Comparison of impact forces measured by different instrumentation systems on the CCGS Terry Fox during the Bergy Bit Trials

A comparison of the forces that were measured by different instrumentation systems during a number of ship collisions with bergy bits is presented. Three types of instruments were used to measure ship impact forces: an inertial measurement system called MOTAN, a 5.4 m2 strain gauged area and a 3.5 m2 external Impact Panel. Forces on the Impact Panel were consistently lower than forces on the strain gauged area, for different impacts. The maximum force on the Impact Panel was 1.2 MN, and the highest force on the strain gauged area was 5.1 MN. Forces from MOTAN are calculated using two different approaches; one requires information about where the impact occurred, and the other does not. The agreement in forces from MOTAN and the strain gauged area or Impact Panel depended upon how completely the hull instrumentation captured the loaded area. When the strain gauged area provided good coverage of the loaded area (7 impacts), forces from MOTAN were from 17% lower to 33% higher than forces on the strain gauged area, if the impact location was used to calculate the forces. When the impact location was not used in the calculation, forces from MOTAN were up to 120% higher than forces on the strain gauged area. The four most significant impacts to the Impact Panel, all of which extended beyond the Panel's sensing area, indicated that forces from MOTAN were from 11% lower to 233% higher than forces on the Impact Panel.