Testing of hull drag for a sailboat

Transcription

1 Testing of hull drag for a sailboat Final report For Autonomous Sailboat Project In Professor Ruina s Locomotion and Robotics Lab, Cornell Jian Huang jh2524@cornell.edu Mechanical Engineering, MEng student 2016/5/12

2 Table of Contents 1. Background Testing procedures Straight ahead motion Motion with angle of attack Pool test Testing results Straight ahead motion Moving with an angle of attack Pool test Conclusion Reference... 12

3 1. Background The drag of a sailboat hull is an important parameter that can aid hull design process and needs to be taken into consideration when navigating the sailboat. Previous a drag test was conducted by the Autonomous Sailboat Team, the results of which was used in calculating the maximum boat velocity with various attack angles between the sailboat and the wind.[1] In that test, it was found that the overall drag force was approximately as follows: F hull = 2.48 V boat 3 That test was not very detailed, however, because only drag while moving straight ahead without attack angle was measured. Also, the amount of data collected is limited. After the previous test, new hull designs have been completed, and a new hull drag test on them would be beneficial to simulating and navigating them. 2. Testing procedures Before testing the real-sized hull in a swimming pool as the previous test did, several tests were conducted with small models of a sailboat in a bathtub. They are useful for testing whether the planed method for testing will work, and getting a rough impression on how the results of such drag tests might look like Straight ahead motion The basic structure for testing is shown in Figue.1. Figure.1 Basic structure of testing hull drag.

4 The boat is connected to a weight m by a wire, with two pulleys in between. The weight is free to fall, exerting a tension T on the boat. The drag D acts on the boat and increases with the boat speed. Eventually the weight falls and the boat moves at a steady state speed, and the drag equals the tension and the gravity of the weight. By measuring steady state velocity, recording the weight, and repeating with different weights, a relationship between drag force and boat velocity can be made clear. Figure.2 shows the actual testing structure. The pulleys are hung on the sprayer with wires, with additional wires preventing spinning and swinging. Figure.2 Pulleys hung in the air. Figure.3 shows the model boat used. A wire is attached to its head through a hole. Some coins are attached to the other end of the wire as the weights in Figure.1. After the weight is free to fall, the boat starts moving and approach a steady speed before the weight reaches minimum possible height. The motion of the boat is recorded as a video which can be later used to measure the steady state velocity. The time between points A and B on the boat pass a particular point on screen can be measured with any video player, and with the distance between A and B, the velocity can be measured.

5 Figure.3 Model boat used for testing Motion with angle of attack Besides drag when moving straight ahead, the drag force when the motion of the model boat does not align with the body of the boat is measured in another test. The rest of the test structure is the same, except for the way the boat is connected by the wire. Figure.4 shows the modification, where the arrow indicates another wire keeping the motion of the boat in the center of the bathtub. This wire disables the measurement of lift force, but it prevents the boat from swinging in the direction of minimal drag force. The model boat is connected by two wires at both the head and the stern, which are then connected to a single wire. The location of the intersecting point determines the angle that the boat would be moving at. By moving this point and reconnecting the wires, steady state velocities at various angles can be measured.

6 Figure.4 The model boat connected with an angle of attack Pool test Multiple methods including the one used in bathtub tests above were attempted, and the final test method is shown in Figure.5 below. Figure.5 Test method of the pool test. As shown in the figures, a weight is falling through the water, pulling the boat in the process. Thus the problem of fixing the pulley system is avoided, and the test is easy to conduct. Meanwhile, without using the pulleys, the friction between the wire and the metal rod results in an error. The weight is now subject to buoyancy and hydraulic drag, and the computation of these forces can also contain error. The buoyancy force can be calculated after calculating approximate volumes of the weights. The drag force is calculated as such: (1) let the weight fall without pulling the

7 boat; (2) measure the pool depth and the time it takes the weight to fall to the bottom; (3) calculate an average drag coefficient with Gravity Buoyancy = Cv 2 ; (4) after measuring boat speed, calculate the drag force of the weight with C and v. The drag force while moving at an angle was not included in the pool test. 3. Testing results 3.1. Straight ahead motion The main results of the straight ahead case are shown in Figure.5. Drag force with friction measured and removed is plotted against the steady state velocity that the model eventually achieves. Figure.6 Drag force changing with velocity, when the boat is moving straight ahead. While there are noise in the data, it can be seen that the drag force increases exponentially as velocity increases. In order to compare the results to the previous tests, Figure.6 plots the drag force against the cube of steady state velocity. After ignoring the last point with a large drag force and connecting the remaining first and last points with a straight line, the drag force seems to correspond to the previous conclusion that it is proportional to the cube of velocity. The correspondence would require more data to confirm.

8 Figure.7 Drag force plotted against cube of steady state velocity. However, shown in Figure.7, the model boat clearly does not obey the coefficient of 2.48, most possibly simply due to the difference between the model boat and the full sized boat used previously. Figure.8 Comparison of this test and the previous test.

9 3.2. Moving with an angle of attack Figure.10 shows the main results in this test. When the angle of attack increases, the steady state velocity decreases as expected. However, in a bathtub test, there is little room for the model boat to pass the phase of swinging and enter a status of a steady motion at an angle, therefore it is difficult to gain more accuracy or sufficient data points in this test for a detailed relationship between the two. Figure.9 Steady state velocity plotted against the angle of attack Pool test Figure.10 shows the results when there is no additional weight placed on the empty hull, which weighs kg. The hull is made of leaking materials, and the water leaking into the hull is uncontrollable and unmeasurable, and can possibly outweigh the hull itself. Therefore, much error and noise can be noticed in the figure. A general trend that the drag force is larger at higher speed can still be observed.

10 Figure.10 Drag force changing with velocity, without additional weight. Figure.11 shows the results when there is an additional weight of 2 kg placed in the hull. With the hull being heavier, the water is leaking more heavily. An estimation value of the total weight for reference including the weight of the hull is kg. Due to error and noise, the results show no relationship between the drag force and the cube or square of boat velocity. Meanwhile, the general trend is more explicit than the case without additional weight, which means water leaking in caused much of the error in the previous case, and that the additional weight does stabilize the results. Figure.11 Drag force changing with velocity, with additional weight.

11 4. Conclusion The tests on the model boat shows trends as expected, thus validating the methods used to test the drag force. However, the pool test is not as successful, with possible causes being: (1) Performing the method used in bathtub tests failed due to being unable to fix the pulleys tight to the standing rod; (2) Ignoring the pulleys in the new method caused additional friction; (3) The motion of the falling weight is complicated, and approximations of the additional drag and buoyancy are not exact enough. Any future attempts to measure hull drag should take these causes into considerations.

12 Reference [1] Bo Baker, Jesse Miller, et al., 2015, Polar Plot Generation.