There is an error in Jacobsen et al. (Reference Jacobsen, Bakker, Uijttewaal and Uittenbogaard2019), equation (3.2). The correct form is

$$\begin{eqnarray}\displaystyle \frac{\unicode[STIX]{x2202}^{4}x_{s,\ast }}{\unicode[STIX]{x2202}z_{\ast }^{4}} & = & \displaystyle \frac{\unicode[STIX]{x03C0}^{2}}{2}C_{M}\frac{CaL}{KC}\left(\frac{\unicode[STIX]{x2202}u_{\ast }}{\unicode[STIX]{x2202}t_{\ast }}-\frac{\unicode[STIX]{x2202}^{2}x_{s,\ast }}{\unicode[STIX]{x2202}t_{\ast }^{2}}\right)+2\unicode[STIX]{x03C0}\frac{\unicode[STIX]{x1D6FF}_{x}}{\unicode[STIX]{x2202}_{y}}\frac{CaL}{KC}\left(\frac{\unicode[STIX]{x2202}u_{\ast }}{\unicode[STIX]{x2202}t_{\ast }}-\frac{\unicode[STIX]{x1D70C}_{s}}{\unicode[STIX]{x1D70C}}\frac{\unicode[STIX]{x2202}^{2}x_{s,\ast }}{\unicode[STIX]{x2202}t_{\ast }^{2}}\right)\nonumber\\ \displaystyle & & \displaystyle +\,\frac{1}{2}C_{D}CaL\left|u_{\ast }-\frac{\unicode[STIX]{x2202}x_{s,\ast }}{\unicode[STIX]{x2202}t_{\ast }}\right|\left(u_{\ast }-\frac{\unicode[STIX]{x2202}x_{s,\ast }}{\unicode[STIX]{x2202}t_{\ast }}\right).\nonumber\end{eqnarray}$$

Here, $x_{s,\ast }$ is the non-dimensional horizontal displacement of the flexible stem, $z_{\ast }$ is the non-dimensional vertical coordinate, $C_{M}$ is the inertia (added mass) coefficient, $CaL/KC$ is the inertia-to-stiffness ratio, $u_{\ast }$ is the non-dimensional horizontal fluid velocity, $t_{\ast }$ is the non-dimensional time, $\unicode[STIX]{x1D6FF}_{x}$ is the thickness of the stem, $\unicode[STIX]{x1D6FF}_{y}$ is the width of the stem, $\unicode[STIX]{x1D70C}_{s}$ is the density of the stem, $\unicode[STIX]{x1D70C}$ is the density of the water, $C_{D}$ is the drag coefficient, and $CaL$ is the drag-to-stiffness ratio.