Rate of fuel consumption means the amount of fuel consumed as a mass in a given time to develop one unit of power.

Influence of External Factors on Fuel Consumption

The fuel consumption plays a major part in the day-to-day running of ships. Hence, saving in fuel is a very important aspect for all shipping companies and their vessels. Sometimes external factors influence the consumption of fuel. Following are some external factors which directly affect fuel consumption.

13.7.1 Ship’s Hull Condition

Hull condition is a major contributory factor to frictional resistance. Thus, it is very important to keep the hull clean. As a ship’s hull condition degrades due to marine fouling, more power and fuel are needed to maintain service speeds.

Hence, the increase in underwater hull fouling or propeller fouling will increase hull frictional resistance, which causes an additional power requirement and is the largest single cause of excess fuel consumption and cost to maintain vessel speed.

13.7.2 Weather Condition

Depending upon the area of trading, weather conditions keep changing along with the condition of the sea. In bad weather conditions, more opposing forces act on the ship against its motion. Due to this, the engine is required to provide more power to maintain ship speed. This, in turn, causes it to burn more fuel to produce the required power. Hence, it can be stated that the condition of the weather is another factor which affects the ship’s fuel consumption.

Seasons such as summer, winter, or monsoons of an extreme nature are very common in the trade of shipping. If climatic/weather conditions are favorable, it may result in a positive slip (i.e., the ship travels more than the distance given by the engine) and vice versa. Bad or heavy weather conditions may result in a negative slip, resulting in extra fuel consumption due to higher power demands and overloading of engines.

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 as a percentage.

Example

Engine distance: 320 NM Observed distance: 305 NM

Find the percentage of slip:

Answer:

Tutorials

Find the slip in the following cases.

1. Engine distance: 320 NM Observed distance: 300 NM

2. Engine distance: 380 NM Observed distance: 305 NM

3. Engine distance: 280 NM Observed distance: 285 NM

13.7.3 Damage to Propeller Blade

Damage to the propeller decreases its efficiency, and this will cause increased fuel consumption. The propeller should be maintained in top condition at all times. The main factors that detract from optimum condition are fouling, cavitation, and physical damage. However, any distortion from its true shape can cause an imbalance and hence vibration, which in turn causes increased cavitation, loss of thrust, drive shaft damage, wear on the stern tube bearing, and increased fuel consumption due to decreased efficiency.

Propeller Cavitation

During propeller operation, sea water passes across the back of the blade and meets a high suction region. When the net pressure of the water falls below the vapour pressure of the water at that temperature, cavity bubbles are formed. When these vapour bubbles reach a relatively high-pressure area close to the propeller blade, the voids implode. This implosion suddenly creates a vacuum and it causes the removal of metal particles from the blade. This is called propeller cavitation.

Physical Damage

Propellers can be easily damaged if they strike foreign objects. The damage is usually to the tip, which usually takes the form of cracks, bends, or breaks and may include the loss of a small section of the blade, or can even have large chunks of metal taken out of it. Physical damage of this nature causes vibration and performance losses.

13.7.4 Methods Employed to Reduce Fuel Consumption

1. Use of antifouling paint on hull structure to minimize hull fouling Appropriate measures should be taken to reduce the extent of hull fouling on the ship. Antifouling paint of an approved type and a well-maintained antifouling system, along with periodical inspection and cleaning of the hull, play an important role in a ship's regular operating period between dry docks. This minimizes marine growth and helps reduce hull resistance.

2. With the aid of present-day technology, use of weather routing services to avoid rough seas and head currents will help to optimize voyage efficiency, thus reducing fuel consumption.

3. Periodical polishing of the propeller is also employed nowadays to improve fuel efficiency.

4. Nose Job – Modifying Ship’s Bulbous Bow Modifying a ship’s bulbous bow is an efficient way to reduce fuel consumption of ships.

   

    A bulbous bow is a peculiar, bulb-like projection at the forward end of the ship.