Boat fuel tank ??

G-Spot

Captain
Mar 14, 2008
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John
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53’ Hatteras STEEL N TIME
Tossing this out for some feedback... I have googled and read, until I’m tired of it... One more set of opinions would be great to consider....

Current project is an old former Navy vessel, looking at the fuel tank(s). Originally had two fiberglass 90 gallon tanks, cylinders, basically a beer can on each side of the vessel midship. They don’t have baffles, and one has been removed....

I’m looking at options for new fuel tanks.... one option is a set of Semi truck saddle tanks. 26” diameter, 72” long, similar size as existing, 100 gallons each, so I would be 200 gallon capacity rather than 180 original to the boat. I like the beer can cylinder style tank, as it will significantly improve your ability to use the fuel compared to a large flat bottom tank where a lot of fuel can get lost on the bottom while you suck air.....

So.... then comes Baffles... The current tanks don’t have baffles. The aluminum Semi truck tanks also do not have baffles.... are they that important on this style of tank?!? The side to side movement would be minimal, front to rear maybe different...

I have read that marine tanks require baffles every 30 inches.... then I read contradictory info that plastic tanks don’t have baffles...

So... what does the collective think?
 

kapnd

Almost A Member
Jul 22, 2007
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haleiwa Hawaii
Name
don
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ss minnow
When it’s half empty or so, it can generate some serious punching effort, possibly damaging to the tank and/or the boat, and definitely making air ingress a problem.
I’ve made some insert baffles by cutting a slot in the top of he tank
 
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G-Spot

Captain
Mar 14, 2008
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John
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53’ Hatteras STEEL N TIME
When it’s half empty or so, it can generate some serious punching effort, possibly damaging to the tank and/or the boat, and definitely making air ingress a problem.
I’ve made some insert baffles by cutting a slot in the top of he tank


The slots are a good idea....

I don’t think the sloshing will damage the tank at all.. these things hang off of Semi truck cruising around the country, fast stops, down hill, up hill, windy roads.... the tanks hold up.... the other good thing about these as a replacement is the strapping system...

I don’t think it will damage boat since it has been that way for so many years already...

I do understand the force we are dealing with and the worst time would probably be around half full..... but it is front to rear, so that will minimize the slosh too...
 

kwik_wurk

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Apr 10, 2012
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kwik_wurk
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If metal, ensure grounding and proper ventilation. Repurposing semi tanks sounds great, but they do get lots of air around them on the trucks. That may not be the case in boat compartment (sealed or not). So that different environment (corrosion) would be a concern. (Probably don’t want to foam them in, doesn’t sound like you’re going to anyways.)

Baffling is for the pickup (you know) and fittings which includes the fuel vent. If the vent is at the forward end of the tank (unlikely now, but may be if the tank is angled) pumping out fuel (and pulling in salt spray) and will cause headaches. (And I’m not talking about an improperly installed vent line.) — Small separate baffle for vent lines can be done, akin to really small wet exhaust basically. (Again not preferred as it’s more parts/pieces, but workable.)

Also if the sloshing is too much, you can break the fuel gauge (especially the lever arm type). So you may need to hunt or fabricate a cylinder to go around the collar type sender. (Assuming you need to redo the fuel tank sender.)

Also, is this going to be a private boat or potential commercial (charter)...if the later may want to check the ABYC (USCG really) guidelines/requirements.
 

G-Spot

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Mar 14, 2008
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53’ Hatteras STEEL N TIME
If metal, ensure grounding and proper ventilation. Repurposing semi tanks sounds great, but they do get lots of air around them on the trucks. That may not be the case in boat compartment (sealed or not). So that different environment (corrosion) would be a concern. (Probably don’t want to foam them in, doesn’t sound like you’re going to anyways.)

Baffling is for the pickup (you know) and fittings which includes the fuel vent. If the vent is at the forward end of the tank (unlikely now, but may be if the tank is angled) pumping out fuel (and pulling in salt spray) and will cause headaches. (And I’m not talking about an improperly installed vent line.) — Small separate baffle for vent lines can be done, akin to really small wet exhaust basically. (Again not preferred as it’s more parts/pieces, but workable.)

Also if the sloshing is too much, you can break the fuel gauge (especially the lever arm type). So you may need to hunt or fabricate a cylinder to go around the collar type sender. (Assuming you need to redo the fuel tank sender.)

Also, is this going to be a private boat or potential commercial (charter)...if the later may want to check the ABYC (USCG really) guidelines/requirements.

Good points! I am not planning on commercial, but I do not want to limit the boats use. A 36’ Long 13’ wide battle wagon may be used by future owners for commercial use, so I will need to check into that aspect...

corrosion should not be an issue any more than any aluminum tank in any boat. It rests on two curved supports and would be strapped in. Lots of air around and not mounted right on the bottom; they sit about 6” off the bottom. The fuel gauges are on each end and a filler on each end, currently, so it’s not a floating arm...

I thought the baffles were more about the live load sloshing and The impacts of the weight movement from surge... Didn’t think it was about the pick ups or vent?.... expound on that... I understand in theory that if the fuel was surging a lot it could effect the pick up, but I actually did not think that was the real reason for baffles...
 

goatram

Notable Member Gate Keeper to the Great Northwest
  • Apr 3, 2008
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    grrrrrrrr
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    New tanks can be modified. Used tanks hell no.
    Two weeks ago in Ballard a young gentleman attempted to repair a fuel tank. He's hurting now.

    I have a 8ft by 14inch rectangle tank with baffles in it. 108gls gave the second tank to a friend. He has yet to put it in use. My new 208gls tank bad a crisscross of baffles every 15inchs. Corners top and bottom notched. Two vent ports for and aft.
    .190" 5052 al. I initially hung the tanks by 6 L angles welded to the top. They moved and cracked out. Tank was not compromised. They would need cradles to sit on. New 5'x6' tank is sitting on Structure and fermly attached. Lesson learned. Working on my Masters from the U of Hard Knocks
     

    Captain Decent

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    Cylinder style tanks don’t need baffles because they don’t suffer from oil can fatigue. Since the outside walls are convex they don’t oil can like a rectangular tank with flat sides under slack. That’s really the main reason for baffles. Of course they also reduce sloshing as well which can result in loss of prime but I believe this is a secondary thing.
     
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    G-Spot

    Captain
    Mar 14, 2008
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    New tanks can be modified. Used tanks hell no.
    Two weeks ago in Ballard a young gentleman attempted to repair a fuel tank. He's hurting now.

    I have a 8ft by 14inch rectangle tank with baffles in it. 108gls gave the second tank to a friend. He has yet to put it in use. My new 208gls tank bad a crisscross of baffles every 15inchs. Corners top and bottom notched. Two vent ports for and aft.
    .190" 5052 al. I initially hung the tanks by 6 L angles welded to the top. They moved and cracked out. Tank was not compromised. They would need cradles to sit on. New 5'x6' tank is sitting on Structure and fermly attached. Lesson learned. Working on my Masters from the U of Hard Knocks

    I go to that university as well!

    I’ve repaired several used tanks, steel and aluminum. diesel not near as much of a concern as gas... I usually run. Hose from one of my cars and flood the tank with exhaust and leave it running while welding.

    My cross box tank for the back of my truck had two baffles, but shitty welding in a production environment and they were laying on the bottom. I could look in and see them... I cut access and put them back properly welded.. That one I didn’t run exhaust because having it opened up I emptied and cleaned it well..
     

    G-Spot

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    Mar 14, 2008
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    Cylinder style tanks don’t need baffles because they don’t suffer from oil can fatigue. Since the outside walls are convex they don’t oil can like a rectangular tank with flat sides under slack. That’s really the main reason for baffles. Of course they also reduce sloshing as well which can result in loss of prime but I believe this is a secondary thing.

    So.... you think the baffles are more about structural integrity and not about the slosh? That makes sense... I just read so much about the load shifting... of course those conversations are all about square tanks...

    I’ve opened up and repaired some boat tanks and the baffles usually have a large crescent cut out of the bottom, they probably don’t do much the last 1/4 of tank... they would allow same slosh impacts on pick up as non baffle tanks..
     

    G-Spot

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    Mar 14, 2008
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    Typically coated with coal tar epoxy to prevent external issues. Be careful about what is in contact with the tanks at the supports, crevice corrosion is a thing.

    Yes sir... that’s exactly what I was thinking... coat the tanks and then use wide rubber strap under the hold downs.... Weld a good bond connection as well.... I will also use a crossover hose to keep the tanks equal, so I don’t list... From my research this is common and acceptable with diesel, not allowed with gas...

    Right now I am looking at several... the dimensions the guy gave me suggest they are 150 gallon, but he swears they are 100 gallon semi trailer mounted tanks... 26” x 72”...... Wondering if the tanks mounted on a trailer are a tank inside of a tank for safety... the volume doesn’t equal his dimensions....
     

    liltrouble

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    Jun 9, 2012
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    Maritime academy they told us baffled fuel tanks prevent the hang time on rolling seas. 100 gal x 8ish lb/gallon sloshing is like having 800 lb going side to side. If you have 800 lb rolling to one side, take a wave/swell at the same time can lead to vessel instability. The volume force can overwhelm the righting arm of the vessel. that is why they have baffled liquid tanks. Dry tanks have shift barriers. Does the same thing, just different contents.
     

    Omakase

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    Rubber pads or strips used to secure alloy tanks have a tendency to trap moisture against the metal which, over time, can cause pitting (that you can't see), particularly if the rubber used has carbon black in it's make-up. If possible safest way to mount the tanks is using brackets welded to the tanks that are then lagged to hull framing.
     
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    G-Spot

    Captain
    Mar 14, 2008
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    Maritime academy they told us baffled fuel tanks prevent the hang time on rolling seas. 100 gal x 8ish lb/gallon sloshing is like having 800 lb going side to side. If you have 800 lb rolling to one side, take a wave/swell at the same time can lead to vessel instability. The volume force can overwhelm the righting arm of the vessel. that is why they have baffled liquid tanks. Dry tanks have shift barriers. Does the same thing, just different contents.


    Thanks! Good info... Just not sure if the round tank is the same as a square for the emerita, since it comes up and the rounds shape sends the fluid on a rotation back.... Unlike square where it clearly can hammer against the sides....

    This boat came with two non baffled 90 gallon tanks, my old 1979 Chris Craft had a 60 gallon steel cylinder tank without baffles under floor along the transom side to side...

    The more I looks it seems the 100 gallon semi truck tanks are 26” X 45”....... I’m thinking it will be ok....
     

    G-Spot

    Captain
    Mar 14, 2008
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    53’ Hatteras STEEL N TIME
    Rubber pads or strips used to secure alloy tanks have a tendency to trap moisture against the metal which, over time, can cause pitting (that you can't see), particularly if the rubber used has carbon black in it's make-up. If possible safest way to mount the tanks is using brackets welded to the tanks that are then lagged to hull framing.

    That’s a good idea... since I’m doing it, I can totally do that... I think I won’t be able do do much about the ribs that it sits on, but if I coat it and use a membrane to isolate It should be ok...
     

    liltrouble

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    We sell neoprene in 1/16, 1/8 and 1/4 thickness from bulk rolls 12 and 36" wide. Always good to put that on you tank cradles before mounting your tanks. Absorbs vibration and stops tank chatter. All the guys who install tanks and do repairs use a lot if that stuff.
     
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    kwik_wurk

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    Apr 10, 2012
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    Indianola & Browns Point
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    Is this going to be a new tank, or a used tank?

    If the used, suggest not coating/painting it unless you intend on blasting, etching, etc etc. (Not to lecture on proper chemical vs mechanical coatings.) If the aluminum is good, it’s much easier to visually inspect 10-15 years down the road. (vs hidden under the coating)

    Nice thing about an unbaffled tank, is you can inspect all of it with a mirror or scope.

    As for the vent line comments I made earlier. - Side/Saddle tanks often have shorter vent/fill lines, due to location within the vessel. Accordingly, vent line can’t be too small or short. My guess i what was there was working fine, just don’t shorten it. (Not talking gallons of fuel, but enough to smell when you’re motoring.) And there’s nothing worse than someone on aft deck saying “I smell fuel”...

    Food for thought, how many fittings are on a typical truck tank, 2; or can you easily install (or order) more. — Cause my guess is 4 are needed, Fill, Vent, Fuel to, Fuel return. (And are you going to install a manifold to transfer fuel around?)

    As for Neopreme, suggest pure rubber (oil/fuel resistant) or similar solid synthetic. A closed cell neoprene is easier/cheaper to get, works for a few years, but it packs out, and you’ll be left with the thickness of the material without the air bubbles.
     

    G-Spot

    Captain
    Mar 14, 2008
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    53’ Hatteras STEEL N TIME
    Is this going to be a new tank, or a used tank?

    If the used, suggest not coating/painting it unless you intend on blasting, etching, etc etc. (Not to lecture on proper chemical vs mechanical coatings.) If the aluminum is good, it’s much easier to visually inspect 10-15 years down the road. (vs hidden under the coating)

    Nice thing about an unbaffled tank, is you can inspect all of it with a mirror or scope.

    As for the vent line comments I made earlier. - Side/Saddle tanks often have shorter vent/fill lines, due to location within the vessel. Accordingly, vent line can’t be too small or short. My guess i what was there was working fine, just don’t shorten it. (Not talking gallons of fuel, but enough to smell when you’re motoring.) And there’s nothing worse than someone on aft deck saying “I smell fuel”...

    Food for thought, how many fittings are on a typical truck tank, 2; or can you easily install (or order) more. — Cause my guess is 4 are needed, Fill, Vent, Fuel to, Fuel return. (And are you going to install a manifold to transfer fuel around?)

    As for Neopreme, suggest pure rubber (oil/fuel resistant) or similar solid synthetic. A closed cell neoprene is easier/cheaper to get, works for a few years, but it packs out, and you’ll be left with the thickness of the material without the air bubbles.


    Thank you! I appreciate all comments, no lecture understood.

    I think I am looking at used simply because of economics..... Tonight I found a set of 140 gallon 23” x 72” for $100.00 each..... New would be $800.00 each, so as long as they are sound, that pretty economical.... I can add bungs as needed for vents or pick ups... I won’t have much of a manifold, probably feed and return in one tank and have a crossover tube for equalizing... I have ready many a thread and this is a common practice...

    A used tank would be etched properly with acid etch primer, prior to any coating.... it’s a double edge sword I will wrestle with... leave them and have the ability to visually monitor corrosion issues, or..... Properly prep them for coating and help the tanks survive corrosive environment.... Probably would lean towards coating the tank.... I like having the extra protection. Of course if the corrosion gets beneath the coating it’s an issue.... either way it will likely outlive me...