Frictional Resistance: The Dominant Component of Drag

In continuation to our series for vessels' calm water resistance let's continue with the second part of our series for the Frictional Resistance component:

Foundation of Drag: Frictional Resistance's Dominance

Frictional resistance (RF) is a primary component of a vessel's total calm-water drag. It typically accounts for a significant portion, ranging from 75-85% in new, slow-speed ships and up to 50% in high-speed vessels. The ITTC-57 model-ship correlation line (CF = 0.075 / (log Re - 2)^2) is an established method for estimating this critical resistance component. This line shows its dependency on the Reynolds number (Re), wetted surface area (Sw), water density (ρ), and ship speed (U). Frictional resistance originates from viscous shear stresses. These stresses develop within the boundary layer—a thin region of water adjacent to the hull where velocity gradients occur. For full-scale ships, the flow within this boundary layer commonly transitions from laminar to turbulent at Reynolds numbers exceeding 1x10^6, influencing overall resistance.

The Impact of Hull Roughness & Biofouling

The condition of the hull surface, often quantified by Average Hull Roughness (AHR), directly influences frictional drag. An AHR of 65 µm generally indicates very good operational efficiency. Values significantly exceeding 200 µm, however, suggest a suboptimal hull condition. Industry research shows increased hull roughness can measurably increase required propulsion power, leading to higher fuel consumption throughout a vessel's operational life. Moreover, severe biofouling substantially increases power requirements. In some cases, it can double power needs compared to a clean, smooth hull. This effect typically worsens between dry dockings due to the accumulation of corrosion and marine growth.

Correlations: Hull Condition, Fuel, and Emissions

The power required to overcome total (and hence frictional) resistance is approximately proportional to the cube of the vessel's speed. This relationship implies that even modest increases in speed can lead to disproportionately higher power demands; for example, doubling a ship's speed can necessitate up to eight times the power. Consequently, increased frictional drag directly leads to higher Shaft Horsepower (SHP) requirements and a corresponding rise in fuel consumption. Therefore, poor hull condition contributes directly to increased operational costs and Greenhouse Gas (GHG) emissions. This impacts vessels' ability to comply with maritime regulations.

Optimizing hull performance is a critical strategic imperative for improving sustainability and economic viability in maritime operations.

How is your organization addressing hull efficiency to meet regulatory demands and reduce operational costs?