Fuel flow between tanks

[Doug]

Gang,

 

I have a 1984 46’ Uniflite with 4 fuel tanks. Two smaller fwd and two main tanks aft. My understanding is excess fuel not consumed by the engines (671 Detroit diesels) goes to the fwd tanks and gravity feeds back to the main tanks. I’ve been sitting dockside for quite some time now and the fwd tank gauges read they are ¾ full, but the aft tank gauges read ¼ full. I feel nervous leaving the dock with only ¼ tank in the main tank. Can I rely on the fwd tanks feeding the aft tanks while underway?

 

The last time I took the boat out, the aft gauge read ¼ tank. It then dropped to empty while I was out on the ocean and I went into a panic! We did get back to home port, so the fwd tanks must have been feeding the engines. But it is VERY unnerving! Is this normal? Why doesn’t the fuel in the fwd tanks eventually replenish the aft tanks back at dockside?

 

Please set my mind at ease that this is normal and the way things are supposed to work.

 

Doug

[Kevin McIntosh]

Doug ... I have a '79 42' Double Cabin Sedan (w. 6-71N's) that only ever had 3 tanks .. two 200 USg saddle tanks and a 100USg aft center tank ... the aft center tank started leaking with the previous owner so he had it removed ... if your aft tanks are sitting in a bilge area that you cannot easily access (as mine was), you need to ensure they are not sitting in residual bilge water that the pump or pumps cannot scavenge out of there ... if you don't, and, if your tanks are aluminum, as mine was, you could eventually have corrosion-perforated leaking tanks as well ... you also need to ensure you are running fresh fuel through them on a regular basis (sounds to me like you have been, even if you don't fully understand how) ... many DCS owners isolated and ignored their aft, center tank for too long and I've looked at some boats where the tank is still there and intact but full of sludge ...

Anyways, I've got the same engines as you and I should point out (in case you didn't already realize) that those engines are always returning fuel to the tanks - based entirely on the shutoff valves configuration (mounted to the aft engine room bulkhead on my boat).

I've never heard of this gravity feeding arrangement on a Uniflite that you speak of, but of course yours is an entirely different model and obviously has more tankage ... my tank source(s) of feed to the engines and generator as well as fuel return from all 3 motors is/are controlled entirely by the shutoff valves ... I run the Port engine from the Port saddle tank and the fuel return is directed back to that same tank, ditto for the Stbd. side ... with my generator feed line normally shutoff (my boat does not have a generator return line shutoff valve), unless I am using the genset of course ... however, should I wish to (i.e. trimming), I can transfer fuel from one tank/side to the other based soley on the shutoff valves configuration.  This capability can quickly become a critical issue if you don't understand what's happening - you can overfill one tank if both returns are feeding into it while the engines are being supplied by the other tank ... sounds far-fetched, however, the previous owner of my boat managed to find a way to do just that ... for some unknown reason - I assumed a possible bad load of fuel in the isolated tank but that has not turned out to be the case.

I believe you need to study your specific owner's manual regarding fuel control (if you don't have that, you need to find someone who does have a copy - from a model and year as close to your own as possible), find your fuel manifolding center (shutoff valves array) and study it at length ... until you are dead certain of which lines feed to (larger I.D. copper line) and return from (smaller I.D. copper line) each engine and your generator ... then ... which other shutoff valves can be used to isolate each individual tank.  Once you have all that figured out, draw up a logical diagram and physically label everything, if that's not been done already ... I have shutoff valves config. diagrams placarded on my e. r. aft bulkhead for each possible fuel control scenario ... in case someone other than myself needs to quickly make sense of it in order to make changes while underway.

Kevin Mc

[Doug]

First, and most important - does anyone have a fuel flow/valve diagram for a Uniflite around 1984 and 46'? I could use a copy of those pages.

My tank configuration is as follows: Two main tanks, I believe 182 gal. each, under aft stateroom bed. They are high and dry. Two saddle tanks, 68 gal ea., halfway fwd.

Finally, the user manual that came with the boat is totally useless. Very small, no real useful info and even starts out describing my "gasoline" engines!

Doug