
88
Scripps Ranch
Name
Eric
Boat
25' Skipjack, Albiebac II
I dug out all the foam on the back of the tank all the way to the bottom so I could get a cargo strap underneath it. I then used a 26" hand saw to cut the foam the full length of the tank down to the hull to relieve the pressure away from the tank. I then attached a cargo strap on the tank and a rope around my hardtop. I then attached a come along between the cargo strap and rope and started to put steady pressure on it. I continued to cut the foam to relieve the pressure and the hold it had on the tank (having a hardtop on my boat made it a easier). I also used a few wood wedges that I placed at the bottom back of the tank and would alternate the pressure on the come along and driving the wedges further under the tank to get it moving upward. This was more tedious than the pictures made it look. It took me a solid 10-12 hours over the course of two days to get it done, but I was working it alone. If you don't have a hardtop you can use a 4x4 the width of your boat as the anchor for the come along.
Thanks for the follow-up. I like the strap idea, and your 10-12 hours wrestling with it sounds familiar. I'll be doing a 4x6 across the gunnels to get mine up. Mine is 31yo, so I imagine it is in similar shape.
Thanks!
 

peter7883

Newbie
Apr 21, 2012
29
34
San Diego
Name
Peter
Boat
Skipjack
I got the new tank from American Tanks and completed the installation. The new tank uses a thicker gauge aluminum .190 vs the original .125. I had them weld "L" brackets on top of the tank for mounting since I was not going to use foam to encapsulate it like the original installation. There are also 2" wide aluminum strips welded on both sides of the bottom length of the tank with 1/4'' neoprene glued on the same strips where the tank would make contact with the bottom of boat. I then used 1/4" angles to secure the tank to the stringers running from the forward to rear bulkhead. I used Blue Removable Thread Locker LOCKTITE for all fittings attached to tank. I also changed the 30 year old fuel feed hose. Fill and vent hoses were changed about five years ago. Thoughts on the final installation?

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Albiebac

Member
Jul 23, 2004
288
88
Scripps Ranch
Name
Eric
Boat
25' Skipjack, Albiebac II
Take this for what is worth, I'm not a professional and have not done a fuel tank install yet.
You need to consider the weight (600+ lbs) of fuel in the tank and what it will be doing while on the water. The tank is baffled to minimize the sloshing and movement, but the forces are still strong. Not to mention the flexing and settling that will happen just adding the fuel into the tank. That being said, you need to through bolt all fasteners, which will add 12 (or 24 if you consider both sides) possible points for water intrusion into your stringers. The screws you have in now will pop out or break as soon as you fill the tank. Are the other holes drilled through the stringers sealed? You certainly don't want any exposed wood.
I believe this is why most manufacturers end up foaming the tanks in (now with sealing the aluminum).
I would wait to see if others with real world experience chime in.
Otherwise, you're getting close, nice job.
 

MYNomad

Heading South
Dec 12, 2007
4,046
4,488
Pacific Northwest / West Coast Mexico
Name
Rick
Boat
Yes
Thoughts on the final installation?

What, if anything, is supporting the bottom of the tank? Nice for it to breathe, but it may need support. I suggest you ask the manufacturer.
If support is recommended, I would put some kind of corrosion inhibitor on the tank (tar may be best), and let the tank rest on pour-in, closed cell foam.
 

peter7883

Newbie
Apr 21, 2012
29
34
San Diego
Name
Peter
Boat
Skipjack
Take this for what is worth, I'm not a professional and have not done a fuel tank install yet.
You need to consider the weight (600+ lbs) of fuel in the tank and what it will be doing while on the water. The tank is baffled to minimize the sloshing and movement, but the forces are still strong. Not to mention the flexing and settling that will happen just adding the fuel into the tank. That being said, you need to through bolt all fasteners, which will add 12 (or 24 if you consider both sides) possible points for water intrusion into your stringers. The screws you have in now will pop out or break as soon as you fill the tank. Are the other holes drilled through the stringers sealed? You certainly don't want any exposed wood.
I believe this is why most manufacturers end up foaming the tanks in (now with sealing the aluminum).
I would wait to see if others with real world experience chime in.
Otherwise, you're getting close, nice job.
Take this for what is worth, I'm not a professional and have not done a fuel tank install yet.
You need to consider the weight (600+ lbs) of fuel in the tank and what it will be doing while on the water. The tank is baffled to minimize the sloshing and movement, but the forces are still strong. Not to mention the flexing and settling that will happen just adding the fuel into the tank. That being said, you need to through bolt all fasteners, which will add 12 (or 24 if you consider both sides) possible points for water intrusion into your stringers. The screws you have in now will pop out or break as soon as you fill the tank. Are the other holes drilled through the stringers sealed? You certainly don't want any exposed wood.
I believe this is why most manufacturers end up foaming the tanks in (now with sealing the aluminum).
I would wait to see if others with real world experience chime in.
Otherwise, you're getting close, nice job.
Thanks for the feedback. I did not do any detailed calculations on the screw strengths but you peeked my interest to do a deep dive into the topic. First let me say that I filled the tank about half way before I mounted the brackets to compress the neoprene sitting on the hull at the bottom of the boat. This was to minimize the possibility for downward movement on the tank specifically for the reason you mentioned. This leaves the potential for forward or aft movement. In this case the only thing holding the tank are 12 1/4" screws. Tensile values for the lowest grade 1/4" screw I could find is 840 lb. of tensile strength. The screws holding my tank would see more of a sheer load. Industry guidance of sheer load is approximately 60% of tensile strength. For round numbers and conservatism lets say 420 lb. of sheer (Proof load) per screw. This equates to 5040 lb. of holding power on a boat that gets 25 knots cruise on a flat calm day. I don't believe the tank is going anywhere even when full. But would appreciate additional feedback if you think my calculations are off. I could easily install through bolts at least on the rear mounts if convinced it is necessary. On the other topic, none of the holes going through the stringers for fuel lines/wires were sealed from the manufacturer and there is no evidence of any water intrusion. However all those holes are about 24" from the bottom of the boat where any water from the bilge would have to rise to get them wet. I guess If the water level was to get that high, there's probably a bigger problem at hand.
 

peter7883

Newbie
Apr 21, 2012
29
34
San Diego
Name
Peter
Boat
Skipjack
What, if anything, is supporting the bottom of the tank? Nice for it to breathe, but it may need support. I suggest you ask the manufacturer.
If support is recommended, I would put some kind of corrosion inhibitor on the tank (tar may be best), and let the tank rest on pour-in, closed cell foam.
Regarding supports on the bottom, the tank manufacturer said this is a typical installation when I discussed it with them before they built the tank. This is because it is a square shaped tank sitting on a "V" bottom with no lateral movement of the tank possible. The tank is sitting on 1/4" neoprene pads on both sides for the length of the tank. Forward to aft movement is the main reason for the 12 1/4" screws. Which mount to the stringers running from the forward to rear bulkhead. I did not speak to the the boat manufacturer, but they would likely not recommend anything other than the original foam installation. Which is a good installation until the tank needs to be pulled. I would like others to offer their thoughts as well. Thanks for the feedback.
 

MYNomad

Heading South
Dec 12, 2007
4,046
4,488
Pacific Northwest / West Coast Mexico
Name
Rick
Boat
Yes
Regarding supports on the bottom, the tank manufacturer said this is a typical installation when I discussed it with them before they built the tank. This is because it is a square shaped tank sitting on a "V" bottom with no lateral movement of the tank possible. The tank is sitting on 1/4" neoprene pads on both sides for the length of the tank. Forward to aft movement is the main reason for the 12 1/4" screws. Which mount to the stringers running from the forward to rear bulkhead. I did not speak to the the boat manufacturer, but they would likely not recommend anything other than the original foam installation. Which is a good installation until the tank needs to be pulled. I would like others to offer their thoughts as well. Thanks for the feedback.
Whether you need support from the bottom is really an engineering question heavily dependent on the design of the tank. There will be a lot of force on the bottom center of the tank, which will be transmitted to the sides through the welds joining the bottom and the sides. You could greatly reduce that force by adding a third pad strip down the center. But, I think you want something that won't have much give in it in order to limit the deflection of the center of the tank ideally to less than 1/8". I am skeptical that unreinforced neoprene can do that. Perhaps a block of treated wood cut to fit within 1/8" of the bottom of the tank, with neoprene on top of that. And I should have been more clear -- I wasn't suggesting you talk to the boat manufacturer, only the tank manufacturer. And the way I would phrase the question isn't whether you need bottom support, but whether it would be better to have it. If they say no, ask how much deflection they anticipate with a dynamic load of a full tank of fuel. If they have an answer, great, if not you might suspect they are winging it.
 

peter7883

Newbie
Apr 21, 2012
29
34
San Diego
Name
Peter
Boat
Skipjack
I understand your concern now. It is not that the tank is not secure and may move. It is about the structural integrity of the tank under dynamic loads without support across the bottom as was provided with the foam in the original installation. Let me give you a little more information about the tank design and construction and will look forward to your opinion. This tank material is .190" thick which is thicker than the original tank built at industry standard of .125" thickness. The tank is 56" long and has two baffles of the same thickness welded on three sides. The tank was welded as two pieces length wise with one weld across the bottom and one on the opposite side on top. The front and rear parts of the tank were welded across the entire perimeter. With the two welded baffles the tank can essentially be evaluated as three separate parts approximately 20" x 20" x 19" each holding about 34 gallons and weighing about 210 lb. each. The center part of the tank has only one weld across the bottom and would likely be the strongest of the three sections. The other parts on each end have two welds each. One on the length of the tank and one across the width. I doubt the tank manufacturer did any calculations on tank dynamic pressure, they built the tank to same dimensions as the previous one. Regarding the support on the bottom, I would guess they would say it would be better to have one vs not. I will give them a call and at least try to understand their experience with tank deflection.
 

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