3 Important Calculations Every Marine Engineer Must Know On Ships

In ship’s engine room, a number of gauging instruments display various important parameters such as level, pressure, temperature etc. But there are some essential parameters which cannot be read directly through any instrument as they depend on a number of dynamic factors.

This demands the marine engineer working onboard ships to do some formula based calculations by considering all those factors and with possible available inputs. Of all the important calculations that are to be done on board ships, there a few ones which marine engineers must know without fail.

Related Read: 8 Engine Terms Every Marine Engineer Should Know – Part 1

Following are 3 important parameters that are most often required to be calculated by marine engineers for record-keeping:

1. Bunker Calculation:

Bunker terms to fuel oil in the maritime industry. Bunker quantity calculation is the most important calculation which every marine engineer should be familiar with throughout his career. Bunker fuel, being a highly valued product, has to be very carefully and accurately calculated for determining the quantity.

The volume of a definite quantity of bunker increases with an increase in temperature whereas its weight remains the same. For this reason, the bunker is always ordered and measured in weight and not by volume.

Related Read: The Ultimate Guide to Fuel Oil Bunkering Process on Ships

Also, all the energy and efficiency calculations of fuel onboard ships are calculated in terms of the mass of the fuel rather than its volume. (Calculation of bunker fuel quantity in weight involves many factors to be considered making it more complex.)

Mostly, the fourth engineer will be the deputy of the Chief engineer for receiving bunker & measuring the bunker quantity for maintaining records.

Mentioned below is the procedure for measuring and calculating the bunker quantity:

• Once the total quantity (tonnes) of bunker fuel to be received is confirmed, take a sounding of ship’s bunker tanks & calculate the available quantity of fuel oil onboard to formulate a bunkering plan denoting how much tonnes of fuel to be bunkered on each tank & the tank sequence of bunkering.
• Before commencing of bunker operations, confirm the temperature at which the bunker is to be received and note down the standard density of the fuel oil. With this parameters calculate the volume of the bunker to be received in each tank as per bunkering plan and note down the final sounding level of each tank after bunkering using the capacity table for the ease of stopping & change over to next tank.

Related Read: 15 Practical Tips For Bunkering and Storage of Fuel Oil On Ships

The basic formula used for calculating the bunker quantity in weight is:

Mass = Volume x Density

• It is to be noted that in the above formula, the density and volume of bunker fuel should be known at the same temperature.
• After receiving the bunker, take sounding/ullage of all the bunker tanks using sounding tape and note down the tank temperature. Use sounding paste in the tape while measuring distillate fuel such as MDO for easy reading.
• The ship always does not float with an even keel so the floating conditions of the ship such as trim and list should be well noted while taking the sounding of bunker tanks.
• Every ship is provided with a tank capacity table in which each tank capacity in volume is marked against the successive levels of sounding/ullage with correction factors under various trim and heel conditions of the ship. The volume of fuel oil at tank temperature for the corresponding tank sounding is thus measured using tank capacity table, which gives the actually sounded volume.
• The density of fuel oil (in kg/m3) at a standard reference temperature of 15⁰C is always provided by the supplier in Bunker Delivery Note. With this, the density of fuel oil at tank temperature can be determined using ASTM table or using software most commonly installed on all ship’s computer.

The formula used to calculate the Temperature Corrected Density is:

= (Density of Fuel Oil @ 15⁰C) x [1- {(T-15) x 0.00064}]

Where:
T = temperature of oil in bunker tanks in degree Celsius,
0.00064 = Correction factor

Since the bunker oil is normally supplied to the vessel at a temperature higher than 15⁰C, the formula used for calculating the bunker quantity in weight will be:

Metric Tonnes =  (Actual Sounder Volume) X (Temperature Corrected Density)

 The corresponding values of each tank are tabulated for easy reading and the total weight of bunker quantity is calculated.

2.  Specific Fuel Oil Consumption (SFOC):

Specific fuel oil consumption is the measure of the mass of fuel consumed per unit time to produce per KW. The marine engine efficiency is usually determined using the SFOC.

In order to achieve accuracy, the fuel consumption and power developed is always measured over a suitable time period on a good weather. The formula used for calculating SFOC is:

SFOC (g/kwh) = Mass of fuel consumed per hour / Power developed in KW

•  The readings of the flow meter to the main engine should be noted over the specified time interval say 1 hour. With the difference in readings, the volume of fuel consumed is obtained. It can also be measured by noting down the HFO service tank reading provided the oil is being supplied only to the main engine.
• The mass of the observed volume of fuel consumed can be determined by following the above-said bunker calculation procedure.
• The horsepower can be measured using dynamometer if fitted on the shaft of the engine which will indicate the BHP in the digital indicator. If not, the horsepower can also be calculated using engine rpm and average pump fuel index with the aid of engine characteristic curve of various sea trials which is supplied by the manufacturer. However, the calorific value of the fuel used for sea trial may differ and hence compensation factor has to be determined to obtain the accuracy in calculation.

Related Read: 14 Terminologies Used for Power of the Ship’s Marine Propulsion Engine

3. Percentage of Slip:

Slip is considered as the difference between the speed of the engine and the actual speed of the ship. It is always calculated in percentage. Positive slip is influenced by various reasons such as fouled bottom or hull part which offers resistance to the movement of the ship, environmental factors such as water current and wind against the ship direction. The slip may be negative if the ship speed is influenced by following sea or wind. Engine slip is calculated daily onboard the vessel and recorded in the logbook.

Related Read: Various Methods Used to Minimise Resistance on Ship’s Hull

Engine distance – Observed distance

Percentage of Slip = ————————————————– X 100%

Engine distance

• The actual distance (nautical mile) covered by the ship from noon to noon is measured using the ship’s log.

• The total revolutions of the propeller from noon to noon is obtained using the revolution counter. The engine distance can be calculated using the pitch of the propeller provided by the manufacturer. Care must be taken in the unit conversion of the pitch from meter to a nautical mile (in general 1 NM = 1800m).

Engine distance in nautical mile = (Pitch x revolutions per day)

Obtaining the accuracy of value in all the above calculations are always challenging onboard as the parameters recorded are more sensitive to dynamic conditions of the ship and also depends on various environmental factors.

Related Read: 8 Engine Terms Every Marine Engineer Should Know – Part 2

Over to you…

Do you know any other important calculations that can be added to this list?

Let us know in the comments below.

References & Image Credits:

M.V. Jag Rahul – Ship’s tank capacity plan manual.

M.T. Jag Preeti – MITSUI B&W – Operation & Data Manual

Reeds Naval Architecture for Marine engineers.