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Annex 1 177<br />
Main engine power output<br />
In steady state (constant speed), the thrust produced by the engine and propeller is in equilibrium with forces<br />
opposing the ship’s motion. These forces include both hydrodynamic and aerodynamic resistance. Both forces<br />
are modified by the weather; e.g. sailing into headwinds or head seas (waves) increases resistance. In both<br />
calm and rough weather, total resistance is dominated by hydrodynamic resistance, which in turn is dominated<br />
by viscous (friction) and wavemaking resistance.<br />
Naval architects have progressed methods for estimating resistance from ship characteristics for a ship in<br />
ideal conditions (negligible wind and waves, clean hull), which reveal that in these conditions, resistance is<br />
strongly related to the speed of the hull through the water. However, in operation, a hull rarely stays “clean”<br />
and the surface properties are modified over time as coatings deteriorate, macro- and microfouling grows on<br />
the hull and the plating deforms through wear and tear. This modification of surface properties can have a<br />
significant impact on viscous resistance and needs to be taken into account in any calculation of operational<br />
fuel consumption.<br />
Further influences to a ship’s resistance and propulsion are its draught and trim, which are in turn determined<br />
by the ship’s loading condition (the amount and distribution of cargo and variable loads). A greater draught will<br />
increase the wetted surface area of the hull and typically increase the resistance (although both bulbous bow<br />
and propeller performance can sometimes counteract this trend of increased power demand with increasing<br />
draught). The approximation used in this model is to represent the effect of draught through the use of the<br />
Admiralty formula, which assumes that power is related to displacement to the power 0.66.<br />
The formulated equation to encapsulate all of these effects on resistance and therefore main engine power is<br />
given in equation (1).<br />
P<br />
P t<br />
= <br />
ref(<br />
__<br />
t t <br />
t ref<br />
) __<br />
( 2 3 )<br />
___<br />
( V t n<br />
<br />
V ref<br />
)<br />
____________<br />
η w η f<br />
eq. (1)<br />
In equation (1), P t , V t and t t are respectively the instantaneous power, speed and draught at time t, P ref is<br />
the reference power at speed V ref and draught t ref (both taken from IHSF). n is an index that represents the<br />
relationship between speed and power, and η w is the modification of propulsion efficiency due to weather<br />
and η f is the modification of propulsion efficiency due to fouling (discussed above). For the bottom-up model,<br />
the same assumptions have been used as in the Second IMO GHG Study 2009: that n = 3, an assumption<br />
discussed in greater detail in Section 1.5, and evaluated with respect to quality in Section 1.4.<br />
Auxiliary engine and boiler power demands<br />
The power outputs required by both the auxiliary engine and the boiler are both found using look-ups from<br />
input tables described above in the section “Assumptions for auxiliary and boiler power demands”. The<br />
corresponding mode is calculated for each ship and each hour of operation, from its instantaneous observed<br />
speed.<br />
Emissions subroutine: Emissions_at_op<br />
The emissions produced by machinery are a function of the amount of fuel consumed and the specifics of<br />
that fuel’s combustion. The former (fuel consumed) is found from the power, SFOC and time, and the latter is<br />
found from the use of an emissions factor – in the case of CO 2 , a carbon factor. The calculation of SFOC and<br />
emissions factors is detailed in Section 2 and Annex 6. Given this information, the formulation for this model’s<br />
calculation of emissions of main, auxiliary and boiler machinery is given in equation (2).<br />
CO 2 = P t × sfc × C f × t eq. (2)<br />
In equation (2), P t is the instantaneous power output at time t (obtained from Power_at_op), sfc is the specific<br />
fuel consumption (for a given engine with a given fuel at a given load factor), C f is the carbon factor (for a<br />
given fuel), and t is the length of time the instantaneous power was observed to be constant. The values of C f<br />
specific to different fuels are reported in Section 2.2 along with the other emissions species.<br />
The sfc is found from the combination of a default assumption for a given engine type, size and age, sfc e and<br />
a modifying factor obtained from a look-up table to account for variations in sfc as a function of fuel type and<br />
engine load factor.