Directional Stability And Control

It was seen in an earlier chapter that when a ship, at rest in still, water is disturbed in the horizontal plane there are no hydrostatic forces to return it to its original position or to increase the movement. The ship is in neutral equilibrium. When a moving ship is disturbed in yaw it is acted upon by hydrodynamic forces which may be stabilizing or destabilizing. If stabilizing, the ship will take up a new steady line of advance but unless some corrective action is applied, by using the rudder for example, this will not be the original line of advance. The vessel is said to be directionally stable in these conditions but clearly this stability differs from that discussed in considering inclinations from the vertical. A ship is said to be directionally stable if, after being disturbed in yaw, it takes up a new straight line path.

An arrow is an example of a directionally very stable body. If gravity is ignored the flight of an arrow is a straight line. If it is disturbed, say by a gust of wind, causing it to take up an angle of attack relative to its line of motion, the aerodynamic forces on its tail feathers will be much greater than those on the shank. The disturbing force will push the arrow sideways and the moment from the force on the tail will reduce the angle of attack. The arrow will oscillate a little and then settle on a new straight line path. The arrow, like a weathercock, has a high degree of directional stability.

For a ship form it is not clear from looking at the lines whether it will be stable or not. By analogy with the arrow, good stability requires that the resultant hydrodynamic moment following a disturbance should tend to reduce yaw. The disturbing force is said to act at the hull's centre of lateral resistance. For stability this must be aft of the centre of gravity and it is to be expected that a cut away bow, a large skeg aft and trim by the stern would all tend to improve stability. That is about as much as one can deduce from the general hull shape at this stage. A degree of directional stability is desirable otherwise excessive rudder movements will be needed to maintain a straight course. Too much stability makes a ship difficult to turn.

Consider a small disturbing force in the horizontal plane. In general this will be the net effect of external forces (due to wind, say) and ship generated forces (due to propeller forces and rudder movements, say). The fore and aft component of this force will merely cause the ship to slow or speed up a little. The transverse component will lead to a sideways velocity and acceleration, and angular velocity and acceleration in yaw. As the ship responds, additional hydrodynamic forces will be brought into play. When the disturbing force is removed these hydro-dynamic forces will persist for a while. They will either tend to increase the deviations in course already experienced, or decrease them. In these


cases the ship is said to have unstable or stable directional stability, respectively.

Ignoring the fore and aft components a small applied transverse force can be regarded as a transverse force at the centre of gravity and a moment about that point. The point at which it effectively acts is usually termed the centre of lateral pressure. Following a short period of imbalance the ship will settle down with a steady transverse velocity and yaw velocity, at which the hydrodynamic forces induced on the hull balance the applied force and moment. There will be a point along the length of the ship at which an applied force leads only to a transverse velocity with no yaw velocity. That is, the ship's head will remain pointing in the same direction. This point is commonly called the neutral point and is usually about a third of the length from the bow.

If the sideways force is applied aft of the neutral point and to starboard the ship will turn to port. If it is applied forward of the neutral point the ship turns in the direction of the force. The greater the distance the force is from the neutral point the greater the turning moment on the ship. Thus rudders placed aft are more effective than rudders at the bow would be, by a factor of about five for typical hull forms. Aft they can benefit from the propeller race aft as well and are less vulnerable in a collision.

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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  • janne
    What is meant by saying that a vessel hull is directionally unstable?
    1 year ago

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