On the generation of surfable ship waves in a circular pool

Abstract

Surfing as a sport is growing. Locations with good surfing conditions are limited and therefore crowded. The growing interest for the sport led to a new concept, surf pools. Surf pools mimic good surfing conditions found along the coast. This thesis is about the technical feasibility of a new surf pool concept, creating surfing waves in a circular pool by towing ship-like hulls around. The hulls are towed along the outer wall of the pool and waves generated by these hulls propagate inwards and break on an island in the middle of the pool. The waves break around the island which theoretically leads to endless surf rides. The initial design as suggested by Webber is based on a pool diameter of 200 m and a water depth of 3 m. The objective of his design to generate 2 m high waves for intermediate to expert surfers. Because of the lack of knowledge about ship induced waves in such an enclosed environment, a physical and a numerical model study are carried out. Various hull sizes are towed in a towing tank and the surface elevations are measured and analyzed. From this physical model study it was concluded that the wave pattern behind a hull is the result of a complex interference pattern and is easily disturbed when the velocity beside the hull becomes large. The difficulties of wave generation by a hull therefore lie in the return currents. The magnitude of the return flow is a function of the blockage, the percentage of the cross section of the pool blocked by the hull. In case the blocking and the velocity are too large, water accumulates in front of the hull and the wave field behind the hull is disturbed, the so-called trans-critical regime. Maximum wave height is found when to towing a hull at the onset of this trans-critical regime. The initial wave pool design is evaluated, and it was concluded that the water depth of 3 m is insufficient to generate the desired 2 m waves. Therefore an amended design is presented. Water depth is increased to 7 m and the generation area is enlarged to reduce the blockage to 5%. The large generation area causes a weak return flow and therefore less turbulence in the pool. In such a cross section it is considered technically possible to generate 2 m high surfable waves.