Direct Evaluation of Ship Speed Loss and Fuel Consumption in Actual Sea Condition
— Development and application of a self-propulsion test device simulating the main engine characteristic —

　A ship running in waves will run at a lower speed than one running in calm water. There are several causes for this, but the main ones are the increased resistance caused by waves and the decrease in thrust due to the drop in propeller rotation speed caused by the increased load on the ship's engines. If we want to know the ship speed in the expected waves, we generally predict the component of increased resistance due to waves by theoretical calculations. Theoretical studies on the calculation method have been conducted from the past to the present, and it has become possible to predict accurately. However, the calculation method itself is complicated, and it is still difficult to forecast if the sea is in rough conditions. On the other hand, considering the situation where the ship is sailing at the same speed in the tank test, the resistance component in calm water is not directly the same as that of the full-scale ship, as described in “Development of Auxiliary Thrust Device for Model Test”, but the increased resistance component due to waves can be regarded as basically the same as that of the full-scale ship. In other words, in tank tests, if we take into account the effect of changes in engine speed on thrust and make the apparent resistance the same as that of a full-scale ship by use of an auxiliary thrust device, the measured speed reduction of the “model ship” in waves can be treated as the speed reduction of the full-scale ship sailing in the same waves.

　Therefore, NMRI has developed a theory and test method to measure full-scale ship propulsive performance, including reduction of ship speed, by tank model tests. There are three main elements required for the test method (Figure 1 is a schematic diagram.); 1. Use of motor control system for model ships (main engine characteristic self-steering device) to control the propeller rotation speed by reproducing the variation of engine rotation speed against the load fluctuation, 2. An auxiliary thrust device to make the resistance of a model ship in calm water the same as that of a full-scale ship. 3. To apply a correction method for the input values for the control of the main engine characteristic self-propulsion device the same as those of the full-scale ship.

　Using this test method, we conducted speed trials in our Actual Sea Model Basin. A speed test is a test to examine ship speed at a given engine speed and is used as the test for sea trials of actual vessels.

　Figure 2 shows the result of extracting 10 seconds of data acquired while navigating through regular head waves. The green line shows the results of the developed test method, and the red line shows the results of the conventional method, in which the propeller rotation speed is kept constant. From top to bottom, the figures show vessel speed, propeller rotation speed, propeller rotation resistance, propeller thrust, and fuel consumption (normalized to the maximum value of the full-scale ship). First, we grasp the propeller rotation speed fluctuation in response to rotational resistance, reproducing how a real engine changes its rotation speed in response to load fluctuations. In addition, the variation of rotational resistance and thrust by the developed test method is slightly lower than the result of constant rotational speed, which means that data closer to the actual phenomenon can be obtained. It is now possible to grasp the complex behavior of fuel consumption in response to fluctuations in rotational resistance. Thus, the developed test method can measure phenomena that are difficult to evaluate with conventional methods.

　Figure 3 shows the results of our analysis of the average values for the test results within the same wave. The horizontal axis of the figure is the ratio of the regular wave length to the model ship length, and the vertical axis is the ratio of ship speed with and without waves, propeller rotation speed, and fuel consumption. The green dots are the results of the test method we developed. The red dots are the results when the propeller speed is kept constant, the apparent drag is the same as that of a full-scale ship with an auxiliary thrust device. The blue dots are the results when the propeller speed is kept constant without using an auxiliary thrust device. We set the propeller rotation speed so that all colors have the same ship speed when sailing in calm water. Looking at the green results, we see that the ship's speed is much lower than the other results when the value of the abscissa (ratio of wave to ship length) is around 0.9. In the same situation, the fuel consumption takes the maximum value, and the rotational speed is lower than in other situations. This result explains why the ship could not maintain its rotational speed because fuel consumption had already reached its maximum with the increased propeller rotational resistance, and the reduced rotational speed resulted in a significant decrease in ship speed due to the reduced propeller thrust.

　Thus, by adapting a test method that considers engine characteristics, we can obtain more helpful data than conventional methods, and it is now possible to evaluate the speed and fuel consumption of a real ship using only tank model tests. In addition, we plan to apply this test method to develop research on approaches to engine design, safety evaluation of engine operation in consideration of wave conditions, etc.