Fuel draw with twin tanks on single diesel

[J.T kearney]

Hi. Trying to balance my fuel drawdown on our mainship 350 with twin 150 gl fuel tanks and a single 315 yanmar. Tried to adjust fuel return line to reduce fuel flow to starboard tank but still drawing down port side tank.
Another question is if one tank becomes empty will and both tank lines are
open will the engine draw from only the tank with fuel ?

Cheers
J.T.

 

[tiltrider1]

If one tank is higher than the other, the higher tank will draw down first until fluid levels are at an equal height. This happens when tanks are located fore and aft.

If the tanks are at the same hight then something isn’t right. Most likely one of the returns is clogged or a valve that you are unaware of. Possibly there are more valves in the system and one is partially closed.


Are you drawing anything from the starboard tank?

 

[rgano]

Apparently you have return fuel lines to both tanks since you tried to throttle the stbd return in an effort to balance tanks. If so, shut off the port return line and open the stbd to full open to see what happens. I have the same engine and p/s 90-gallon tanks. Fuel draw to the engine is over on the port side which seems to be the preferred side to draw from based on how my tank level checks. Engine returns 12 GPH to the tanks at 2800 RPM and burning 8 GPH. I had to add a return line to stbd tank because the boat came with only return to port. Nowadays I run with both returns wide open, and all seems well, but if things get uneven, I can fix it quickly by closing one side's return.

 

[GoneDiving]

Are your fuel lines in good condition? Does one side have significantly more restriction than the other?

You would think that tanks would naturally balance to the same level but my port return lines have significantly more restriction so will return far more to the stbd side on a long motor.

 

[O C Diver]

Depending on the boat, not all boats draw equally from both tanks. If the fuel level in both tank is equal, and the boat has a port or starboard list, fuel in the tank opposite the list will be higher and tend to shift to the other tank or be drawn first. Before going through other solutions, balance the fuel in the 2 tanks and check for a noticeable list. Contributing factors can include tanks such as water and waste that are off center line and stuff loaded on the boat.

Ted

 

[Hippocampus]

We also have twin saddle tanks. There’s a cross over. The main engine draws from the port tank. The genset from the starboard. You would think with a crossover they would be at exactly the same height on the sight guages. They are not in several situations.
First if the boat is out of trim disregarding fuel the fuel tank on the low side will fill more.
Second if the throttle is really opened up it takes awhile for the cross over to catch up. It does but there maybe a very slight asymmetry.
Third if we are listing due to wind the tanks match that list.
The boat is set up so you can open or close the cross over but the main engine draw is always from the port tank.
I’m not clear from your description if your tanks do or do not have a crossover. If it does the above maybe applicable or it may be something simple with the valve that controls the crossover. I’m confused. Is the engine fed from both tanks or only one even though you have separate returns. Is there a crossover between the tanks. If you have separate draw and returns for each tank I apologize for barking up the wrong tree.

 

[rgano]

I suspect his MS is set up the way my MS is with the engine feed coming out of the crossover line and probably more toward one side than the other. Each tank has a shutoff valve. I never noticed that I was having issues with the boat wanting to list to port until the boatlift was destroyed and the boat sat afloat for 18 months waiting for the lift to get rebuilt. At one point I suspected the crossover line and replaced it - no change. I'd even out the fuel levels in the tanks getting the boat on an even keel and come out the next day to find the same list with fuel level a bit higher in the downhill tank. Interesting how even tanks (using sight gauge) gives an even keel but left to its own it flops over to one side with both tanks open to the crossover. Once I got it back in the lift, I quit worrying about it because I can adjust the lift to make the clinometer read zero if it comes down off plane and idles into home base with a list due to fuel, and the fuel will then self level between the tanks before the next run.

 

[J.T kearney]

Hi Rich.Been following your blogs from back in the yahoo days. When both tanks are full and return lines are open I get diesel coming out of my starboard fuel overflow outside the boat. So I closed the starboard return line to 2/3 open and fuel stopped flowing out starboard breather. Unfortunately my fuel gauges are not working so I'm est. 6 gph @ 2000 rpm. We ran for 34.5 hrs and came up with about 200 gallons used. Tapping on the port side tank it sounded bone dry but didn't have time to check out the starboard tank but I think there should be about 100 gallons . If all fuel lines are open and one tank is dry would I be sucking air out of the dry tank?
thanks for any info
Cheers J.T

 

[syjos]

I have valves ro select port or starboard tanks for fuel in and fuel return to my single engine.

I switch tanks every 4 hours to keep the boat balanced.

Drawing from both creates issues with fuel return and uneven draw due to differing fuel line lengths between sides.

 

[rgano]

Hi Rich.Been following your blogs from back in the yahoo days. When both tanks are full and return lines are open I get diesel coming out of my starboard fuel overflow outside the boat. So I closed the starboard return line to 2/3 open and fuel stopped flowing out starboard breather. Unfortunately my fuel gauges are not working so I'm est. 6 gph @ 2000 rpm. We ran for 34.5 hrs and came up with about 200 gallons used. Tapping on the port side tank it sounded bone dry but didn't have time to check out the starboard tank but I think there should be about 100 gallons . If all fuel lines are open and one tank is dry would I be sucking air out of the dry tank?
thanks for any info
Cheers J.T

JT, here's a stream-of-consciousness piece about your issue:

How is fuel drawn from your tanks, through the top with a standpipe or from the bottom?

The venting overboard of fuel is cause for concern that something is wrong, especially if you end up with a huge disparity in fuel levels after running both tanks with their supply valves open. Without a verifiable fuel level indicator for the tanks, you are sort of shooting in the dark, but my first guess is that the problem lies within the high-fuel-level tank, either an air leak in a standpipe or its valve and connections on top of the tank or a clog in the bottom of the standpipe or clog in the exit at a bottom feed system, if that is what you have.

Like you, I suspect that were you to continue to draw fuel from the low-level tank while carrying a high fuel level in the other that you could eventually draw air into the engine's fuel. Shutting off the low tank to force draw from the high tank might do the same if there is indeed a slow air ingestion issue with a standpipe system. Can you get full RPM at WOT drawing from the high tank? Do you have a vacuum gauge in the fuel supply like on top of a Racor filter? A high vacuum with an otherwise clean filter would indicate a clog in the tank.

My boat model has some inherent off-center weight which results in a little bit of a list when the two fuel cross-connection valves are both open with the boat off the list and in the water. I got a bit frustrated and replaced the cross-connection hose to no avail. I checked the flow out of both my bottom-feed tank valves by momentarily opening them into a quart jar. All good.

This double checking of shifting fuel was made possible by the sight gauge I installed into the cross-connection hose. Just close the tank valves and cut the hose to insert a brass or bronze tee with appropriate hose barbs and a valve leading to a vertically oriented clear vinyl hose with another valve at the top end a bit higher than the tank tops. Open the two sight gauge valves and one of the tank supply valves to get its level. Reclose the tank valve and open the opposite one to get its fuel level. Then close both valves in the sight gauge and finish with both tank valves open. I think the installation took me 30 minutes.

I find your 2000 RPM fuel use rather high compared to my 315-HP Yanmar 6LPA-STP which gets the following (3,2 GPH burned at 2000) as measured by my Floscan:

RPM Input Return Burned
725 8.0 7.4 0.6
1000 9.0 8.2 0.8
1500 11.1 9.3 1.8
1800 12.4 9.9 2.5
2000 13.3 10.1 3.2
2500 16.6 10.8 5.8
2800 18.6 11.2 7.4
3000 20.0 11.4 8.6
3200 21.8 11.8 10.0
3400 23.7 11.9 11.8

The return flow is quite brisk and can cause overflow in a nearly full tank which is not draining via a cross connection hose in short order.

 

[Lepke]

One solution bigger boats use is a day tank to run from. Mounted on the center line. Then fuel would be pumped from other tanks as needed and to balance the boat.
Another way is a crossover line with a 2 way pump and tank sensors. When tank levels get too unequal the pump comes on.
My day tank holds 2 days fuel. It's not on the center line, so that's a pain. I have to balance with the water tanks.