3 Important Calculations Every Marine Engineer Must Know On Ships!

In a 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 that the Marine Engineer working aboard ships, do some formula based calculations by considering all those factors, and including all 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.

Following, are 3 important parameters which are most often required to be taken into the calculations by Marine Engineers and for recordkeeping:

1. Bunker Calculation:

Bunker terms for 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, when determining the quantity.

The volume of a definite quantity of bunker increases, with 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.

Also, all the energy and efficiency calculations of fuel on board ships are calculated in terms of 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 Third Engineer will be the deputy of the Chief engineer for receiving bunkers & measuring the bunkers 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 the 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 the bunkering operations, confirm the temperature at which the bunker is to be received and note down the standard density of the fuel oil.

With these 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 ease of stopping & change over to the next tank.

The basic formula used for calculating the bunker quantity by 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 mostly does not float on an even keel, so the floating conditions of the ship, such as trim and list should be well noted while taking the sounding of the 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 the tank capacity table, which gives the actual sounded volume.

The density of fuel oil (in kg/m3) at a standard reference temperature of 15⁰C is always provided by the supplier in the Bunker Delivery Note.

With this, the density of the fuel oil at tank temperature, can be determined by using the ASTM table or using software most commonly installed on all ships' computers.

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 produced 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 good weather.

Vessel Slip.
It's to be calculated relatively to the speed by LOG, not GPS.

When the slip is negative, there is nothing about favourable current, only wind or some unknown circumstances may affect it.

Sometimes negative slip is the source of incorrectly provided or calculated by manufacturer figures, such as propeller dimensions, pitch, etc.

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 of 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 the 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.

3. Percentage of Slip:

Slip is considered as the difference between the speed of the engine and 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 log book.

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 ship’s log.

The total revolutions of the propeller from noon to noon is obtained using 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 nautical mile ie. 1 NM = 1852m.
Engine distance in nautical miles = Pitch x revolutions per day.

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

Note: Vessel slip.

It's to be calculated relatively to the speed by LOG, not by GPS!