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 (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.
Bunker/ Fuel Oil Calculations
Mass = Volume x Density
Metric Tonnes = (Actual Sounder Volume) X (Temperature Corrected Density)
Volume: tank capacity table: Cal. trim and list
Density: (kg/m3 at a standard reference temperature of 15⁰C, using ASTM table/software)
Temperature Corrected Density = (Density of Fuel Oil @ 15⁰C) x [1- {(T-15) x 0.00064}]
T = temperature of oil in bunker tanks in degree Celsius,
0.00064 = Correction factor
Calculated Carbon Aromaticity Index (CCAI) is an index of the ignition quality of residual fuel oil. … For spark-ignition engines the fuel has an octane rating. For diesel engines it depends on the type of fuel, for distillate fuels the cetane numbers are used.
Once the CCAI goes above 860, it is an indication that some combustion problems may occur.