Objectives  The deep-sea Crawling-Swimming Vehicle(CSV)is a new type of underwater robot, of which the multi-legged pose will change the distribution of the surrounding flow field and its center of gravity.

  Methods  Based on the structural characteristics of the CSV, i.e. left-right symmetry, front-back approximate symmetry and up-down asymmetry, the maneuvering vertical plane motion equation of CSV was established and the corresponding stability criteria and critical speed were obtained. The hybrid mesh was used to compute the hydrodynamic coefficients of CSVs' pure heaving motion and pure pitching motion, and the results of computation were compared with the experimental results. According to the stability criterion, the static stability and dynamic stability of CSVs in three poses including the longitudinal expansion pose, lateral expansion pose and landing pose were judged, and main influence factors of motion stability were analyzed.

  Results  The results show that CSVs in all three poses are in the state of static instability and relative dynamic stability. The design speed is lower than the critical speed, which can satisfy the requirement of linear stability. The static stability mainly depends on the position derivative related to vertical force. The static stability in lateral expansion pose is the best, while that in landing pose is the worst. The dynamic stability mainly depends on the position derivative related to vertical force, the initial height of stability center and the structure layout. The dynamic stability in lateral expansion pose is the worst, while that in landing pose is the best.

  Conclusions  The hydrodynamic and stability laws of CSVs' multi-legged pose can better guide the design of control system and make it run safely.