Wind Tunnel Tests of a Trimaran Hull

Abstract

The U.S. Navy is developing technologies to produce surface warships with higher speeds and varying mission capabilities. An enhanced understanding of the hydrodynamics of high speed multi-hull ships is one of the key technologies. The experimental set-up and very limited sample data reported in this paper were developed for tests of a trimaran hull in a large cross-section, Mach 0.3 capable, closed circuit wind tunnel. The purpose of the paper is principally to describe the technique used to design and execute the test program. The hull model is of the “reflex” type in which a mirror image of the hull below the waterline is used to create a symmetric model representing the ship at zero pitch and yaw and with no waves at the plane of the “free-surface” To evaluate the influence of the hulls, main hull and side hulls, on the frictional and form drag resistance of each other, the model was configured with two longitudinal and three transverse side hull (outrigger) spacings relative to the main hull. The paper discusses the model design, mounting technique and drag and pressure distribution measurements. The wind tunnel model hull geometry is a nominally 1/30 scale model of a preliminary version of the Royal Navy Technology Demonstrator R.V. TRITON (2001- 2005). This geometry was chosen since it closely represented an actual trimaran ship that had been designed and built and for which technical information existed in the unclassified literature. This preliminary hull geometry had been previously tow tank model tested (Gale, et al, 1996) with a main hull waterline length of 6.0 meters. The wind tunnel model had a main hull length of 3.0 meters and side hulls 1.2 meters long at the waterline. Non-dimensional maximum side hull spacing corresponded to the towing tank transverse location. The minimum sidehull transverse spacing was as close to the mainhull at the waterline as felt reasonable. An intermediate spacing was selected midway between the maximum and minimum. The minimum sidehull spacing was actually so small as to negate the roll stability improvement which might lead one to choose a trimaran configuration over a monohull in the first place, but was selected to insure that at least some influence of the hulls on each other could be measured in the current tests. The longitudinal locations of the sidehulls relative to the mainhull matched two of the five non-dimensional locations used in the towing tank tests. A third intermediate longitudinal location was provided for in the wind tunnel model, but was not used due to budget and schedule constraints. The wind tunnel speed was varied from 27.5 to 95.7 m/s or 62 to 215 mph to provide a range of Reynolds numbers on the model and the mounting system. Mounting the model in the wind tunnel test section provided a challenge. The model needed to be mounted in a rigid configuration that could accommodate alignment and utilize the wind tunnel external balance six-component force measuring system (Barlow, et al, 1999, 2009). Supplementary drags of the sidehulls and the sidehull mounting system were measured by a force gage mounted inside the mainhull. Pressures at 91 locations on the mainhull were measured by pressure transducers inside the mainhull. Signals from the internal instrumentation were transferred out of the transom of the model mainhull and into the aft sting mount on the on the wind tunnel external balance system. To support the model forward of the sting mount, faired cables or “flying wires” were utilized from the main hull to the external balance system. Removable duplicate cables and struts were used to allow dummy drag values to be determined for tares to correct for mounting system drags.