INFLUENCE OF EXTERNAL FACTORS ON FUEL CONSUMPTION

Bunkers have become now a day very significant expense in day to day running of ships. Hence saving in fuel is a very important aspect for all shipping companies and their vessels. A small saving in bunker expense surprisingly contributes a significant amount in the long run for the ship. Sometimes external factors influence the consumption of fuel. Following are some external factors on which fuel consumptions depends a lot:-
1) SHIP’S HULL CONDITION:- Ship’s hull condition definitely influences the fuel consumption. Ship’s movement is restricted by resistance experienced by heship, which is comprised by two types called frictional resistance and residual resistance. Frictional resistance is a function of density of water, hull roughness and length of ship. Residual resistance is due to wake forming tendency caused due to movement in water and shape of ship. Frictional resistance can be up to 70% of total resistance of the ship and hull condition is a major contributory factor in frictional resistance. Thus it is very important to keep hull clean. Hull can be cleaned by several methods while ship is afloat, but majority of these are not effective in long run. So, regular dry docking is the best solution in this regard.

      So any increase in underwater hull roughness will increase hull frictional resistance or vessel drag resulting in additional power requirement with increased fuel consumption and cost to maintain vessel speed. Factor responsible for hull fouling are:-
a) Use of improper techniques in applying paint.
b) Poor quality of applied paint.
c) Long port stays or anchorage.
d) Damaged hull surface.
e) Poor maintenance of hull protective system.
Appropriate measures should be taken regarding the above points to reduce the extent of hull fouling of the ship. Antifouling paint of approved type and well maintained antifouling system plays an important role in ship’s regular operating period between dry docks.
2) WEATHER CONDITION:- Ships are designed and constructed to withstand the forces of nature up to certain extent for a certain time. Depending upon the area of trading, weather conditions keep changing along with the condition of sea. Seasons of extreme nature are also common in the ship’s routes. If climate/weather condition are favorable, it may result in a positive slip i.e. ship travels more distance than given by the engine and with adverse weather condition, the slip may be negative, that means that the actual distance travelled by ship is lesser than the engine distance, resulting in extra fuel consumption due to higher power demands and overloading of engines.
                   Good judgement and regular updates regarding weather condition helps the master in choosing a route to avoid adverse weather condition. This may result in lesser fuel consumption in long run. Engine manufacturer’s guideline should be strictly followed in severe weather conditions. Governor load index, hunting, RPM, scavenge air limits, torque limits must be taken into account to avoid thermal and mechanical overloading of the engine.
3) MAINTENANCE OF DIFFERENT ELEMENT IN FUEL OIL SYSTEM:- Although the condition of the hull and weather play a significant role in regulating fuel consumption, elements which directly control fuel have a proportional relation to the consumption of fuel. The different elements of fuel oil system are:-
a) Fuel oil injector:- It should be regularly changed after fixed running hours as per maker’s guidelines. Overhauled and tested valves to be used.
b) Fuel pumps:- Injector timings to be checked regularly and adjusted as required.
c) VIT:- Adjusted as per requirement.
d) Fuel oil filter:- Regular cleaning.
e) Viscotherm:- Regular maintenance and maintaining proper fuel oil temperature to achieve desired viscosity as per fuel oil analysis report.
f) Service tanks:- Proper temperatures to be maintained.
g) Greasing of fuel linkage
h) Operation and maintenance of purifiers should be always carried out for proper operation.
4) DAMAGE TO PROPELLER BLADE:- Propeller should be maintained in top condition at all times. The main factors that detract from optimum condition are fouling, cavitations and physical damage. But, any distortion from their true shape can cause an imbalance and hence vibration, which in turn causes increased cavitations, loss of thrust, drive shaft damage, wear on numerous bearing and increased fuel use due to decreased efficiency.

                                                        Damaged propeller blade can be repaired in dry dock. Propellers can be easily damaged if they strike a buoy, or hit floating debris or ice. The damage is usually to tip which can become bent or which can even have large chunks of metal taken out of it. Physical damage of this nature mostly causes vibration. The solution in this case is to trim the blades equally to remove the damage and achieve proper balance, and reduce excessive cavitations. But this should be done carefully as bad trimming can result in even worse problem.
              There are 3 types of modification:-
a) Diameter reduction:- Easily and inexpensively performed underwater, this is the usual method for increasing RPM and balancing the ratio. The blade tips are cropped and faired.
b) Pitch reduction:- This involves twisting of blades and can only be accurately done in a workshop as blades need to be heated to prevent cracking. Although more expensive this is most effective modification as there is no loss of blade material. It is ideally suited to blades smaller than 4,000 mm diameter.
c) Trailing edge modification:- This is achieved by either bending the trailing edges or by cutting them. Both operation can be performed in water and can achieve an effect on the RPM of approximately 5%.