Nylon is a very promising candidate to replace steel chain moorings for Marine Renewable Energies applications like Wave Energy Converters or Floating Wind Turbine in shallow-water. However, test data for nylon ropes in a wet environment at intermediate scales are lacking in the literature except in a recent study by Sørum et al 2022. This article proposes a new set of experimental data on nylon subropes with a detailed test procedure. This work focuses on the dynamic stiffness of nylon mooring line and its experimental evaluation at realistic orders of mean load, load variation and frequency. We also examine the accumulated strain after successive test procedures. The experimental campaign highlights the stiffness non-linearity with respect to both the mean tension and amplitude. A simple bi-linear model taken from the work of Huntley2016; Pham 2019 is considered here, and is shown to provide a good simulation of the experimental results.