Ship resistance and hull roughness measures - Energy Efficiency considerations
The ship resistances due to wetted surface areas are composed of frictional and wave-making resistance. Frictional resistance, in particular for slower speed ships, is the primary component of total resistance. A tanker at its design speed will use the majority of its fuel overcoming frictional resistance when sailing in calm water.
When the hull moves through the water, water will be dragged along, creating a body of water following the ship and forming what is referred to as a "boundary layer." In the forward part of the ship, this boundary layer will be comparatively thin, but it grows in thickness along the hull's sides. A boundary layer will form even on completely smooth hull surfaces. Increasing the roughness of the hull surface tends to increase the boundary layer, consequently increasing the hull's frictional drag.
In ship resistance, another major player is the ship speed relative to the water close to the hull surface. The effect of surface roughness on the resistance depends on the effective speed of the water relative to the hull, and this varies over the hull surface. For example, increased surface roughness in the bow area will cause greater resistance than in the aft areas or under the hull bottom because the effective speed of the water will be less. So in reducing the roughness, a different part of the ship will have a different level of impacts even if their original roughness is the same.
The smoother the hull, the less resistance the ship will have, and thus the faster it will go for the same power output, saving fuel and reducing Green House Gas GHG emissions. Fouling will reduce the smoothness (increase roughness) of the hull and even may add weight to the vessel reducing the cargo-carrying capacity. These impacts make the hull roughness and fouling a major issue of control for energy efficiency.
It is not uncommon for a new ship to be delivered with surface roughness as low as 75 µm and later in life enter dry-dock with a roughness of 250 µm. Historical records have shown that even with good maintenance practices average hull roughness can increase by 10 to 25 µm per year, depending on the hull coating system, even when fouling is not included [ABS 2013].
Fig:Types of under water fouling: green-weed
Causes of surface roughness
Hull surface roughness comes in many forms and from many sources, which can be broadly categorized as physical or biological. These sources are further grouped based on the size as either micro-roughness (less than 1 mm) or micro-roughness (greater than 1 mm). The physical micro-roughness can be increased in service by mechanical damage, failure of the applied coating, and even improper preparation of the surface and/or improper application of a new coating. Biological roughness (fouling) also has a significant impact on resistance, even at the micro-level (slime, algae, etc.). See
The Figure for some types of fouling.
Predictions based on model tests of a light displacement fineform
ship indicate that a light slime covering the entire wetted surface can increase total resistance by 7 to 9 percent. A heavy slime results in a total increase on the order of 15 to 18 percent. Small barnacles and weeds push this up to a 20 to 30 percent increase in total resistance [ABS 2013].
Fig:Types of under water fouling: burnacles & mussels
Figure - Example of fouling [International Paint]
Biological fouling is a very complicated process that depends on factors such as the ship's loading
condition, its operating zones, the effectiveness of anti-fouling paint, and environmental conditions.
If a ship is always moving, it will not gather as much marine growth as one that spends long
periods in port or at anchor. If a vessel is left static for extended periods, it will allow the marine
growth, which causes fouling, to attach itself to the hull and propeller, which will reduce the ship's speed and increase fuel consumption. Hull cathodic protection also tends to work better when
the ship is moving.
The main factors that influence hull fouling rates are:
Initial roughness of the hull
Quality of hull coating
Robustness of the coating with respect to mechanical damage
The hull areas where there is sunlight, along the sides of the hull and near the
waterline.
Temperature of water (colder water generally means less fouling)
The salinity of the water (performance coating will be a function of salinity of water)
Amount of algae in the water
Ship speed and its operation profile (hull moving, speed, at berth, at anchor, layby, etc.). or
static)
Hull maintenance
During the operation of the ship, surface roughness can increase due to cracking and damage to the
coating as well as due to corrosion, which can also attract marine growth. The growth of organic
species will include slime, weed fouling, and barnacles as examples shown in Figure. Current antifouling paints tend to last for a maximum period of 3–5 years when the self-polishing coating must be renewed; however, its performance is reduced gradually over time, as explained later. The hull will also require cleaning/brushing that can either be performed by divers or automatically with either the whole hull or just critical parts being targeted. Having a shorter interval between the applications of coatings may, therefore, reduce energy consumption. Still, there are some problems as the ship will require an additional dry-docking that is very costly.
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