The effect of tides and vessel parameters of fishing boat generated waves

Abstract

Ship generated waves (SGW) can cause bank erosion as well as disturbance to moored boats especially in the river at estuary due to the restricted area, very high boat traffic and shallow water. In this river, the dominant environmental effect is tidal, but unfortunately in-depth investigation of this effect has not been conducted before. Tidal effect may present the critical issues that are closely related to the different water depth, as well as current speed and direction of tide flow due to the boats movement at flooding and ebbing condition of the river. The aim of this research was to determine characteristic of SGW in the river that have tidal condition, also to obtain allowable speed of fishing boat while sailing in the river. Results of a full-scale field experimental work during rising and ebbing tides in the Mersing river are presented. The displacement hull fishing boat has dimension 14.05 x 4.35 x 1 m (length x breadth x draft) and the planing hull fishing boat has dimension 6.5 x 1.46 x 0.8 m (length x breadth x draft) and both were run at 6, 9 and 12 knots. Wave heights and wave spectral energy from SGW of the two fishing boats were analysed and the energy obtained are compared to one of the accepted river criteria known as Brisbane River Criteria. The effect of assessing the SGW beside the boats at different distances from the sailing line (SL) was also studied using Computational Fluid Dynamic (CFD) simulations. The results show that higher energy was created at the shallowest water depth (h=3.6 m). As SGW is propagated from 5 m to 13 m off SL with 12 knots, the wave energy density was reduced by 1687.6 kg/s2 (38 %) for the displacement hull fishing boat and 29.75 kg/s2 (83 %) for the planing hull fishing boat. During the field experiment, it was found that the SGW has the highest energy when the boat and tide condition are in opposite directions. The findings also indicate that in order to minimize river bank damage, the displacement hull fishing boat should not exceed a limiting speed of 4.8 knots during the flooding, slack and ebbing conditions. However, for planing hull fishing boat, the allowable speed is less than 9 knots. The findings can be useful for the fishing boats operators and local authorities to regulate fishing boat traffic in rivers.