Objective Surf-riding followed by broaching is one of the five stability failure modes in IMO's Second Generation Intact Stability （SGIS） framework and represents a critical safety threat to high-speed craft. This paper quantifies how the propulsor type—waterjet versus conventional propeller—alters the onset probability and underlying mechanics of surf-riding, thereby supporting rational selection of propulsion systems during early design.

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

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

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