  High-speed water entry of projectiles and diving systems induces high forces
and jerk to the entering bodies due to the development of large hydrodynamic
pressure. Previous research has shown separately that the peak forces can be
reduced by improving the aerodynamic shape of the head (nose) or, recently, by
introducing a spring element between the head and body. This study seeks to
understand whether the aerodynamic shape or spring stiffness coupling is most
important for force reduction by combining both in one study. The experiment
combines the nose cone aerodynamics and spring stiffness with a rear body and
examines the forces acting on the nose and body. Three parameters are varied:
the nose angle, spring stiffness, and impact velocity. An unsteady
semi-analytical formulation is developed to estimate the water entry forces and
coupled body dynamics. We find that the peak force reduction due to the spring
is highest when the slamming force is most significant, particularly at higher
impact velocities and with blunter nose angles. The spring coupling enables
periodic fluctuations between the kinetic and potential energy throughout the
duration of impact, which can be tuned by varying the stiffness. These findings
can allow engineers to control the dynamic response of water entry.
