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Old 01-05-2014
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Fuel Tank size error

I have a 1985 Catalina 30 with the MX25 diesel. The tank does not hold the full amount of fuel it is supposed to. The tank is stamped that is holds 18 gallons. When i measured it, I calculated its volume at 17.5 gallons.I ran out of fuel about 10 miles out of the harbor. I sailed in , in deteriorating weather, but I would have wanted the engine.

The fuel gauge read full when the tank was full and empty when I ran out. When I filled the tank, it topped out, absolutey full at 12 gallons.

What is causing this. Could I have 6 gallons of sludge in the tank? The fuel looks clean after the the filters and the engine runs good.
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Re: Fuel Tank size error

3 things you need to know:
- Is the tank square? If not have you compensated for the shape?
- The fuel pick up tube - how far off the bottom is it?
- The gauge sender may not be hitting the bottom either.
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Re: Fuel Tank size error

I believe the fuel pick-up is located above the bottom which accounts for some (maybe 10%?). While I've never run completely out, I've run 2 Catalina's as far as I dared with the gauge on E and never been close to the refill being near tank capacity.
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Re: Fuel Tank size error

Quote:
Originally Posted by glassdad View Post
I have a 1985 Catalina 30 with the MX25 diesel. The tank does not hold the full amount of fuel it is supposed to. The tank is stamped that is holds 18 gallons. When i measured it, I calculated its volume at 17.5 gallons.I ran out of fuel about 10 miles out of the harbor. I sailed in , in deteriorating weather, but I would have wanted the engine.

The fuel gauge read full when the tank was full and empty when I ran out. When I filled the tank, it topped out, absolutey full at 12 gallons.

What is causing this. Could I have 6 gallons of sludge in the tank? The fuel looks clean after the the filters and the engine runs good.
"Topped Out" according to the fuel gage or by squirting over-flow from the breather tube?
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Old 01-05-2014
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Re: Fuel Tank size error

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Originally Posted by glassdad View Post
I have a 1985 Catalina 30 with the MX25 diesel. The tank does not hold the full amount of fuel it is supposed to. The tank is stamped that is holds 18 gallons. When i measured it, I calculated its volume at 17.5 gallons.I ran out of fuel about 10 miles out of the harbor. I sailed in , in deteriorating weather, but I would have wanted the engine.

The fuel gauge read full when the tank was full and empty when I ran out. When I filled the tank, it topped out, absolutey full at 12 gallons.

What is causing this. Could I have 6 gallons of sludge in the tank? The fuel looks clean after the the filters and the engine runs good.

#1
Your pick up is NEVER on the dead bottom of the tank and is usually 1" or more off the bottom. This means you will never get the full rated capacity.

#2 Sailboats heel and slosh back and forth in a sea thus exposing the pick up to air.. As tanks get low it is very easy to suck air. You may not have been out of fuel but may have very likely sucked air especially if motor sailing..

#3 My rule of thumb is to never let my tank get below 1/3 full to prevent sucking air.

#4 You could also have a low spot/trap in your fuel vent thus causing you to "burp" full early....

#5 Don't EVER count on a boats fuel gauge to be accurate, unless you have specifically calibrated it.

#6 To calibrate:
A) Know actual tank volume measure and calculate.

B) Drain tank 100% by removing sender and sucking it 100% dry.

C) Fill in 25% known increments and mark gauge glass

D) Use a Sharpie, paint marker, tape etc. at 25% increments E, 1/4, 1/2, 3/4, F....

E) Now you KNOW the gauge is accurate, but this will still NOT be accurate when heeled....
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Last edited by Maine Sail; 01-05-2014 at 01:30 PM.
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Re: Fuel Tank size error

Quote:
Originally Posted by SVTatia View Post
3 things you need to know:
- Is the tank square? If not have you compensated for the shape?
The tank is not square. I did compensate for the shape.
- The fuel pick up tube - how far off the bottom is it?
The pick up tube is rubber and does hit the tank bottom. It curves so it seem to lie flat on the bottom or very close.
- The gauge sender may not be hitting the bottom either.
The tank shape is such that the bottom of pick up tube is lower down than the bottom of the sender.

#2 Sailboats heel and slosh back and forth in a sea thus exposing the pick up to air.. As tanks get low it is very easy to suck air. You may not have been out of fuel but may have very likely sucked air especially if motor sailing..

The tank was empty. When we returned to the dock, I tried to start the engine and got nowhere. After filling, it started up fine.

When we filled it, We did get fuel flowing out of the vent and the gauge showed full. I overfilled it deliberately.
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Re: Fuel Tank size error

When I built a new fuel tank, I calibrated the gage by filling the tank a gallon at a time and making up a table of gage reading and fuel height height above the bottom measured with a stick. Also measured the pick up tube length compared to tank depth. That way if the gage doesn't appear right you can measure reliably with a dip stick and you know at what level the tube will start seeing fuel.
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Re: Fuel Tank size error

I am fairly confident about my tank capacity being 18 gallons. but holding only 12 usable gallons. So my real question is, do you think I could have 6 gallons of algae, shmootz, dirt, gunk built up inside? And if so, how does one go about cleaning out said gunk?
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Re: Fuel Tank size error

Glass, loosing six gallons to sludge would be pretty extreme, but frankly I wouldn't consider it impossible if the tank has never been cleaned. But it would be the worst case I have heard of. However since you didn't have a filter problem, motoring in rough weather, I highly doubt this is a sludge problem, my guess is you pick up hose is just shorter than is should be or has a hole somewhere above the bottom... Say at around the 12 gallon mark.
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Re: Fuel Tank size error

Thanks for all your suggestions. I'll have to put some more investigation into it. I'll let you know if I learn anything. Meanwhile, I'll sail based on a 12 gallon range.
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