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.

 

[Jasper]

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.

Okay, thanks to all!
Looks like it's back to doing the average, logging the hours.
Then thumb in the air and I should be within a few gallons or soooo!
Again, thanks.
Cheers,
Jasper

 

[jim rosenthal]

The only way to measure this kind of thing accurately, as far as I know, is to install diesel FloScans. They measure fuel coming into the engine and going back out. You WILL get an accurate figure from FloScans, if they are properly installed, calibrated and maintained, but they aren't cheap and it's a LOT of money to spend to get the information that you want.

You might get some information on this by joining boatdiesel.com and starting a thread with this question. They are ALL about marine diesels and someone else may have already researched this. good luck; it's an interesting problem.

 

[Ross Macdonald]

The previous owner of our 60 EB with 12-71s at 650 HP. installed floscans and who ever calibrated them was really close based on fuel taken after a long run. Anytime we fueled after a 24-40 hr. run at about 1400 RPM and approximately 10 Kts. It showed between 14-16 GPH not including a little less than a gallon an hr. for the genset. For longer trips they are sure nice to have if, as Jim said, they are calibrated properly. The last trip when we arrived Cabo from Mag Bay it showed we used about 275 gallons and we took 295. So including genset, it was very close..

 

[Diver Down]

Two years ago we ran out of fuel during a transfer from one tank to another. That's when I learned that the flow rate was about 2 gal/minute at 1000 RPM (total through both 8v71 engines). I'm still doing fuel transfers underway but I'm much more cautious about the timing!

 

[OceanGaugesLLC]

Hey Guys,

My Fort Lauderdale based company has dealt with this problem for years, and there is no easy solution. Return fuel varies as was previously mentioned, so accurate math based rate measurement is difficult. Even Flo-Scans have many errors at different flow rates (lots of shocking testing info found in google). We believe the most effective and safe fuel management is accuracy in quantity measurement. Installing probes and gauges that have have high resolution and that display "Gallons Remaining" (liters for the EU folk) is the safest way for long passages and capacity optimization. Calibration as was mentioned, is also absolutely necessary as fuel tanks are rarely symetrical in position and shape. Our company can help out with affordable accurate digital gauges that are programable but, the probes would have to be recommended by factors such as the depth of the tank and quantity. I trust this helps. Cheers,
-Daryl

 

[SKYCHENEY]

Gauging is the best answer. I have the Tank Tender and I have a detailed chart of my tank in inches. This give very accurate measurement and is relatively inexpensive and also very easy to install.

 

[Jasper]

Firstly, many thanks for all the responses, really appreciate it.
We have had to return a bit earlier than expected due to a bit of bad weather. Queensland is being hit by very bad weather. We finally moored at our home just beating 15 foot waves and 5 to 7 foot swell with 65 knot gusts. Still we had 40 knot gusts and mooring was another story which involved quite a hassle with some chains and bouys caught in between the Stbd rudder and prop which meant dropping anchor and getting the SCUBA out and I was down there for over an hour.
Still had a great week and once again a reminder to check the weather forecast, add contingency and get the hell out if in doubt, when we left Tangalooma there were still a few boats happily enjoying the weekend, I hope they are okay. As it was we had the occasional 12 footer and 40 knot gust and all this in relatively shallow water with sand banks limiting the way I could tackle them.
Back to the fuel:
I did an average and it worked out that at an average speed of 8 to 10 knots over a 6 and 1/2 hour cruise we transferred 662 litres (175 gallons) in 55 minute period. I was going to do this on the return trip a few times to average the transfer out but obviously had my hands full with just my wife and I on board.
By the way, in 20 years this was our FIRST boating or any holiday without our two Sons! Kind of a bitter sweet experience that soon turned SWEET!
Cheers Guys, have a good week.
Jasper