High-Speed Planing Hull for Rough Watev 



Dynamic Course Stability 



Dynamic stability relates to the track of a vessel following a 

 small disturbance in, for example, heading angle when no corrective 

 action is taken (i. e. , controls fixed). A ship is said to be dynamically 

 stable when, having suffered a disturbance from an initial straight 

 path, it tends to take up a new straight path. The vessel may perform 

 diminishing oscillations about the new track. The degree of stability 

 is measured by the magnitude of a stability index which is negative if 

 the vessel is stable and vice versa. 



The course stability may be found from linear differential 

 equations governing the craft's motion. The coefficients in these 

 equations may be found from the forces and moments measured dur- 

 ing steady state turns, as described above. At both 14 and 45 knots, 

 the craft is statically stable, that is to say that when run at a constant 

 yaw angle the yaw moment tends to reduce the yaw angle. Static sta- 

 bility is measured by the coefficient N^- and is positive for static 

 stability. However, the degree of static stability is not enough to im- 

 pair maneuverability. Since it is statically stable, it follows that the 

 craft is also dynamically stable, though oscillatory. These oscilla- 

 tions decay very rapidly, however, being damped to 40 % of the 

 initial disturbance by the time the craft has traveled one boat length. 

 The stability index has been calculated to be 



Speed Stability Index 



14 knots o - - 0. 2 9 



45 knots a - - 0. 22 



Turning Performance 



The straight course tests with the rudders deflected showed 

 that the longitudinal position of the center of pressure coincided with 

 the quarter chord point of the mean rudder chord and the vertical lo- 

 cation coincided with the depth of the mean rudder chord. Thus, the 

 effect of the rudders can be represented by a force acting at the aero- 

 dynamic center of the rudder. The magnitude of the rudder "lift force" 

 was calculated from aerodynamic theory and confirmed by experiment 

 to be represented by a lift curve slope of 0. 0373 per degree. 



The forces acting on the boat when making a steady turn to 

 port are shown in the following sketch. 



441 



