Explained: propeller fouling and ‘the power penalty’

The impact of calcareous (hard shell) fouling is particularly critical. Researchers in the past have sought to replicate calcareous fouling through casting, three-dimensional (3D) scanning and printing technologies, etc., to investigate the effect of biofouling without introducing real fouling into experimental facilities.

CFD is seen as an advanced and more accurate tool, in-part because the parameters are dynamically computed for each cell and therefore the variations in parameters can be accounted for. 3D effects can also be taken into account, and the simulations are free from the scale effects if they are modelled in full-scale, according to the authors of the study.

A roughness function model was employed in the wall-function of the CFD model to represent the barnacles on the hull, rudder and propeller surfaces. A proportional-integral (PI) controller was embedded in the simulation model to find the self-propulsion point.

The simulations were conducted under combinations of different fouling scenarios and surface conditions (fouling severities). The fouling scenarios consisted of three fouling configurations: ‘fouled-hull/clean-propeller’; ‘clean-hull/fouled-propeller’; and ‘fouled-hull/fouled-propeller’, as well as a ‘clean-hull/clean-propeller’ condition as a reference case. Ten different surface conditions were then applied for the three fouling scenarios, except for the clean-hull/clean-propeller condition.

The surface conditions varied on barnacle type, height and coverage of the ship surface. For instance, B20%, which is the most fouled scenario, describes a surface condition with big barnacles of height 5 mm covering 20% of the surface. Some 31 different simulations were carried out in the study (1 smooth and 3 × 10 fouled conditions).

The impact of biofouling on various parameters such as propeller speed, thrust co-efficients, thrust deduction factor, wake fraction and hull efficiency was evaluated. The overall power penalty was evaluated too.

Hull fouling and propeller fouling both increased the propeller speed at the self-propulsion point, but the impact of hull fouling was more severe than that of propeller fouling. For a fouled-propeller case, a higher propeller speed was required to achieve the same thrust. The increase in propeller speed under fouled-hull/clean-propeller, clean-hull/fouled-propeller, and fouled-hull/fouled-propeller conditions at the most severe surface fouling condition were observed to be 8.1%, 2.0%, and 10.1%, respectively.

The increases in required shaft power values obtained from the simulations, under fouled-hull/clean-propeller, clean-hull/fouled-propeller, and fouled-hull/fouled-propeller conditions at the most severe surface fouling condition (B20%) were 57.8%, 19.2%, and 81.8%, respectively.

The results showed that hull fouling increases the total resistance coefficient up to 52% with the most severe surface condition (B20%), whereas the effect of propeller fouling on this parameter is negligible. The propeller rotational speed at the self-propulsion point was observed to increase due to both hull fouling and propeller fouling.

Holistic assessment

Improved hull efficiencies were observed for the fouled-hull cases while the effect of propeller fouling was minor. On the other hand, the behind-hull efficiencies showed decreases due to hull and propeller fouling. The overall propulsive efficiency for fouled-hull/clean-propeller case increased, whereas the other cases showed decreases.

As expected, the clean-hull/fouled-propeller showed a 22.2% decrease in propulsive efficiency at the most severe surface condition. The increase in shaft power for the fouled-hull/fouled-propeller case under the most severe surface condition was 81.8%.

The roughness effect on the flow characteristics was also examined. The velocity fields at the stern showed a strong dependency on hull fouling, whereas the effects of propeller fouling were relatively minor. As expected, fouled-hull cases showed thicker boundary layers on the hull. The effect of propeller fouling on the surface pressure on the hull and rudder was observed to be negligible, while the effect of hull fouling was more marked. Decelerated flow upstream to the propeller and enlarged wake field due to the hull fouling was also observed, which is consistent with the increased wake fractions.

Vorticity downstream to the propeller increased, due to both hull fouling and propeller fouling. This can be seen as a consequence of the increased propeller revolutions of those cases. When it came to the vorticity on the leading edge of the rudders, however, the vorticity magnitudes for the fouled-propeller cases were lower compared to the clean-propeller cases, although they had even higher propeller rotational speeds. This finding can be seen as a positive, since it would reduce cavitation and noise.

Verification and validation
A verification study was carried out to assess the numerical uncertainties of the CFD model and to determine sufficient grid-spacings and time steps. Spatial and temporal convergence studies were performed using the Grid Convergence Index method.

For the validation of the CFD model, four different simulations were conducted: ‘model-scale/with-rudder’; ‘model-scale/without-rudder’; ‘full-scale/with-rudder’; and ‘full-scale/without-rudder’. The model-scale/without-rudder simulation was compared with the experimental result and showed good agreement. The simulation results of the four different cases were compared to examine scale effect and interactions with rudders.

You can see the original paper Penalty of hull and propeller fouling on ship self-propulsion performance here