Fuel draw with twin tanks on single diesel

[J.T kearney]

fuel draw with twin tanks

Thanks everyone for the insight on the fuel tank balancing act. There's a lot of info floating around in this forum. My boat has virtually no list with my dinghy and motor on a Sea wise davit system with everything topped off.
So now I'm going to work on my fuel gauges not working. Starting off by using a jumper wire between the center stud and the black wire on the fuel sending unit. From what I have heard with all engine switches on, If the gauge reads full its a bad sending unit. If it doesn't move it's a bad wiring issue or both sending unit and wiring problem.

Thanks again everyone for sharing your knowledge and experience.
Cheers
J.T.

 

[Cold Duck]

There are as many fuel configurations as boat mfgrs. Problem with the MS 350, tank valves on each tank but just a "T" on return line so cannot control return flow. My Willard Vega had same configuration. Both vessels made it very difficult to open hatches underway and switch valves. Now my 26ft Daytona Daycruiser had two 70 gal tanks and one valve under engine hatch on transom, again not convenient to switch. I installed an electric fuel switch that can now be controlled on instrument panel without leaving the helm seat. Keeps my tanks equal always. It just boils down to how much effort and $$$ you want to expend to solve this problem. Also, when activating numerous switches, to run off one tank or another, return to one tank or another, the real risk of forgetting or setting wrong the configuration and dumping fuel overboard or running a tank dry are probable (spoken from experience). It's embarrassing to be towed in and find you have 100 gallons onboard when you thought both tanks empty. If I decide to put electric valves on the MS350, both the flow and return for each tank will act in unison, a simple Port/Stbd switch on inst panel, nothing complex. And make sure fuel gauges work for sure.

 

[backinblue]

Why not just feed from both tanks simultaneously. Still trying to understand why that's a bad thing. Seems to solve a lot of problems with zero intervention.

 

[Cold Duck]

On our MS350 we run off both tanks but return goes to the port more for a number of possible reasons thus at end of long run a big list to port. If it would draw equally, you bet we would do it.

 

[backinblue]

On our MS350 we run off both tanks but return goes to the port more for a number of possible reasons thus at end of long run a big list to port. If it would draw equally, you bet we would do it.

I get that, but why don't they self-level due to gravity if they are simply T'd together? I realize that a list will not correct itself, and in fact this can worsen the list, but when running I would think the 2 tanks should equalize. Sometimes if I'm not taking on much fuel (say 30-40 gal), I can just fill 1 tank and after a short time of running, if the boat is level, the tanks equalize as the higher one feeds the lower one. What am I missing? I would not want to have to manually manage the levels in each tank and as you are thinking it would tank extra plumbing and gauges to do so and I don't see the advantage.

 

[Cold Duck]

A lot has to do with the vessel configuration. If they drain equally, great. I had a 70 fter, 3 tanks for 1800 gal, port, stbd and belly. They all drained into the belly tank and did so equally, no issues. But for what ever reason, the MS350 tankage configuration doesn't. It's possible there is an inherent port list regardless of the fuel burn then when fuel is burned from both tanks, the stbd tanks drains faster possible due to the initial list so that at end of run there's a serious list to port. Now with a big list, the fuel may equalize after a time, but that doesn't bring the low side up. It just equalizes the levels with one tank now lower than the other due to the list and it never levels out the boat. Could this possibly be corrected with replumbing, possibly. Just a matter of how much effort and $$ you want to spend on the issue. The issue is real, and everyone deals with it a little different. Also, there are real reasons not to draw from all tanks at same time ,that is a cross feed. A boat in Baja woke up and found they had a fuel leak that totally drained one tank. The other tank was still full and they could get back to port since the crossfeed had valves shut so there was no cross feed at the time. If both tanks had been connected they might have lost all their fuel in a very desolate location.

 

[rgano]

I found that my MS 30 Pilot II (it's important that one specify Pilot or Pilot II because the fuel tankage is quite different in them) has p/s tanks and will NOT self level when moored due to an inherent off-center weight. I never noticed it until I was forced off my boatlift for 18 months by hurricane damage to the lift. The port-only return was corrected by me in about an hour with a wye valve and some fuel hose connected to the existing but unused stbd tank. Now I can allow port or stbd or both return and adjust return flow rate to both tanks to get them to end up even after a long run, and the boat lift forces them to stay that way; otherwise, remaining in the water, I'd have to shut off one tank at the end of the day.

 

[backinblue]

I wonder if some people assume its a fuel imbalance causing the list when it could be some other uneven weight distribution. I can only speak to my boat, but it has P&S saddle tanks mounted symetrically. I do get a noticeable list to starboard, but I notice that is due to me emptying the holding tank and topping off the water tank which are on opposite sides. Not to mention the galley and fridge are also on the startboard side along with the starting batteries, so there could be other things that are not quite perfectly balanced. Before I re-plumbed my fuel system I'd look at shifting other weight. But again it seems mostly due to my fresh water tank level and it's not so much to bother me as it's easily trimmed out underway.

So again, with my tanks mounted symetrically and both feeding from lines from the bottom of the tanks that feed into a T, I fail to see how gravity would not level the tanks when the boat is running and trimmed properly, regardless of the return fuel feed. Why would the higher tank not push fuel through the feed line into the lower tank? I know this happens because as I said earlier, I can add fuel to only 1 tank and after running for a short time I can see the fuel gauge change as the tanks equalize.

 

[rgano]

I wonder if some people assume its a fuel imbalance causing the list when it could be some other uneven weight distribution. I can only speak to my boat, but it has P&S saddle tanks mounted symetrically. I do get a noticeable list to starboard, but I notice that is due to me emptying the holding tank and topping off the water tank which are on opposite sides. Not to mention the galley and fridge are also on the startboard side along with the starting batteries, so there could be other things that are not quite perfectly balanced. Before I re-plumbed my fuel system I'd look at shifting other weight. But again it seems mostly due to my fresh water tank level and it's not so much to bother me as it's easily trimmed out underway.

So again, with my tanks mounted symetrically and both feeding from lines from the bottom of the tanks that feed into a T, I fail to see how gravity would not level the tanks when the boat is running and trimmed properly, regardless of the return fuel feed. Why would the higher tank not push fuel through the feed line into the lower tank? I know this happens because as I said earlier, I can add fuel to only 1 tank and after running for a short time I can see the fuel gauge change as the tanks equalize.

My bottom-fed side tanks are connected by a crossover line, and the fuel to the engine is tapped off of this crossover - inches from the port tank and a number of feet from the stbd tank. Before I did the minor replumb to allow return to both or either tank, the return was only to port, and the port tank level still went down faster than the stbd, even after I replaced the crossover line. Drove me NUTZ!