Introduction: The following is an analysis of fuel consumption onboard my vessel the Pacific Odyssey, a 44’ Vantare twin Cat semi-planing hull yacht with twin CAT 3208 diesels.  The vessel is used as a personal yacht, marine research platform, and charter yacht through Delta Charters, Richmond, BC.

I recently installed a FloScan fuel monitoring system and integrated into my Raymarine E-Series GPS electronics navigation system so that accurate liters consumed per nautical mile can be displayed directly. This was accomplished by using the NMEA 0183 GPS output as a data input to the FloScan system.  My VDO tachometers were replaced with the identical diameter FloScan gauges so no additional console holes were required.

Overview:  With the rising cost of diesel fuel, now as high as $1.25 per liter in Canada, a logical approach would be to invest a bit of time and money into figuring out just how to minimize or at least reduce fuel costs. I noted several comments from individuals that chartered the Pacific Odyssey that they had no idea filling up the diesel tanks would cost that much for their one-week vacation cruise.

 As the vessel owner I already provided RPM vs. speed curves, but this was also easy to obtain from the onboard, state-of-the-art integrated Raymarine Navigation System.  I know I would be doing vessel charters a big favor by providing accurate fuel consumption data to minimize the “Sticker-Shock” when filling up at a fuel dock. With the installed FloScan system, I now had the technology to provide these answers.

Fuel Consumption Calibration: I ran all the calibration tests in calm water, vessel fully loaded with minimal wind in a protected channel. I started at 1500 RPM and continued every 100 RPM until 2600 RPM. At each RPM interval I allowed sufficient time, approximately 1 minute, for the GPS and fuel readings to stabilize.  I started the first calibration run at 1500 RPM and went to 2600 RPM then immediately repeated the test in the reverse direction starting at 2600 RPM and then reducing speed to 1500 RPM.  I did this to give me an idea of repeatability.

I placed all data into an Excel spreadsheet on my Sony laptop and started to analyze the data immediately after it was recorded from the calibration runs. The analysis of the data is provided in the following section.

Excel Spreadsheet Explanation: The following lists the tabular results by describing each column in the spreadsheet:

A = RPM per the FloScan digital tachometer. Both port and starboard engine set at the identical RPM within 10 RPM.

B = Liters of fuel consumed per nautical mile in the increasing RPM test sequence

C = SOG as measured by my Raymarine GPS and E-120 series display in the increasing RPM test sequence

D = BLANK

E = Liters of fuel consumed per nautical mile in the decreasing RPM test sequence

F = SOG as measured by my Raymarine GPS and E-120 series display in the decreasing RPM test sequence

G = BLANK

H = Difference in l/nmi readings between the UP and Down RPM sequence (The data shows excellent repeatability)

I = Difference in SOG readings between the UP and Down RPM sequence (The data also  shows excellent repeatability)

J – Average SOG of the Up and Down RPM test sequence

K– Average L/nmi in the Up and Down RPM test sequence
L = The cost in Canadian Dollars per nautical mile at the specific RPM.  This is based on a fuel cost of $1.25 Canadian per liter.
NOTE the tremendous change as a function of RPM, an increase of over 300% over the test range as vessel speed increased from 9 to almost 16 knots.

M = The cost in Canadian Dollars per  knot of speed at difference RPM’s
It is important to note how the value peaks out at around 2100 to 2200 RPM.  The reason is based on the hull design.  At lower speed it acts like an efficient displacement hull and at very high speed it approaches the efficiency of a planing hull.  However in the middle of this range, 2100-2200 RPM, the boat is trying to get up on step and is rather inefficient.  This would be nearly impossible to detect without the FloScan instruments.

N = This is the predicted cost in Canadian dollars for a voyage or cruise of 100 nmi.  I picked this value as sort of an average for a reasonable coastal voyage.  Note that this distance can be covered for a fuel cost of $225 if you are willing to travel at 9 knots, (1500 RPM), but increased to almost $700 if you want to travel at almost 16  knots, (2600 RPM).  Presenting the data in this way gives the owner a good idea of just how expensive it is, based on the price of fuel today, to travel at elevated speeds.

O = This is the time it would take in hours to travel the 100 nmi voyage or cruise I selected for this analysis.

P = The MOTHER of all columns….This shows the increased cost in Canadian dollars per the reduced travel time in hours for the selected 100 nmi voyage.  This data dramatically shows just how expensive it is just to save an hour or so of transit time.  Again, in the mid range of speeds. we see costs as high at $180 per hour just to get to the destination a bit faster.  This is incredible…and something I would have never known if I had not installed FloScan instrumentation onboard by vessel!  How can any boat owner ever afford to be without one based on the high fuel costs of today?  All data is relative to traveling at 1500 RPM and a SOG of 9 knots.

Q = This table represents the absolute bottom line. Put in simple layman’s terms and based on a 100 nmi voyage, it details the increased fuel costs incurred relative to the time saved as a function of travel speed or RPM.  In this column, 1500 RPM and 9 knots was used as the base. Amazing to think you could save almost $500 in fuel costs for a relatively short 100 nmi cruise if you have the patience to arrive there in 11 hours and not in a fuel-guzzling 6.5 hours.

Summary:  I always knew speed costs…………….and now I can finally put an exact dollar value on it!

Professor Joe Valencic,
Onboard the Pacific Odyssey.