Usable gallons of a fuel

BillD

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Here's a question or two for the guys that have shallow "below" deck fuel tanks in the boats.

I'm debating on whether to install a single above deck stern fuel tank (90-100 gals.) or "twin 50ish gal" tanks below deck.

There are advantages and disadvantages to each install.

My question is "your experience" in "usable gallons" in long shallow rectangular tanks.

The largest "best size" tank for decent clearances under the deck of my 25 T Jason are a pr. of :

98-102 inch long, 18 inch wide and 7 inch tall tank. These measurements are "inside the tank" measurements.

This size adds up to 55 gals.
Rectangular Tank Storage Capacity Calculator - Tanks Volume Calculation Online Physics Calculators

The tanks will be mounted such that the front end will be "up bubble from level" about an inch with the boat @ rest. Obviously when running the boat the tanks will be tilted up more in the front.

I'll be able to switch easily back and forth between tanks. Obviously at cruise speeds with the boat up on plane I'd be able to run a tank almost dry.

The tanks will be baffled every 16", pickups will be 3/4 copper in the rear of the tank and will be 1/2 to 1 inch from bottom.

My question? One tank full...puttering along with 1-2 foot wave action...what's your experience with "how many gallons" out of the total in one tank is usable in the size tank above before the engine starts to sputter and suck air??

Thanks, Bill D
 

twister

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i have had tanks with similar shape and volume and I could get 52 gallons out of the 55 total. they were "bent tube" construction and I think it would have been a better use but the tanks had a triangular bottom side.
 

oldshell55

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i have had tanks with similar shape and volume and I could get 52 gallons out of the 55 total. they were "bent tube" construction and I think it would have been a better use but the tanks had a triangular bottom side.


what is bent tube construction ?

1" would be a lot off the bottom, I have one with 9 " depth and 1/2" is sufficient, no less. that shallow you need everything you can get!
 

twister

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"bent tube" is the pick in tube starts in the middle of the tank then bent in an L shape towards the low spot. not sure how well it worked but it did ok
 

BillD

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Thanks,

The pickups on the tanks I;m considering have pickups 1/2" off the bottom.

The bottom of the tanks are flat.

A triangular bottom would increase the usable fuel volume.
 
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oldshell55

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Thanks,

The pickups on the tanks I;m considering have pickups 1/2" off the bottom.

The bottom of the tanks are flat.

A triangular bottom would increase the usable fuel volume.

just to make it clear, I once put a pick up to close to the bottom of the tank and the engine starved for fuel at high rpm, it took a little work to figure out where it was starving from :confused:, so yes I think 1/2 " is ok unless its a large engine pulling a lot of fuel
 

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starrfish

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can you slope the tanks to the hull? I did with mine and put the pickups in the forward 1/3 of the tanks in the deepest part. out of 2 tanks I hold 325 gallons, and can burn 300. that leaves about 12.5 gallons in each side..on a buddys boat he has a "day tank" about 6 gallons, the pickup is in that and the 2 mains gravity feed into it. has valves to isolate both tanks, feeds and returns, and can isolate the day tank, which has a decent inspection cover if you ever need to get into it. (ie) if something made it through the main tanks, it would end up there.
this method also gives him 100% of usable fuel.
 

Toolate

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Are you doing some kind of bilge ventilation Bill? THink I would be as concerned about corrosion as I was usable capacity if not.

What thickness/kind of metal for the tanks? Think you could gain some volume and do away with metal in the bilge by making FG tanks, no? Heh, its not my time I am suggesting using up.:D
 

Raider Ronnie

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Keelboater

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As Jersey mentioned, you can "sump it" if you have the room. Otherwise you can add trap door baffles to create a "flat sump" around the pickup tube for lack of better terminology. Just like they use in high performance road race oil pans. Simple to make with piano hinge, and very functional. The boat can rock and roll, but you will always have fuel within this area as the trap doors swing open and closed. Even if you used small fixed baffles to create a "flat sump" instead of trap doors, it would still effectively trap fuel near the pickup tube.
 

BillD

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All good things to consider. Thanks for all the input.

A sump pocket can only be in the front of the tank.

A full length "V" bottom (maybe 1 inch down in the center over the 18 inch width of the tank may be doable.
 

captjohn

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I know it's probably not per Coast Guard regulations, but I would be very tempted to install some kind of a drain in the tank, to get water out. Just my 2 cents.
 

BillD

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I know it's probably not per Coast Guard regulations, but I would be very tempted to install some kind of a drain in the tank, to get water out. Just my 2 cents.

In 22 years of boating I've never had a water issue with either gas or diesel.
My "gut" tells me all threaded or welded fittings @ the top of the tanks or top side fill... Nothing on the sides or bottom other than welded seams.

I plan on plastic tanks below deck. Lots more work to level mount, run lines, deck hatches etc., but a good way to use below deck space and free up top deck space in the 25 footer.

LOTS to do before tank/s are considered.
 

MASTERENEGADE

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BillD: I hope you knocked on wood when you wrote you never had a fuel/water problem.
Every spring I have to pump out approx a gallon of water out of the tanks on my privateers. I have tried full or empty for the winter and I always seem get some water in the gas.
I have found leaving the tanks mostly empty for the winter and then just pumping the primer ball with the hose into a clear jug for a few gallons of mixed gas/water in the spring before moving the boats, then refilling with fresh fuel has solved this problem the best. And of course replacing the Racor fuel/separator elements at the beginning of each season.
 

Toolate

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BillD: I hope you knocked on wood when you wrote you never had a fuel/water problem.
Every spring I have to pump out approx a gallon of water out of the tanks on my privateers. I have tried full or empty for the winter and I always seem get some water in the gas.
I have found leaving the tanks mostly empty for the winter and then just pumping the primer ball with the hose into a clear jug for a few gallons of mixed gas/water in the spring before moving the boats, then refilling with fresh fuel has solved this problem the best. And of course replacing the Racor fuel/separator elements at the beginning of each season.

Sounds like a fuel source or boat specific issue. I have owned 8 boats of every gas configuration possible and never water in my fuel once (except when the MFG sank at the dock...).
 

BillD

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BillD: I hope you knocked on wood when you wrote you never had a fuel/water problem.
Every spring I have to pump out approx a gallon of water out of the tanks on my privateers. I have tried full or empty for the winter and I always seem get some water in the gas.
I have found leaving the tanks mostly empty for the winter and then just pumping the primer ball with the hose into a clear jug for a few gallons of mixed gas/water in the spring before moving the boats, then refilling with fresh fuel has solved this problem the best. And of course replacing the Racor fuel/separator elements at the beginning of each season.

My new Cummins 370 will have a SMX Double/Double Fleetguard filter system.
Each filter cannister (one 20 mic the other 10 mic) has a water drain. Very simple to drain off water/fuel using your finger and thumb.

SMX Multi-Stage Fuel Fueltrationâ„¢

I have this filter package on each 370 in the Blackfin.
Each week during the boating season I check and drain off a cup or two of fuel. I never seen water.
If I did that's how any water or moisture would be removed from the tank.
 

southshore30

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sandytows

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MY TJ 28 has three tanks.
Port and starboard below deck 35g each usable about 30
Stern 70G usable about 50.
The small tank pickups are at the back and it is easy to strip most of the fuel out of them, without a probem. The stern tank pickup is dead center and with the sloshing effect I suck air with 3+" of fuel in the tank.
My boat had FW and waste tanks below deck when built which is what limited the size of the fuel tanks. without them I could have double the under deck fuel capacity.
If my under deck tanks were bigger I would delete the stern tank.
when full it adds 500lbs right at the transom, not good for efficiency.
If you really want max capacity add the stern tank and only use it when you need the extra range.
 
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