Improvement of Seaworthiness of Fast Catamaran by Hydrofoils Support

Abstract

The multihull vessels provide better stability, seakeeping and performance conditions than mono-hull crafts, this is one of the reasons whereby during the last 33 years the fast catamarans, able to reach 30 to 50 knots or more, have been used widely in important naval applications such as short- middle distance passenger fast ferry transport and offshore operational support, like oil platform and wind turbine farms.

Even with their good ones operational conditions the fast catamarans have limitations, as insufficient seakeeping qualities in rough seas, specially at high accelerations in head and oblique waves, mostly due to the slender hull shape that they use. The fast ferry transport and the offshore operational support normally take place, where the sea conditions are unsteady, to navigate through the irregular and multidirectional waves implies, among others, the increasing of the slamming force effects, higher amplitudes in the heave and pitch movements of the ship, major vertical accelerations at LCG and bow, vibrations and wave added resistance also.

The above situations produce uncomfortable rides, longer duration of the travels because low velocity operations, higher fuel consumption, lower safety levels, more engine power request, drag rising and electrical device damages, which increases the costs. Go as faster, economical and comfortable as possible always have been an imperative factor for passenger ferry transport and offshore operational support activities, for those reasons multiple options have been tested along the years in order to overcome the difficulties, already expressed, of sailing across adverse sea states.

The aim of this work is to show the benefits that the hydrofoils have to damp the longitudinal oscillation in rough seas, increase performance to the decrease the frictional drag in calm water and how it can be worth economically for catamarans supporting vessels. The foils exert a force in the hull upwards, moving it up, this event produce a vertical acceleration reduction due to the lower amplitude motion response reached at heave and pitch and the minor wetted area exposed to have the hull in a higher position from the undisturbed free surface level.

A non linear time domain mathematical model have been implemented, using C++ programming language, the model is based on the Zarnick´s method to predict vertical motions of a planing hull in regular and head waves, it was extended to incorporate the influence of the hydrofoils in the movements in order to perform comparisons among a single fast hull and hydrofoil configuration.

As it was predicted the results were satisfactory making evident the advantages that the hydrofoils have to improve seakeeping, worth noting that at higher wave amplitudes and wavelengths a fixed hydrofoil arrangement has a trend to follow the wave movement, therefore a lift control device is required to achieve the longitudinal damp expected.

From economical point of view the hydrofoil allows the ship navigate more time under rough seas, enhancing the rides comfort, saving money, now an analysis between the amount saved and the cost of installing, operation and maintenance takes place.