fuel tank level issues while running

I have noticed a strange condition with my fuel tanks and feeds while running and am wondering if anyone has seen similar conditions.

I have a 2002 52SS. It has 5 fuel tanks: 2 saddle tanks and 3 under floor tanks. The two tanks on each side are connected in series to the center floor tank (the port saddle feeds the port under floor which feeds the center under floor; same for stbd.). Fuel passes from the center under floor tank to a manifold that feeds the engines and generator. There are two fuel gages: the port gage measures fuel on one of the saddle tanks; the stbd. gage measures fuel in the center under floor tank. If all tanks are full and I run for say 4 hours I have noticed that the saddle tank level will drop as expected but the center under floor tank drops also. If I stop for overnight, the next day the saddle tank level will be lower and the center under floor gage will read full. Seems that as I am running the center tank does not fill as fast as the fuel is drawn out. Longest time I have run is 7 hours and I have never had a problem with running out of fuel but I have concluded that it would be possible to run out of fuel in the center tank and still have fuel in the saddle and under floor side tank.

Has anyone encountered anything similar?

Ran compressed air into the fuel fill and air came out the vents. Slightly more on one side than the other but a good flow of air on both sides. Putting the boat away for the winter but next spring I will experiment with the fuel caps off and see if there is a difference (has to be a very nice day of course. Thanks for the response.

Have to wait until spring. Boat is stored in Michigan. Will post final result when I have it figured out.

Return lines go to the saddle tanks. Fuel lines that connect the saddle tanks to the 3 under floor tanks are the same diameter as the fuel line coming from the center floor tank that feeds the manifold that feeds the engines. Therefore there should be no problem with the saddle tanks always keeping the under floor tanks full unless a vacuum is present, eg. vents are clogged, but I had no problem running compressed air thru the fuel fill pipes and out the vent. I plan to run the boat with the fuel caps off (nice day in the spring) and see if this makes a difference.

I don’t think there are separate vents for the lower tanks; just vents port and starboard off the top of the big saddle tanks. Since the tanks are interconnected one vent per side should suffice I would think. If there were vent lines from the lower tanks they would be full of fuel up to the level of fuel in the saddle tanks.

I am not sure how the tanks are set up on a 48. My previous boat was a 45 with just saddle tanks that were dedicated to each engine. In this set up if there was a change in ship attitude I would consider the possibility of one of the engines consuming a lot more fuel than the other if the boat started to list while running. If it was a long run my generator was fueled by my starboard tank and that side always took more fuel.

I still do not have an answer for mine. If I run for 4 hours both the saddle tanks and the center tank drop. I have taken the fuel caps off of both sides and the center tank does not seem to fill any quicker so I don’t believe I have a vent issue. I have no gunk in the fuel filters or canister to indicate clogging in the fuel lines and since I have owned the boat (2 years) never any water in the fuel filter. I think I could probably run at least 12 hours at cruise (24 knots) before I emptied the center tank so I am not going to worry about it any more.