FIGURE 3-9. Norman Pin Use.
Sheering may be initiated by an external force or disturbance, such as wind or wave action. Tows with bulbous bows
tend to sheer more than those with "fine" bows. Improperly-rigged bridles can also cause sheering. Yaw, of course, can
lead to sheering. Depending on the tow's inherent maneuvering characteristics, the amount of yaw and sheer may range
from small to substantial. In general, the tow is considered directionally unstable if the sheer angle continues to increase
from swing to swing, despite an absence of the force that initially caused the motion. The following paragraphs discuss
operational factors that, properly used, influence the yawing and sheering of a tow.
3-5.4.1 Trim. Before undertaking the tow, the towed vessel should be trimmed by the stern slightly as described in
Paragraph 3-2.7 3. Trimming by the stern makes the towed vessel less susceptible to yawing.
3-5.4.2 Speed. Yaw of the tow may be increased or decreased with a change in speed; a range of tow speeds may be
attempted in an effort to obtain a desired reduction in yaw.
3-5.4.3 Use of Rudder or Skegs. If the tow is tracking poorly but is steerable, use of rudders can reduce or eliminate
yawing and sheering. However, active use of the rudder increases drag and adds the risk of steering machinery failure at
a permanent rudder angle. Hull damage may cause the tow to take up a permanent sheer angle. In this case, permanent
adjustment of the rudder can significantly improve the tow's behavior.
If excessive yawing on a movable twin-skegged tow occurs, each skeg can be splayed at an outboard angle. Although
the drag will increase, the directional stability should improve Out-