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Old 22-04-2013, 17:45   #1
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Surprise Fuel Tank/Line on Catalina 36

I have a 1995 Catalina 36 Mk II with a surprise fuel tank. The survey report indicate one fuel tank, the manual reports one, and when I went to fill the tanks I discovered two deck-fills, and two fuel vents, and when the boat bounces at sea, one of the fuel vents sputters diesel.

I investigated further, and found that a small storage compartment had been replaced with another 20 gallon tank. The new tank is directly below the original tank.

The fuel drain from the upper tank T's into what appears to be a fuel drain from the lower tank before going (combined) to the Racor filter. I say "appears" there because the "drain" is on the top of the lower tank. I'm guessing that the "drain" contains a tube that runs to the bottom of the tank such that it can be sucked out. There is also a hose running from the injector assembly of the aft-most cylinder back to the top of the lower tank. I have no idea what that hose is for. The lower tank also has its own fuel vent line (on the top).

I'm guessing that as long as there is fuel in the upper tank, the lower tank will be completely full. Fuel will drain from the upper tank into the lower tank, displacing the air that will escape through the fuel vent. The fuel will then flood into the tank vent and rise until it is at the level of the upper tank.

This system is problematic because it appears as though the lower tank will almost always be full, and diesel will always spill out of the fuel vent. I also have no idea what that second hose is running from the engine to the top of the lower tank.

Does anyone know what the second hose might be?

Would it be reasonable to place a fuel valve on the lower tank's vent line such that diesel does not spill as the boat tosses?

When (not if) I forget to open the vent valve before motoring or a hot-summer day, would that represent a hazard?

Can anyone recommend some good reading to better understand things? I'll be looking into books on marine diesels in general.

Thanks,
Novice Sailor
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Old 22-04-2013, 18:11   #2
Jd1
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Re: Surprise Fuel Tank/Line on Catalina 36

I would think the hose from the injectors to the tank is a fuel return hose.
I would try to connect the second tank vent line to the original tank fuel return port in the hopes that the entire system only uses the original vent line. to the outside. I haven't thought that through completely so don't take my word for it.

Would it be possible for you to post a picture of how the bottom tank is installed and possibly any manufacturer info on the bottom tank? I have been contemplating on how to extend my diesel storage and this seems like an excellent solution to the problem !
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Old 22-04-2013, 18:22   #3
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Re: Surprise Fuel Tank/Line on Catalina 36

the hose coming from the back of your block is most likely diesel fuel return line . since you have two fuel fills.and two.vent lines. it would be easy to isolate those two tanks with a manual select valve . that would solve your overflow problem
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Old 23-04-2013, 18:36   #4
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Re: Surprise Fuel Tank/Line on Catalina 36

Many Thanks to Jd1 and motion30.

Based upon what I'm reading in Nigel Calder's Diesel Engine book, and what I remember seeing, I believe that you are both correct and the diesel return line is hooked into the lower tank.

I won't be able to post photographs for another couple of days, but I have some ascii art below:

Quote:
ASCII art doesn't work in a non-monospaced environment, and use of the Courier New font doesn't seem to be taking effect here.
The upper tank T's into the lower tank's line. Presumably, if the lower tank's vent line is closed, and the upper tank empties:
* the lower line will fill with air up to the apex
* the fuel lines will be filled with air

If the valve from the to the upper tank is closed, and the lower tank's vent line is opened, fuel should eventually start flowing, though the lines would have to be primed.

If you're looking to add tanks, I'm guessing that a better configuration would be to run a line from the bottom of the upper tank to the top of the lower tank, and then hook the lower tank's vent line to the upper tank's vent line.

I could also probably, for now, get away with closing the valve on the lower tank's fuel line, and adding/closing a valve on the lower tank's vent line.

Thanks!
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Old 07-05-2013, 14:32   #5
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[SOLVED] Surprise Fuel Tank/Line on Catalina 36

For all who find this thread and for all who have replied to it, I think I know what's going on.

As mentioned previously, there are two tanks, one directly on top of the other. The apparent intended purpose is for the lower tank to be a "day" tank and the upper tank to be a "reserve" tank.

The day tank will be isolated from the reserve tank during operation until the fuel level drops. Then the valve between the two tanks can be opened permitting fuel to flow freely from the reserve to the day tank.

If you allow it to fully fill the day tank, then there will be a bunch of diesel in the day tank's vent line that will slosh out when you sail. To alleviate this issue, I've installed a valve in the day tank's vent line so that in such cases I can simply close it.
Attached Files
File Type: pdf fuel_system.pdf (5.5 KB, 64 views)
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Old 07-05-2013, 17:44   #6
Jd1
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Re: Surprise Fuel Tank/Line on Catalina 36

Still hoping for pictures and info on lower tank manufacturer ......
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Old 07-05-2013, 18:46   #7
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Re: Surprise Fuel Tank/Line on Catalina 36

Be careful of running the upper tank, if the lower tank is close to full, since the return line only runs to the lower tank, and will overfill the lower tank and you will have spitting out of the vent(or a steady stream). If you run your lower tank almost out before you switch to the upper, you will find that the upper doesnt seem to last very long before it is low, but you will discover that the lower tank now has a fair amount of fuel in it. The best solution is to have return lines to both tanks and a valve that changes the feed line and the return line to the tank that is being used. ___Good Luck._____Grant.
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