Single engine, dual fuel tanks, return to only one. General questions about why.

[Circumnavigator]

In ordinary coastal cruising, the comments above are entirely valid. However, if you intend to go offshore or out in conditions that will cause significant roll, then you must worry about free surface. You want to have only one fuel tank (and one water tank) partially full to minimize free surface -- that when a boat rolls, all the liquid in a partially full tank will go to the low side and make the roll worse.



The best solution to this, as well as other issues, is to have a day tank that holds 24 hours of fuel. The return goes to that. It also has the advantage that you have fuel that has been well filtered going into the day tank and then again between the day tank and the on-engine filter. This means that a clogged filter will almost always happen while filling the day tank and not unexpectedly while drawing from a main tank. Even with parallel filters, an unscheduled engine stop can be a serious problem.


If you want to add a valved return line to the second tank, either install a new port at the top or tee into a vent (as suggested above) if the vent goes far enough up so there is no chance of fuel spraying out. Don't tee into the draw -- part of the reason for the return is to help cool the injectors and putting hot fuel back into the draw won't do that.


Jim

 

[Barking Sands]

So far the discussion has been mainly regarding two systems:
1) return to tank
2) return to supply/supply manifold

I just had to troubleshoot my system this week since I could not get my generator to start. I attempted to bleed but only got air and slight fuel. Initially I could see no reason for it. Inspected all the usual suspects and saw nothing obvious for the massive quantity of air coming out of my disconnected gen lift pump outlet (had a hose on it to a bucket). Went down and retraced all the lines for engine and genny as well as returns. Then went home and drew it on paper. See photo and dont make fun of the quick chicken scratch. I just doodled it quickly for troubleshooting.

Anyways...my system, single Perkins with Westerbeke genny, returns to both tanks AND to the supply lines. I am not sure why but it does work fine. It does feed slightly more fuel to the port tank than the starboard. Possibly because the return line is much longer to the starboard side. But its just a slight imbalance after a day of cruising.

One of the problems I uncovered with this setup I uncovered completely by accident and was the reason for drawing this diagram. I left the boat for several weeks and shut off the main tank supply valves at the manifold. I came back later and started the main engine and it ran for about 5 minutes before I remembered the valves where shut. Shut off the engine and opened the valves and had to leave the boat. Came back later and the genny would not run up to speed. The Westerbeke is supposed to self bleed so I held the preheat/prims switch for a long time and nothing. Took outlet hose of gen lift pump and put other end in bucket...nothing. Traced line...drew the fuel system out. Discovered I had the return going to both tanks AND supply. Then I surmised that when the engine was running with the fuel shutoff valve closed it began to suck the fuel out of the return lines just as easily as any other source. Probably would have shutdown the main engine had it run a little longer. At that point turning on the lift pump from the genny was actually just sucking air all the way from the tops of the tanks where the return would usually exit. To test this I put some hose clamp pliers on the return line that goes to supply and then turned on the engine electric lift pump and held the genny electric lift pump. In theory this would send fuel through the returns to each tank, filling the lines back up. So I put the clamp in place and turned on the pump and in about 1 minute fuel began to fill the bucket from the genny lift pump outlet. Hooked up the fuel lines and everything fired up immediately and ran fine. I think I am going to install a shut off valve where I had to clamp the line for bleeding/filling the return lines. I am also going to redraw the fuel schematic and laminate it and put it in my book of info. Glad it happened while doing maintenance and not while cruising.

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