About the most common cause of segmentation failures is if there are dark wave shadows in the background. If the segmentation of the tail from the image does not work very well you can manually assist the process by specifying the positions of a few key points. Click on the manual tab, then select the tail tips, the trailing edge centre of the tail, and the points at which the sides of the tail contact the water line.

If you make a mistake click on the clear button to remove all the points. You can also make small adjustments by clicking close to existing points. Once all points have been added click on the update button to try detecting the tail again.