Abstract: Laminar-turbulence transition in a bow boundary layer is crucial for the performance of the bow sonar of an underwater vehicle. An investigation on the application of the hydrogel metamaterial on natural transition delay is conducted and the underlying mechanism is analyzed. The two-dimensional velocity fields around both rigid and hydrogel SUBOFF models are acquired by a long-distance PIV in a high-speed water tunnel, and drag coefficients are measured as well. Based on the velocity fields, the pressure fluctuations around the SUBOFF models are calculated through the multi-path integration algorithm. Combining the linear instability analysis and PIV measurement, the characteristics of natural transition of the boundary layer over the rigid SUBOFF model are analyzed. The flow-hydrogel coupling is figured out, according to the spectral analysis of local deformation of the hydrogel surface and the velocity above the hydrogel SUBOFF model in a low-speed water tunnel. The intermittency of the instantaneous boundary layer flows are denoted by continuous wavelet transform of the wall-normal velocity components. Eventually, compared with the flow intermittency of the rigid SUBOFF model, the mechanisms of natural transition delay by the hydrogel metamaterial are discussed, under the influence of the flow-hydrogel coupling.