An Empirical predictive on fuel consumption figures per-day(T)of old timer Tug Boats,i.e.Tot.Hp X 4;where does it come fro

by Agung Aang on Thursday, March 19, 2009 at 11:01am

May be some of you have ever heard the above mentioned "rule of thumbs", claimed by our beloved old geeks Marine Engineers (those who sailed onboard Tug Boats & Utilities in the early 70's to 80's). Indeed it's not a kind of Axioms nor Law's resulted from a series of published research and iteration, however in fact the said predictive formulas as a rough based calculation (ball park figure)is quite powerful to get some indicative figure instantly, even for nowadays context.

I had notice myself that I have got the same methods of prediction at least from 5 different engineers who sailed during such type of vessels within such above mentioned period of time (70's to 80's)

I myself so used to utilize it frequently in certain situation where I've got to be answerable to my boss on the issues related to fuel consumption figure within short seconds, otherwise I might got another "Jancuk" if I failed to give an answer instantly.

After some times, I just eventually realized that the nos produced from that numerical operation, was actually not that bad, from my record analysis done very recently, I found that from various size Tug Boats (some also utility vessels), with conventional hull bodies, single screw, single engines, in between 5 to 15 years old built, from various brand of propulsion engines as long as high speed diesels, and irrespective of the applied ratings, after I ran an iteration with a norm distribution, the error margin was only within the range of 4 - 7 % (amazing huh?). Though it is enough for me for such an estimating (rough prediction)purposes only. Even though I found this very much interesting however, yet, I will not dare to test this hypothesis further in to a certain further stage of a scientific research methodology, please I'm not a scientist, it is merely on my curiosity, I do not see it falls within my areas of interest (may be some of you does?), Further on I believed that it is just a purely practitioner's approach, not more not less.

After I ran through further the numbers, I've found another fact that the error nos will increased significantly, it goes up to 10% and over, for the application toward a twin screw-twin engines Tugs, as well as for non conventional hulls (modified bow's hulls, modified flared hulls, etc), it goes between 8% - 10%. Hence ones got to be careful to apply such formula for these kind of configurations. Unfortunately I do not have an ample time to see further correlation of the nos for the application of Z-peller drive (an azimuth thruster), I'd just do not have time. All the above mentioned nos are for the application of conventional thrusters with protruded tail shafts.

Before steps further on our discussion i.e. How and where does it (the formula) come from? In order to avoid any false information, I'd like to re-highlighted that the consumption nos meant was the actual daily consumption figures after the vessels put in operation after some time and not the theoretical nor the designed nos, please take a note on this, after all these are more relevant for practitioner.

Even though the formula did not meant for theoretical consumption figures , in order to trace where does that formula come from I would use still a theoretical approaches, as you might aware, the normal daily consumption figure could be obtained from the formulas of SFOC (gr) x BHp (cont) x 24 (hr), from which we could then understand that in order to get in line with the above predictive formulas, the range of the SFOC in a certain assumption of 85% MCR (or even less) of the engine, should be at least abt.195 - 200 gr/BHp/Hr, which is true from a serial of High speed diesel engines in the output range of 750 - 2000 Hp, the SFOC was in the range of 203 - 213 gr/BHp/Hr.

In spite of various and volatile rpm applied through out the operational regime of a Tug boat, an average no of 75 - 85% MCR of engine performance during long distance voyage (more than day) continuously, is found to be quite representatives and does make senses. Thus we could use it as basis for the average actual daily fuel consumption as aforementioned.

Irrespective of all scientific proofing methodology 'IMHO' we can use the formula still, especially on a certain situation where we have to be able to get an instant idea on a predictive consumption figure of a running Tug boats or other Utility vessels, maybe on a certain situation where your boss asking you to get the answer within minutes while at the same he pointing out a gun tip over your head…then I just strongly recommend you to apply the formula…he..he..he…after all again this is just a rough predictive approaches, therefore DO NOT TRY THIS AT HOME…!

March '09