Scour mechanism caused by the external and internal rotating twin-propellers, and a single propeller is investigated using an experimental method. Powertrain system, twin-propeller, tank, sand, and scour measurement were designed and built enabling two counter-rotating jets to cause scour in a water tank that the scour depths are measured using Acoustic Doppler Velocimetry (ADV). Temporal study found that the counter-rotating twin-propeller scour consists of three stages, which are the initial stage, developing stage and equilibrium stage. Two independent scour holes were found in the initial stage within the first 10 minutes. Two holes merged to be a connecting hole to reach the developing stage. The maximum scour depth, and the scour area continued to expand up to 2 hours to reach the established stage. The established stage shows the equilibrium condition without excessive changes in scour depth and area. The experimental results showed that the maximum scour from an external rotating twin-propeller occurred along the rotational axis of each propeller, whereas the internal rotating propeller scour occurred along the central axis in-between two propellers. The jets from external rotating and internal rotating systems have different flow characteristics leading to different scour patterns. Proportional factors were proposed to relate the scour generated by external-rotating twin-propeller, internal-rotating twin-propeller, and a single propeller.
- twin-propeller; propeller jet; scour; internal rotation; external rotation; counter rotating