Important calculations every marine engineer should know

The success of a merchant vessel’s voyage can largely depend on the smallest details. Therefore, special attention should be paid to accurately operating ship systems. Sensors are installed in each of them, transmitting information to modern computing devices. However, some calculations must be made by the crew members themselves. Calculations can also be performed in certain programs, such as Shell BunkerCalc. 

However, according to the requirements of international conventions, these programs must be certified. Therefore, the responsible crew members should be able to carry out the calculations themselves. In this article, we will look at the basics that every engineer should know about.

Nowhere without fuel

The vessel can travel a certain distance based on the amount of stock on board and the consumption standards during the voyage. The greater the mass of cargo a ship can carry (without compromising carrying capacity) and the lower the consumption rate during the voyage, the longer the ship can stay at sea and the greater the distance it can travel without stopping.

Fuel reserves dominate the ship’s reserves. The type and capacity of the ship’s power plant, its technical condition, type and grade of fuel, engine power utilization factor, and vessel speed all influence fuel consumption.

Stock up on what you need

Before the voyage, the vessel receives fuel, water, lubricants, and food supplies. Their combined mass depends on the duration of the voyage, the possibility of replenishment at intermediate ports of call, navigation conditions, and the fuel type.

To calculate the supplies for the passage, account for the following:

  •       The running time in this passage, taking into account the planned delays:

Tr = l / 24V + Ttd

where l is the distance between the final ports;

24V – daily travel speed; Ttd – travel delay time;

  •       Berthing time at ports, accounting for the time of planned auxiliary operations

Tb = (Q/M + Tad) + (Q/M’ + Tad)

where Q is the mass of cargo received by the vessel (or unloaded from the vessel);

M, M’ are the norms of cargo operations, respectively, at the loading and unloading port.

  •       Fuel capacity:

Gf = Gv*Tr*Ksr + Gb*Tb

where Gv, Gb is the daily fuel consumption, respectively, in voyage and berthing;

Ksr is the storm reserve coefficient.

Determination of the amount of fuel at the time of the vessel’s entry or withdrawal from the charter

When ordering a Bunker Survey in the volume of “ON HIRE” or “OFF HIRE,” the charterer may require data on the number of bunkers at the time of the vessel’s entry or exit from the charter, as well as when the vessel was – or will be – in the position specified in the charter.

In this case, the amount of fuel consumed by the vessel between entering or exiting the charter and completing the task must be determined. Then, subtract or add it to the obtained results. The fuel consumption for the specified period is recorded in the engine log if the vessel has already transitioned. However, it is necessary to evaluate machine log data and demonstrate that the numbers are correct, i.e., correspond to the vessel’s actual operation.

If the ship has not yet made this transition, the fuel consumption is determined by a calculation agreed upon with the master. The specific gravity of the fuel is usually given in the fuel passport issued during bunkering at a temperature of 15 ° C and sometimes at a bunkering temperature. To convert the passport value of the specific gravity dt0 at temperature t0 to the specific gravity dt at measurement temperature t, we use the well-known formula:

dt = dt0 + k (t0 – t)

Example: Passport data of heavy fuel: dtO =  0.926 at tO = 15 °C. Determine the specific gravity of this fuel at its temperature in the fuel tank equal to 80 ° C.

d80 = 0,926 + 0,000607(15 – 80) = 0,887

The preliminary calculation cannot account for the conditions of the upcoming passage of the vessel (for example, stormy weather, forced downtime, etc.). Therefore, the master of the vessel often remarks – in the Bunker Survey act – that the actual fuel costs will be reported to the charterer when the vessel passes the position specified in the charter.

Consumption of specific fuel oil (SFOC)

What is the specific fuel consumption?

Specific fuel consumption (SFC) is a measure of the fuel efficiency of an engine, defined as the amount of fuel required to produce a specific amount of power or thrust over a given period.

The mass of fuel consumed per unit of time to generate one kW is indicated by the specific fuel oil consumption. SFOC is commonly used to calculate the efficiency of a marine engine.

To achieve accuracy, fuel consumption, and power output are always measured in good weather over a suitable period. SFOC is calculated as follows: 

SFOC (g/kWh) = Fuel mass consumed per hour / Power generated in kW

The flow meter readings for the main engine must be recorded within a certain time frame, such as one hour. If the readings differ, the amount of fuel consumed is calculated. It can be measured by recording the readings of the HFO service tank, provided that oil is only supplied to the main engine.

The above-mentioned bunker calculation procedure can be used to calculate the mass of the observed volume of fuel consumed.

A dynamometer mounted on the engine shaft can measure horsepower, which displays the number of hp on a digital indicator. If that is not possible, the power can be calculated using the engine RPM and the average fuel index of the pump using the engine’s characteristic curve from various sea trials. The manufacturer provides these details.

However, the calorific value of the fuel used for sea trials may vary, so the compensation coefficient must be determined to ensure the calculation’s accuracy.

Breezemarine Group can help cut costs and reduce your carbon footprint

You can use our fuel consumption monitoring system to make monitoring easier. The FCM system is a universal tool that makes it easy for ship crews and operators to keep track of bunkering, fuel use, and tank fuel levels.

Our systems are designed to track the ship’s overall efficiency and automatically collect the fuel use data needed for MRV reporting. This approach makes the shipping company’s work more efficient and simplified.