The specific Fuel Oil Consumption (SFOC) of the range of propulsion
powers under consideration, since the SFOC originally measured in the Test Bed was of 158
g/bhp/h for powers between 50 and 75% MRC. As a consequence thereof, the FO comsumption
per hour are proportional to the bhp developed by the main engine, and hence proportional
to the ship's speed raised to a power of 3.
The 1988 drydocking had only a minimum effect on the FO consumption.
This was due to the rather good condition of the hull before drydocking, with no fouling
on the bottom and very little algae and slime, so that the roughness reduction obtained
through the elimination of said algae and slime was more or less compensated by the
roughness of the new paint. In contrast, the 1990 drydocking had a very dramatic effect on
the FO consumption, partly due to the very poor condition of the bottom before drydocking,
partly due to the full shell shotblasting achieved during drydocking and partly due to the
new CLT propeller fitted to the ship. In order to avoid any consideration about the hull
state before both drydocking, it is better to compare the FO consumption obtained after
each of them, as it has been also done above, with the results of 21.18% saving in full
load in ballast. These savings are then only due to the CLT propeller and to the effect of
the aforesaid shotblasting on hull roughness.
It is then necessary to calculate the roughness difference between the
hull just painted without any previous full shotblasting (cleaned only by pressure water)
and the hull just painted after a previous full shotblasting, as well as the effect of
such a roughness difference on the propulsion power.
When the ship sailed after the 1990 drydocking, and assuming that the
aforesaid shotblasting was perfectly done, it can be assumed that her hull roughness was
the minimum to be expected in a 16 year-old ship, that is: construction roughness plus
ageing roughness plus shotblasting roughness plus new paint roughness.
According to Townsin (Ref 1), the percentage of increment in propulsion
power (or FO consumption in this case) due to a difference in roughness as existing when
comparing the hull after the 1988 drydocking with the hull after the 1990 drydocking, is
the following:
It can then be concluded that the CLT propeller fitted to Guardo has
reduced her FO consumption in service at constant speed, by a percentage higher than 14%.
FO consumption as calculated above have been plotted against speeds in
Figure 3, to show the behaviour of Guardo with her old conventional propeller and with her
new CLT propeller.
