Comparative analysis of surf-riding characteristics between waterjet and propeller-propelled ships

Surf-riding, often followed by broaching, is one of the five stability failure modes defined in the IMO's Second Generation Intact Stability (SGIS) framework and poses a significant safety risk to high-speed craft. This study quantifies the influence of the propulsor type—waterjet versus conventional propeller—on the onset probability and underlying mechanisms of surf-riding, providing guidance for the rational selection of propulsion systems during the early design phase.

A single-degree-of-freedom surge equation was coupled with the IMO Level-2 vulnerability assessment. Thrust models for the two types of propulsors were derived from open-water propeller tests and pump head–flow bench data, respectively. Time-domain simulations were conducted in both regular and irregular stern-quartering waves for a 110 m wave-piercing catamaran with a service speed of 24 knots. Identical hull resistance and wave-excitation force formulations were applied to ensure that observed differences could be solely attributable to propulsor mechanics.

At the design speed (Fr≈0.36), the waterjet-propelled variant exceeded the SGIS threshold, whereas the propeller-driven sister ship remained below it. In regular waves (λ/L = 2.1, H/λ= 0.09), the waterjet craft entered sustained surf-riding after approximately 30 s, while the propeller craft exhibited bounded periodic motion. The dominant mechanism is the waterjet's weak thrust–speed gradient: a 20 % speed increase reduces thrust by only 6%, compared with 23% for the propeller, allowing the hull to lock onto the celerity of the overtaking wave.

The intrinsic thrust–velocity characteristic of waterjets reduces surf-riding margins. Designers should either impose operational speed limits or implement active thrust modulation when waterjets are used for high-speed hulls. The methodology, fully consistent with MSC.1/Circ.1627, offers a quantitative tool for evaluating propulsion trade-offs and ensuring regulatory compliance.