Fuel Return Amount PER GALLON USED

[Jasper]

Hi Guys,
I have a question regarding the amount of fuel RETURN on the 12V71n’s. I need a Fuel Return Amount PER GALLON USED.
I have a reasonable idea on the fuel burn of the engines at trolling speed, cruise and at full, but I am trying to get a bit more accurate for a week long trip I am doing this coming week. I will be doing a mix of speeds over the trip with a 20 kw Onan at half load running most of the time.

I have one reliable fuel gauge, directly on the Mid tank. All the others have Tank Tender that was not installed correctly by the PO.

So I have FULL tanks on all of them and have run the MID tank down to half. It is calibrated in 16ths. So I now want to run of the FORWARD tank to even out the weights and if I RETURN the fuel to the MID tank then I can calculate more accurately the fuel consumption on the FORWARD tank, just by checking how much the MID tank re-fills by.

Drawn out question…..sorry, but it will make an interesting excersize.
Thanks.
Jasper

[chris piazza]

I don't recall exactly but I think I read hear that approximately 60-70 % is returned, at least on my 892TI's

Chris
Superior Nights 53C

[Jasper]

I don't recall exactly but I think I read hear that approximately 60-70 % is returned, at least on my 892TI's

Chris
Superior Nights 53C

Thanks Chris.
Heck, that seems a lot. I was expecting 40 to 50% at most!
Cheers.

[Cricket]

Jasper, it's always a crapshoot trying to move the fuel around using the return valves. My forward tank is the biggest, mid is middle, aft is smallest, gen only uses the front tank. So we keep 1 engine on the forward all the time and then start with the other engine off the aft for a very specific time by fuel burn, then put that engine on the mid, knowing we have X gallons in the aft for reserve. Your aft tank was modified by the PO and I'm assuming it's bigger than original. At trolling speed the fuel transfer is going to be very slow, at cruise you can get embarrassed very easily so be careful. Anyway, I'm tired of messing with this and I'm putting in a fuel transfer system with their own pickups and discharges so we can do whatever we want.

[Canuck Dennis]

Phone a dd dealer to be sure, I would not be surprised at 60 galls per hour regardless of speed, don't forget the fuel pump adds or reduces pressure depending on engine speed, let us know if you find out from a DD dealer.

[Jasper]

Jasper, it's always a crapshoot trying to move the fuel around using the return valves. My forward tank is the biggest, mid is middle, aft is smallest, gen only uses the front tank. So we keep 1 engine on the forward all the time and then start with the other engine off the aft for a very specific time by fuel burn, then put that engine on the mid, knowing we have X gallons in the aft for reserve. Your aft tank was modified by the PO and I'm assuming it's bigger than original. At trolling speed the fuel transfer is going to be very slow, at cruise you can get embarrassed very easily so be careful. Anyway, I'm tired of messing with this and I'm putting in a fuel transfer system with their own pickups and discharges so we can do whatever we want.

Agree with you. It is a crapshoot! Last time I did this my timing was out a bit and I checked the ER while I was off the helm for a few minutes and realised if I had left it for another 10 minutes I would have been pumping fuel out the overflow/breather!
Maybe I should get the Tank Tender re-installed, last boat I had with the TT, it worked well.
My AFT tank is new, installed by the PO and large, but only feeds the other tanks, not the engines, just to add a bit of interest.

[Jasper]

Phone a dd dealer to be sure, I would not be surprised at 60 galls per hour regardless of speed, don't forget the fuel pump adds or reduces pressure depending on engine speed, let us know if you find out from a DD dealer.

I will email them today. This makes a lot of sense, thanks C Dennis!

[Jasper]

DD guys tell me about 200 litres per hour at 1800 rpm or 52 Gallons per hr at 1800 rpm.

[saltshaker]

DD guys tell me about 200 litres per hour at 1800 rpm or 52 Gallons per hr at 1800 rpm.

That's seem too low. I was told my 550hp 6v92 pump about 90 gph at 2100 and burn about 25.

[rsmith]

the supply pump is a positive displacment gear pump the flow rate varies by rpm and how worn the pump gears are. I dont have the tune up book with me but I seem to remember 60 gph @2000 rpm. I dont think the method your contemplating is going to be accurate enough. My fuel gauges have never worked and even if they did are not accurate enough for any pratical use. I have a fitting on top of the fwd and mid tanks that I remove and stick the tanks with a calibrated wood dowel. Even then the long flat mid tank can varie by 50-70 gal dependant on how full the water tank and aft fuel tank are. I just ded recon the usage at 42-45gph at 2100 and 8-10 at 850 troll.