**Notation:**

*C* = clearance distance from propeller tip to bed;

*Ct* = thrust coefficient;

*Dh* = propeller hub diameter;

*Dp* = propeller diameter;

*dp* = the distance between twin-propeller;

*d*50 = average sediment grain size;

*F*0 = densimetric Froude number;

*n* = number of revolutions per second/minute;

*N* = blade number

β = blade area ratio;

*t* = time;

<sup>ε</sup>*m, twin* = depth of maximum scour of twin-propeller at time t;

*ZD, twin* = maximum deposition height of twin-propeller at time t;

Ω = experimental coefficient;

Γ = experimental coefficient;

*Xm* = distance to location of maximum scour for single-propeller;

*Xm,twin* = distance to location of maximum scour for twin-propeller;

**Author Contributions:** W.-H.L.'s long term research series in ship propeller jet induced scour with former supervisor G.H.; Y.C. and W.-H.L. contributed to establish 2D scour model to predict twin-propeller scour; Y.C. wrote the manuscript with revisions, recommendations and validations from W.-H.L., T.Z., C.S. and G.H.

**Funding:** This research was funded by the Natural Science Foundation of Tianjin City: 18JCYBJC21900 and The APC was funded by the Natural Science Foundation of Tianjin City: 18JCYBJC21900.

**Acknowledgments:** The authors wish to extend their gratitude to the School of Civil Engineering at Tianjin University for laboratory space; Queen's University Belfast, University of Plymouth, University of Oxford, Dalian University of Technology, University of Malaya (HIR ENG47), Universiti Teknologi Malaysia, and Southern University College for their past support; and professional bodies EI, IEI, IET, BCS, IEM, IEAust, ASCE, ECUK, SCUK, BEM, FEANI, AFEO, MINDS and academy AAET for membership support and available resources.

**Conflicts of Interest:** The authors declare no conflict of interest.
