International Shipbuilding Progress

While sailing in head or bow quartering seas operators on board of small planing boats try to avoid unacceptably large vertical peak accelerations, the main limiting factor for operability, by temporary speed reductions. The operators observe the incoming wave, roughly estimate whether or not the next impact might be too severe, and if so, they choose a certain amount of thrust reduction. Results of full scale trials suggested that an increase of operability may be realised using this so-called thrust control.

In this paper the concept of smart control for small planing monohulls sailing in head seas has been introduced. The idea of smart control is that a solution for the increase of the operability of small planing monohulls may be found by using automated proactive control of the thrust. The main purpose of smart control is to keep the vertical accelerations below a predefined threshold value while striving at the highest possible average forward speed during a trip. Three elements are essential for such a control system: a shipboard wave measurement system, a computational model that predicts the response and a stable control system for the thrust.

A planing monohull sailing in head seas experiences large vertical accelerations, which are considered to be the main limiting factor for operability. Results of full scale trials (Part 1) showed that the operability of a small planing monohull sailing in head seas may be increased when the helmsman actively controls the thrust.

In the present paper a conceptual model of automated proactive thrust control, or smart control, is presented. The response of a small planing monohull sailing in head seas has been mimicked using an elementary model. The essential hydromechanic aspects are incorporated. A control system for the thrust has been included. The response will be predicted continuously and if necessary, an appropriate amount of thrust reduction will be applied. By temporary speed reductions the magnitude of the next occurring vertical peak acceleration is reduced.

Results generated using this conceptual model show that the operability may be increased using automated proactive thrust control if sufficient time to decelerate to a sufficiently low speed before impact is available. This is provided that the vertical peak accelerations are estimated accurately.

This paper investigates experimentally and numerically the effect on the propeller shaft loads of dynamically changing propeller speed and azimuth angle of thruster propellers. Model tests with a six-component shaft dynamometer were performed in the large towing tank at the Marine Technology Centre. A blade element momentum theory (BEMT) model is applied for numerical analysis.

It is found that the side forces and bending moments on the propeller in strongly oblique inflow are quite sensitive to dynamically changing azimuth angle; for instance are the maximum values of vertical side force and horizontal bending moments effectively doubled when the azimuth angle is changed dynamically. On the other hand, dynamically changing the azimuth angle is less important for the thrust and torque of the propeller. For ships, the effect of dynamically changing the propeller speed can be ignored and the quasi-static values used.

This paper presents results and analyses of an experimental study into the effects of static and dynamic azimuthing conditions on the propulsive characteristics of a puller podded unit in open water. The model propulsor was instrumented to measure thrust and torque of the propeller, three orthogonal forces and moments on the unit, rotational speed of the propeller, azimuthing angle and azimuthing rate. The model was first tested over a range of advance coefficients at various static azimuthing angles in the range of −180° to 180°. These tests were followed by tests in which the azimuthing angle was varied dynamically at certain azimuthing rate and propeller rotational speed. A comparative study of the performance coefficients at static and dynamic azimuthing conditions in the range of −180° to 180° is presented. The performance coefficients of the propeller and the pod unit showed a strong dependence on the propeller loading and azimuthing angle. The coefficients in static azimuthing conditions fit well with a 10th order polynomial fit of the data obtained in the dynamic azimuthing condition in the corresponding azimuthing angles and advance coefficient. An uncertainty analysis of the measurements is also presented.

A rudder force model is being developed at Naval Surface Warfare Center, Carderock Division (NSWCCD) for use with real time ship maneuvering and seakeeping tools. The goal of this work is to obtain reliable results with acceptable computation time. The rudder of a navy ship is typically designed with a portion of rudder area located outside the propeller slipstream and a portion in the slipstream. The inflow velocities entering the rudder plane vary significantly from the rudder tip to the rudder root due to the ship hull boundary layer. Additionally, with flow acceleration induced by the propeller loading, the portion of the rudder in the slipstream will experience much higher inflow velocities than the portion of rudder outside the slipstream. In this effort, the calculations are performed separately for several spanwise rudder segments, where the segments correspond to the portions of the rudder outside the propeller slipstream and in the upper and lower halves of the propeller slipstream. A method to compute effective inflow velocities entering the rudder plane at each segment is presented in this paper. A separate paper will describe how at each segment, the rudder forces are computed from effective inflow velocity, effective lift curve slope, and effective rudder angles encountered by the rudder. A generalized actuator disk model is applied to obtain the slipstream velocity distributions with a realistic propeller loading. An induction method is adopted to obtain the shape of the contracted slipstream. The theory is extended to calculate the tangential velocity in the slipstream. The present theory is validated against a set of velocity measurements that were conducted in The NSWCCD Large Cavitation Channel.