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Old 12-03-2012, 04:43 PM   #1
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drawing from multiple fuel tanks

I'll do my best to explain this.....

Three fuel tanks two side tanks of 125 gallons each & one "saddle tank(lowest tank over the keel)" of 30 gallons. When I bought the boat the valves were such that each engine drew off of the side tank closest to it and the saddle tank was shut off.

When I filled the boat with diesel, I filled the saddle tank so full that there was diesel in the vent line. So I opened the valve to the saddle tank, shut off the side tanks and went for a little run. I got the fuel level low enough that there is no longer fuel in the vent line. There are valves on the bottom of the tank, presumable to equalize fuel levels but they don't seem to equalize anything when open, I would image the line is clogged.

So the question is: Assuming I leave the saddle open & open the side thanks, what happens when the saddle tank runs out of fuel? Will it send air bubbles into the system and shut me down or will it automatically pull from the side tanks?

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Old 12-03-2012, 05:17 PM   #2
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Brave man just "filling up" the tank not knowing if it had a leak. Glad it didn't. To help with your question it should draw enough fuel to replace what is lost. Make sure the return line goes to the tank your pulling from.

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Old 12-03-2012, 06:47 PM   #3
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It sounds as I you have two 125 gallon saddle tanks (hence like saddle bags on a horse) and an auxiliary tank of 30 gallons. Either the aux tank is for a genset or left for emergency use. I doubt a 30 gallon tank would be used as a 'day tank' with twins.
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Old 12-03-2012, 06:48 PM   #4
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Having the "equalization" valves open means that the top surface of the fuel in the 3 tanks will be the same height in the boat. if one of the tanks is lower than the other two it will be fuller than the others.
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Old 12-03-2012, 07:05 PM   #5
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Great question. I also a similar problem with listing due to fuel tanks drawing off of one tank. It took some trial and error but I figured it out.
The only way to map the valves is to start at the engine fuel pump and run your hands to the valves.
Once you select the shut off valves put some tape on the sight gauges to verify the fuel flow is even.
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Old 12-03-2012, 07:41 PM   #6
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Originally Posted by Baggiolini View Post
............. When I filled the boat with diesel, I filled the saddle tank so full that there was diesel in the vent line........
I can't answer the rest of your question, but fuel in the vent line will go away as fuel is burned. It's not really a problem unles the vent line has a downward loop (which it shouldn't have).
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Old 12-03-2012, 08:16 PM   #7
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Had a boat once with two tanks & single engine. The boat listed due to PO's removal of an old generator on one side. If both tanks were on, fuel would go from high side tank to the lower tank as it was consumed so the low side only got lower (list increased)... The water tanks did the same thing, further increasing the list. Of course it should have been ballasted back level when the generator was removed but I ran it for several years this way by using one fuel and one water tank first until level, then alternating them.
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Old 12-03-2012, 08:26 PM   #8
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Got two water tanks: one starboard, one port. They will equalize unless one of their valves are shut off.

Got four fuel tanks, upper and lower starboard and upper and lower port. Haven't experimented to see if the upper tanks will drain into the lowers if all valves are open. Have always treated the tanks individually.
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Old 12-03-2012, 09:53 PM   #9
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Very difficult to answer the OP's question without seeing a schematic of the fuel system. El Sea's correction is accurate---- saddle tanks are typically opposed-pair side tanks. We have a 60 gallon tank in the bilge in the center of the boat between the engines. This is a day tank and the engines and generator typically draw from it (and return fuel to it). We fill the day tank periodically by gravity and manual valves from the four saddle tanks which drain from their lowest points.

Whether or not your engines will pull air if all the tank valves are opened and the center tank runs dry will depend on the layout and valving of your fuel system.

On our boat I let a pair of saddle tanks drain completely into the day tank (which is what the engines were running off of) and when the starboard saddle tank went dry the engine pulled air from it instead of fuel from the day tank and the engine shut down.

Not realizing at the time why the engine had stopped I went ahead and closed the transfer valves on both saddle tanks. Fortunately I did this before the port tank had drained dry. So the port engine continued to run on the day tank as it never got a chance to pull a slug of air from its saddle tank. Not until I thought through the fuel system on the boat that evening on the drive home did I realize what I had done by letting the saddle tanks run dry during a fuel transfer.

Depending on how your fuel system is set up, it may be possible for the same sort of thing to happen on your boat, only in this case the engines would be getting their slugs of air from your just-emptied center tank. But without seeing a fuel system schematic, I think it's impossibe to determine exactly what would happen.
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Old 12-04-2012, 06:17 AM   #10
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An Equilization line can be big troubble with the sheen police at fill up time.

Most tanks will have their own air vent , so filling a higher tank may cause fuel to vent from a lower tank.


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