The Mystery of Sinkage and Trim for High Performance Craft

Abstract

For more than a century, displacement ships have been able to exceed “hull speed,” the natural speed of a wave the same length as the ship, or a Froude number based on length of 0.40. That ability evolved in a time frame approximately corresponding to the American Civil War or slightly afterwards, and featured changes in hull form to reduce the pressure changes that trigger a ship’s bow wave, as well as a general elongation of the hull form to reduce the curves flow must follow around the hull. Both these changes reduced the Bernoulli pressure differentials that trigger a ship’s wave resistance and form drag. These same pressure differentials tend to suck the hull down into the water as it approaches hull speed. Hull form features can affect the running trim angle at these speeds in several ways as well. One might suppose that sinkage and trim, therefore, would be somewhat correlated with ship resistance, and the slender hull forms developed for low resistance would therefore sink and trim less, or at least differently. However, this turns out to be incorrect. Because the forces of resistance on the one hand, and sinkage and trim on the other, are directed at right angles to each other, they are almost independent. This paper illustrates their relationships through analysis of model tests of a variety of different hulls.