Possible biases in wind speed measurements from merchant ships

Wind speed measurements obtained from ship-mounted anemometers are biased by the presence of the ship which distorts the airflow to the anemometer. Until recently this bias had only been quantified for a few well-exposed anemometer sites on individual research ships, whereas the magnitude and even the sign of the bias was unknown for anemometers on merchant ships. Three-dimensional numerical simulations of the airflow over a typical tanker/bulk carrier have been performed to quantify the pattern of the airflow above the ship’s bridge. The accuracy of the numerical simulations has been verified by comparison to wind tunnel studies. Typically, the flow is accelerated by up to 18±6 % or decelerated by 100% depending on position. In practice, an anemometer located above the bridge should be mounted as high and as far forwards as possible

Wind tunnel tests on the effect of a ship hull on rudder-propeller performance at different angles of drift

This report presents the experimental results from a series of wind tunnel tests. The tests investigated the influence of various upstream bodies on the performance of a representative ship rudder-propeller combination for different angle of drift. All the tests were carried in the 3.5m x 2.5m low-speed wind tunnel at the University of Southampton and are an extension to basic rudder-propeller tests which have already been carried out and are reported elsewhere. The model rudders tested were all-movable and the model propeller used was based on a Wageningen B4.40. Rudder forces and moments and propeller thrust, torque and revolutions were measured for all conditions tested. A standard series of flow conditions were used. These were free-stream flow (no propeller present) and tests at propeller open-water advance ratio of J of 0.94, 0.51 and 0.36. For these conditions, forces were measured at rudder angles between -40 degrees and +40 degree. In addition, a select sample of tests were carried out for both larger rudder incidence and for tests in the second quadrant and at a low advance ratio J of 0.17.Three types of test were carried out:1) the rudder-propeller combination alone was tested at drift angles of -15 degrees, -7.5 degrees, +7.5 degrees, and +15 degrees;2) at two angles of drift of -15 degrees and +7.5 degrees, three different lengths of centre-board were place upstream of the rudder-propeller combination to simulate the effect of a thin upstream hull on performance;3) at two angles of drift of -15 degrees and -7.5 degrees a representative ship hull form based on the stern of the Mariner class of vessels was mounted upstream of the rudder-propeller combination to assess the influence of hull thickness.Pressures were measured at 200 locations distributed over the rudder surface for the rudder-propeller combination alone and the results are given as both chordwise pressure distribution for eight spanwise locations and as distributions of spanwise sectional load distributions.The results give an essential insight into the behaviour of flow around the stern of a vessel providing rudder force data for use in more realistic manoeuvring simulations and detailed data for the validation of numerical models of the ship rudder-propeller-hull interaction problem

Experimental measurements of hull pressures on fast displacement catamarans during motions in long-crested head-seas

Piezoresistive pressure transducers have been used to measure the pressure variations at six points on a catamaran hull moving in regular, long-crested, head-seas. Preliminary results demonstrate the feasibility of this experimental approach although a more refined experimental procedure may be required to obtain greater accuracy in the pressure measurements.The pressure measurements were carried out for one demihull spacing at two speeds in calm water and also in regular waves of differing wave periods. The mean pressures from both calm water and regular wave tests are presented along with the RMS variation from the mean during the tests in regular waves