Abstract
Twin propellers without a rudder were studied using a physical model with a fixed clearance distance and three different rotating velocities. Experimental results were compared with results from theoretical expressions developed over the past 50 years for the efflux velocity, axial velocity, and maximum bed velocity. It was found that the efflux velocity equations overestimated the experimental results, whereas the computed axial velocities matched the experimental data reasonably well. However, when maximum bed velocity expressions were compared with experimental results, only one method was found to behave better; overestimation resulted if a quadratic superposition of single jets was used.
Highlights
The marine transportation industry and regular shipping lines have been experiencing significant growth over the last 20 years
The equations currently used to compute future erosion are based both on theoretical equations with unrealistic hypotheses far from reality and on experimental studies that use one propeller as the propulsion system (Mujal-Colilles et al 2016)
The maximum bed velocity is always expressed as a function of the efflux velocity, which is defined in the literature as the velocity in the downstream propeller plane; this velocity is used as the first parameter to analyze the seabed erosion
Summary
The marine transportation industry and regular shipping lines have been experiencing significant growth over the last 20 years. The increments in the ships drafts and the power of engines during the docking and undocking maneuvers can generate serious problems in harbors. The expressions for the efflux velocity are based either on the mass continuity equation or on the momentum balance equation, using experimental coefficients [e.g., Broglia et al (2013); Hamill (1988); Lam et al (2012); Stewart (1992)]. Both expressions of the efflux velocity are only valid for single propellers, and the effects of two propellers are not included in them
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