As a ship's trim by the stern increases, she becomes more stable and her tactical diameter increases. The change is minor and of no practical significance to the shiphandler as long as the drag remains within limits. There is a marked increase in the diameter of the ship's turning circle as the bow comes out of the water.
From the shiphandler's point of view, assuming there isn't an excessively strong wind on the higher bow, a ship usually steers better as drag increases.
The steering characteristics of a ship on an even keel vary depending on the ship's hull form. A ship with a large block coefficient steers poorly, tending to be directionally unstable. This condition is amplified if the ship trims by the head as she enters shallow water. A ship with finer lines may be direction ally stable, or have neutral stability when on an even keel. The behavior of a ship with moderate block coefficient can only be determined by trials since there is insufficient data available at present to allow an accurate prediction of her steering characteristics.
A ship trimmed by the head is directionally unstable for almost all hull forms. When in this condition the ship requires large amounts of rudder for excessive periods of time to check her swing. The vessel becomes cranky and difficult to handle. If she trims by the head in shallow water the problem is compounded.
Why does a ship behave in this manner when the difference between the forward and after drafts decreases? One must look at the immersed sections of the ship to better understand this phenomenon, and especially at the location ofthe sections having the maximum submerged area.
The ship is turning as a result of couples formed at the rudder and through the center of gravity. One of the forces forming the couple is the imbalance of pressures about the submerged portions of the hull. As the ship begins turning, there is a new increase in pressure below the waterline on the bow away from the center of gravity, the outward bow. This imbalance of forces in that location, forward and outside of the center of gravity, causes the ship to be directionally unstable.
All ships experience the same imbalance in the initial stages of a turn. If a ship is trimmed by the stern though, the pressures shift further aft along the side of the hull as the ship stabilizes in the turn, while the corresponding pressure drop on the quarter on the inboard side of the turn continues to increase. The resultant couple has then shifted aft of the center of gravity so the ship becomes stable. Ships trimmed by the head experience a larger initial positive pressure at the bow due to the increase in submerged area forward, while the negative pressure is reduced at the quarter due to the reduction in submerged area. The couple remains ahead of the center of gravity throughout the turn and the ship continues to be directionally unstable. This condition is indicated to the shiphandler by the shift forward of the apparent pivot point of the ship, so the ship seems to pivot about a point nearer the bow than normally expected, and by the ship wanting to continue to swing after the rudder is placed amidships.
With this in mind, what should you do to safely handle a potentially directionally unstable ship? First, she must not be trimmed by the head. Keep sufficient drag to assure that the ship maintains positive directional stability, allowable draft permitting. Second, on a ship with marginal directional stability the rudder will have to be used for a longer period of time to start the vessel swinging, after which large amounts of rudder are needed for longer periods of time than normally expected to check a swing. The rudder is put back amidships as soon as the swing begins since the rate of turn will increase even when the rudder is amidships. If the rudder is kept on too long the ship will get away and it might not be possible to check her swing in time to avoid leaving the channel.
Don't overlook of having a trained helmsman who is experienced in steering your particular ship. A directionally unstable ship requires special treatment and the helmsman's experience can be valuable.
