Numerical Study of Nozzle Geometry and Back Pressure Effects on High-Pressure Waterjet Propulsion Systems

Fathin Muhammad Mahdhudhu (UPN “Veteran” Jakarta, Indonesia)
Adilia Apriliana (UPN “Veteran” Jakarta, Indonesia)
James Julian (UPN “Veteran” Jakarta, Indonesia)
Elvi Armadani (UPN “Veteran” Jakarta, Indonesia)
Riki Hendra Purba (UPN “Veteran” Jakarta, Indonesia)
Shirad Riezky Ilahi (UPN “Veteran” Jakarta, Indonesia)

International Journal of Marine Engineering Innovation and Research
Vol. 11 No. 3 (2026)

The performance of Autonomous Underwater Vehicle (AUV) waterjet propulsion systems is highly dependent on nozzle geometry and environmental back pressure. Therefore, this study aims to analyze the effects of nozzle length, length-to-diameter (l/d) ratio, and back pressure on thrust, velocity, flow coefficient, and hydraulic loss. The study was conducted using Computational Fluid Dynamics (CFD) simulations. Three nozzle geometries (Cos, Conical, and Exponent) were evaluated at lengths of 40, 60, and 90 mm. The optimal geometry was then analyzed by varying the l/d ratio (1, 1/2, 1/3, and 1/4) under back pressure conditions from 0 to 1 MPa. The results indicate that the Cos nozzle with a 90 mm length yields the highest thrust and velocity. Furthermore, an l/d ratio of 1/3 demonstrates the optimal performance, achieving a near-ideal flow coefficient (0.999) and minimal hydraulic loss, particularly under low back pressure conditions. Ultimately, a Cos nozzle with a 90 mm length and an l/d ratio of 1/3 is highly recommended to maximize the propulsion efficiency of AUVs.