Imbalance in fuel tanks

Hello, I am new to the forum, and have been reading posts since I joined, but have difficulty understanding some of the technical terms, and also please excuse my English, as I have difficulty finding the terms to explain my situation. and, I am sorry if this question was answered before.
My husband and I recently bought a new 41 Benetau Swift trawler fly. This is our first boat and we are complete beginners.
We took the boat with a captain on a 3 day journey to its final destination in Bodrum, Turkiye.
We refueled 3 times and had no problems.
We are now stationary in a little bay, connected to a concrete block.
We have been there for 1 week and have not driven the boat since.
We have 2 x 320 yanmar motors and a two fuel tanks of 600 liters each.
We have been going on board everyday to check the boat. We used the generator, only
About 4 hours max, in the whole week.
We just noticed that the starboard tank is 98%full whereas the port is down to 18%.
We have asked the dealer for help but they can’t seem to figure it out. Apparently there is a valve between the 2 tanks that was left open on our journey down, which should have been closed. We do t know what the problem is, and would appreciate your guidance. Thank you

Has the valve since been closed?

Does the boat have a list to one side? To the new boater, you may have to measure at the transom from the water level to the boot stripe to see a list of a few inches.

If it's still open, as you develop a list, fuel will continue to flow to the tank on the lower list side of the boat.

If the valve is currently closed and your running the generator periodically, you may be drawing fuel from one tank and inadvertently returning it to the other ( most diesel engines draw more fuel than they use and return the unused fuel through a return line. Generators typically use small amounts of fuel and can return comparatively large amounts. My 9 KW Onan has a maximum consumption of 1 GPH and regardless of load returned 14+ GPH).

Finally, it would be important to understand if you created part of the list and fuel transferring from one tank to another exacerbated it. How you load the boat with your stuff can induce list. If you have multiple water tanks that aren't all centerline, you can induce list by using certain tanks. Also, is your waste tank on the centerline? The one on my boat isn't, which means that there's a modest list when the tank is above half full.

If you can respond with whether the valve is open or closed and if a list, how much there is, that would be helpful in further analysis.

Ted

As complete newbies, I suggest you hire the best generator/engine guy you can find to give you the answer and at least a 4 hour tutorial on the engines and generator systems. Make sure you take notes/videos for later reference and make sure you get someone who speaks simply enough for you to understand.

Not having that kind of knowledge about your boat is immensely dangerous in my opinion.

There may not be enough information known. For instance most boats have saddle tanks, one on each side, together with a separate fill on each side.
You have filled 3 times, but did the filling add an equal level amount to each side, was it a total fill each time.
Cross over lines were popular when there were lines from the outside bottom of the tank. (now most are from top of tank). This would allow the fuel to be the same level as long as the boat was level to start with. If there is the slightest list and the crossover was open you would end up with more fuel on one side.
Now 98% and 18% is an extreme difference and the list of the boat should be easily seen.

Usually there will be a fuel manifold in the engine room with a lot of valves, that should be labeled things like “to port engine,” “from starboard engine,” “to/from generator,” “to port tank,” “from starboard tank,” etc. With those you can control which fuel tank you’re drawing fuel from, and which fuel tank the engine is returning fuel to. Diesel engines draw more fuel than they use, and return the excess to the tank. Generally I draw fuel from one tank at a time, but you need to make sure the fuel return goes back to the same tank you’re drawing from. So the “from” valve controls which tank you’re drawing from, and the “to” valve controls which tank fuel is returned to.

If you draw from one tank and return fuel to another, you will wind up moving fuel from one tank to another, and you can overflow a tank.

Are you sure you weren’t just using fuel from the port tank on your last leg, meaning it was down to 18% when you arrived? Or has it changed since the boat was at the dock?

My husband and I recently bought a new 41 Benetau Swift trawler fly.

Click to expand...

New? I doubt if there is a crossover gravity feed. Most likely valves for draw and return. @SeaDogAK explained it well.

Had a learning experience with twin diesel boat. Calculated VERY carefully useable tankage over a period of years. Figured I could make a run with capacity to spare. Did NOT consider one tank pickup tube was over 2" shorter than the other.............

Thank you all for your valuable advice. Apparently the port tank was full, there was an error in the panel- they said we needed to callibrate once we refuelled. Hope it doesn’t happen again. Thanks again

I had this problem on my Camano - it got slowly worse for several years, and then it was time to deal with it. (Earlier would have been wiser, of course.). My Steps:
1. It can be managed somewhat by adjusting the return lines on fuel being returned from the Engine(s). I have a single engine, two tanks.
2. Some boats have a balance fuel line between the tanks. I had that fuel line replaced.
3. Is there junk slowing fuel feed from one of the tanks? Polishing the fuel can help and cleaning out the tank valves was part of the fix.
4. Have fuel lines blown out with compressed air, or replace fuel lines.
5. Is the air vent to the tank clear? Spider trash, etc., can reduce ability of the air replacing volume of fuel drained - not a problem in my case.

My mechanic found that the brass fitting that separated a single return line from my single engine into port and starboard return lines was at l least part of the problem, and a Varsol bath improved the flow there. Replacing the 19 year old balance line between the tanks helped as well. Running off the tank that tended to fill up seemed to improve the flow through that valve and fuel line. Fuel polishing was part of the effort, too.
Bottom line is that old fuel lines valves and fittings "junk up" a bit, and there were probably several problems in my fuel system, including my last fuel polishing being 6 years ago after my primary Racor filter clogged up on the delivery cruise. Polishing and regular use of Biobor JF helped then, proved by cutting up the used Racor primary filter at the end of the next off season - rolled out the filter element in the driveway - looks brand new. Bottom line time is that there was no one smoking gun - just thoroughly addressing all components of the fuel system. Weight distribution in the boat matters somewhat, but not to the degree of your present problem. Buying good fuel like Valve-Tect from a high volume vendor is favored by boat owners in my area, same with using Biobor JF.

Best wishes for a timely and inexpensive fix.
NC Heaven

These can be trying times! Are you sure there is a crossover valve between the tanks? Our boat does not have such a connection. At the risk of explaining something you already know, diesels return the fuel the pump pumps above which the engine burns. You can be drawing from one tank and returning to the other. That is a handy way to move fuel from one tank to the other, but it is also a way to drain one much faster than you expect if you don't have a grip on the fuel return thing.

On my single engine, twin tank Camano there is a cross over hose, but not a cross over valve. There is an outlet valve on each tank so I can run on one tank to balance the tanks, balance list, etc. Tank valves are way back against the lazerette wall, a long reach.