Need help with 8v71N fuel calculation table...

[Eventide]

Well guys, my brain has frozen over and I seem to be unable to interpret a simple table. I was trying to see what DD says my fuel consumption should be based on their data and I can't convert what they say to fuel consumption in gallons per hour for my DD 8v71N's. Could you please help? Here is the table and diesel fuel is assumed to weigh seven (7) pounds per gallon (right?). Thanks!

RPM .......... BHP .......... Fuel - lb / bhp hr

1000 ......... 145 .......... .482

1200 ......... 185 .......... .440

1400 ......... 218 .......... .416

1600 ......... 248 .......... .405

1800 ......... 275 .......... .401

2000 ......... 296 .......... .403

So, how do I go from this table to how much fuel I consume? Isn't there a simple formula? You can't believe how dumb I feel for not being able to figure this out.

Interestingly, peak torque for these engines is at 1400 rpm according to their data. This rpm also seems to be my cruising sweet spot and drives my Hatt at about 10 nmph. Beyond that I don't know what the significance of the torque reading is in this sense. Any clues?

Again, thanks so much.

 

[TedZ]

Well .482 x 145 = 69.89 then divided by 8 which is a little closer to the weight of diesel per gallon gives you 8.74 gallons per hour which sounds right.

Not a wonderful table but it works i guess. There are fuel burn curves on all of the engines available from DD in gallons and liters per hour. They were on-line...haven't looked lately.

Hope this helps.

Ted

 

[Wild Bill]

Eventide,
I'd like to get the same table for my 6-71N's. Where did you find it?

 

[Brian Degulis]

From the looks of that table you will not get real world fuel burn. It looks like that table is giving you the rated power the engine is capable of producing at any given RPM. This will be very different from the power the prop is actually absorbing which is what you must go by to determine fuel burn. There are very simple charts for this available from any DD dealer no math needed everything is right there in plain english.

Brian

 

[Nonchalant1]

I think going at it from engine graphs is less helpful than just asking others what you might get and then running real tables after you start using the boat a lot. That will get you enough info to not run out of fuel when cruising before you really know your boat's numbers.

With a 50MY and 8V71Ns you'll always be at displacement speeds. Your hull speed is probably about 8.5 knots (figure it out), so a good educated guess is that you'll very likely use about 1.0 gal/NM at about 8 knots, using whatever RPMs it takes to get 8 knots. When you crank them up to go faster than hull speed, you'll start pushing a wall of water and you'll use up something like 2.0 gal/NM trying to go 12 knots.

I's very hard to get accurate real use numbers from accurate "lab conditions" engine graphs. I did actual numbers for my 53MY because it has floscans and I can make 2 way runs at many RPMs and calculate the numbers. They're shown in a number of threads here (do a search), but even those will vary based on weight of fuel, water, gear and heavy things like anchor chain vs. rope on board and/or the bottom cleanliness or wave conditions. There are so many variables, you need to make a table from your real experience.

The most important table you can develop is RPM - Speed - Gal/NM. With those numbers you can see how much fuel you'll use based on how far youll go and how fast. To me, gal/MN is much more useful than gal/hr because you start planning any day or any cruise with how far you'll go.

Doug Shuman

 

[Eventide]

Thanks for the help, guys. Really appreciate it. I think that this table is basically bogus for my purposes, as suggested by you for different reasons.

The table came from a DD site that I Googled, but I can't seem to be able to find it again.

In any case there are apparently better tables, curves and data out there, and nothing is as good as real world measurements on my boat over time. I do not have flow-scans but obviously there are other ways to accomplish the same thing.

 

[AquaNav77]

Eventide:

The numbers that you have presented are actual BSFC meaning Brake Specific
Fuel Consumption readings. I have seen these presented on some DD Engine lit as being for the BHP representations as well as a separate chart for SHP
which would be more in tune for actual use. I do have the formulas for the
interpretation of these numbers as well as the actual horsepower/fuel consumption curves. I also have this for the 6-71N that someone asked for also. This info is very difficult to find now, however I have managed to build
a real nice informational library on the 53, 71 and 92 series. When I get home
next week I can forward this info to you.

AquaNav77

 

[Kerry Alexander]

I with Doug. I have a 50MY with 871Ns.
At 1250 RPM I run about 9 to 10 MPH per GPS and avg 1 MPG.
I use the GPS to keep track of total miles run and reset when we fill up.
We live aboard and She is heavy.

 

[Eventide]

AquaNav 77: I am looking forward to seeing your interpretation of this material as well as the fuel curves. Much appreciated.