A saddle-shaped spring in the arm is latched, loaded, then released. How fast does the club move, does the water boil behind it, and what does it do to an inanimate test piece?
The science
Spring-latch. Muscle slowly loads a saddle-shaped elastic structure; a latch holds the arm; releasing it converts stored elastic energy E0 into rotation. Modelled as a Hookean torsion spring: E0 = 1/2 k theta0^2, I theta'' = k theta - drag torque. Without drag the tip speed is v = sqrt(2 E0 / m) and the swing takes (pi/2) sqrt(I/k).
Water. Drag F = 1/2 rho Cd A v^2 (Cd = 0.8 assumed) and added mass m_a = 1/2 rho V_club slow the club. Peak acceleration is quoted at release, where the spring is most compressed.
Cavitation. Behind a fast club the pressure can fall to the vapour pressure (2.3 kPa); the cavitation number sigma = (p - p_v) / (1/2 rho v^2) drops below 1 when v > sqrt(2 (p - p_v) / rho) (14 m/s at the surface, higher with depth). The cavity size here uses a simple placeholder scaling, R_max = r_club sqrt(1/sigma - 1); real cavity size depends on the flow details.
Collapse. An empty cavity collapses in t_c = 0.915 R_max sqrt(rho / (p - p_v)) (Rayleigh). The simulation integrates the full Rayleigh-Plesset equation R R'' + 1.5 R'^2 = (p_v - p - 2 S/R - 4 mu R'/R) / rho and compares. The released energy is 4/3 pi R_max^3 (p - p_v). Gas content, shock emission and bubble asymmetry are not modelled.
Contact. A sphere on an elastic plane (Hertz): d = (15 m v^2 / (16 E* sqrt(r)))^(2/5), F = 4/3 E* sqrt(r) d^(3/2), contact time 2.943 d / v, peak pressure 3F / (2 pi a^2). Real targets crack or yield, so the elastic pressure is only an upper bound; the outcome compares it with a compressive strength. Targets are inanimate test pieces; material values and the club modulus (50 GPa) are rounded handbook-order numbers, not measurements of any animal.
Honest limits. One-degree-of-freedom arm, no latch release transient, no club flexure, no muscle loading; defaults are tuned to give a speed of about 20 m/s, the order reported for the fastest strikes, and should be checked against the literature before quoting. Contact uses a sphere of the club diameter.