This paper reports the results of two model test programs conducted on ship-shape vessels to determine current drag forces and moments on the hulls and to assess thruster and main propulsion efficiencies as a function of current intensity and angle of attack. Tests conducted on a drillship with below and through-hull thrusters and a main propulsion system, and an icebreaker with a through-hull thruster and a main propulsion system reveal that operating a propulsor in a current significantly alters the fluid flow and resulting pressure distribution around the hull. This creates different effective hydrodynamic restoring forces and moments than would be predicted by treating the propulsors purely as thrust producing devices and computing restoring forces and moments using their thrust capability and location. These forces and moments are a function of the propulsor, the current velocity and angle of attack and can affect the vessel's performance and station keeping capabilities. The results of these test programs demonstrate that in order to assess accurately the effectiveness of propulsor devices on a ship-shape body for station keeping in a current, model tests using a full hull model are necessary for those devices aft of amidships. Estimation of bow thruster effectiveness at low speeds in a current appears amenable to mathematical techniques.
Recently there has been increased interest in maneuverability and station keeping capabilities of shipshape vessels. This interest has fueled the need for more information concerning current drag forces and moments and the select ion of thruster or propulsion assist systems. Considerable work has been done in the area of thruster design and the prediction of effective thrust for various types of systems. However, only limited work has been performed on predicting the performance or efficiency of such systems in a current for various current speeds and angles of attack.
Determination of the current forces and moments are required to estimate the average requirements of a thruster system and to determine the resulting response of the vessel in a current field. There appears to be no reliable numerical method currently available for determining hydrodynamic coefficients for this analysis according to a survey of the state of- the-art conducted by the Naval Civil Engineering Laboratory summarized by Palo [1]. The hydrodynamic coefficients are usually determined by physical model tests and used in preliminary static analyses to size the respective propulsive system and in dynamic, nonlinear, time domain simulations to predict the motions of a moored or dynamically assisted vessel.
A catalogue of hydrodynamic coefficients for tankers was developed from physical model tests performed for the Oil Companies International Marine Forum. The results of these tests were compiled into a document titled "Prediction of Wind and Current Loads on VLCC's" [2]. These coefficients are used extensively in the offshore industry and are referred to almost exclusively as a source of semi-empirical data on current hydrodynamic coefficients. Additional work was conducted by Edwards [3] in 1985 concerning the effects of Reynold's number on the determination of these hydrodynamic coefficients.