Wind and its effect
Wind has a significant effect on a ship. It causes heading changes and leeway. Failure to compensate correctly for wind during berthing is a significant cause of berthing accidents. The difficulty in allowing for wind arises from the variable effect that wind can have on a ship because of changes in a ship’s heading and speed.
Wind has special significance in the handling of high-sided vessels such as car carriers. The effect will vary with the relative wind direction and the speed of the ship. Although wind force and direction can be estimated from information obtained from a variety of sources, such as weather forecasts, VTS information, the ship’s own wind instrumentation and personal observation, local conditions can change rapidly and with little warning. Control of a ship can be easily lost during the passage of a squall. There is an obvious need to understand how wind will affect your ship, and how this effect can be difficult to predict. For example, it might appear logical that the effect of wind on a tanker stopped in the water would cause the bow to swing towards the wind. However, experience shows that a tanker stopped in the water will usually lie with the wind forward of the beam rather than fine on the bow.
It is especially difficult to predict the effect of wind on a partially loaded container ship.
The centre of lateral resistance
The force of the wind causes the ship to drift and, by doing so, hydrodynamic forces act on the underwater hull to resist the effect of the wind. The point of influence of these underwater forces is known as the center of lateral resistance (CLR) and is the point on the underwater hull at which the whole hydrodynamic force can be considered to act. Similarly, there is a point of influence of wind (W) which has an important relationship with the CLR. W is likely to alter frequently as it will change in relation to the wind direction and the ship’s heading.
To anticipate the effect wind will have on a ship’s heading, W must be viewed in relation to CLR.
Ship handlers prefer to refer to pivot point (P) rather than CLR when discussing the effects of wind on a ship with headway or sternway. However, a stopped ship does not have a pivot point and for this reason CLR should always be used. In the discussion which follows, CLR is used for a stopped ship and P for a ship with motion.
The point of influence of wind
The point of influence of wind (W) is that point on the ship’s above-water structure upon which the whole force of the wind can be considered an act.
Unlike a ship’s centre of gravity, the point of influence of wind moves depending on the profile of the ship presented to the wind. When a ship’s beam is facing to the wind, W will be fairly close to the mid-length point, slightly aft in the case of ships with aft accommodation and slightly forward if the accommodation is forward.
A ship will always want to settle into a position where the pivot point and point of influence of wind in are in alignment.
Ship stopped – ship with accommodation block aft
On a stopped ship with the wind on her beam, W will be close to the ship’s mid-length. When stopped in the water, the CLR is also at its mid-length. The difference in location between the two points produces a small couple, and the ship will turn with its head towards the wind. As the ship turns, W moves until it is close to the CLR, when the couple reduces to zero. The ship will settle on this heading, usually with the wind slightly forward of the beam.
Ship with headway – ship with accommodation block aft
If a ship has headway, P is forward and the lever between W and P is large. The resultant force will cause the ship’s head to turn to the wind.
Ship with sternway – ship with accommodation block aft
If a ship has sternway, P is aft of W and the ship’s stern will seek the wind. However, and for the majority of ships, the complexity of the aft-end accommodation structure can cause W to move further aft as the ship turns. Eventually, the ship may settle with the wind broad on the quarter rather than the stern.
Force of the wind
Wind force can be estimated by the formula:
F = (V2/18,000) x windage area,
where F is the wind force in tonnes per square metre, V is the wind speed in m/s (metres/second) and windage area is the area of ship exposed to the wind in square metres. Estimate windage area for a beam wind by multiplying length by freeboard and adding the profile area of the accommodation housing. For a head wind, multiply beam by freeboard and add the area of the bridge front. And, as any mariner will know, double the figure obtained for F and order one or more tugs with the nearest bollard pull.
This calculation gives an estimate of the total force of wind on a ship’s side. It will give an indication of the total power that tugs will need in order to overcome this force.
It should be remembered that a ship will always want to settle on a heading where the ship’s pivot point is in alignment with the position of the wind’s point of influence.When navigating on such a course, a ship will show good course-keeping properties. As a result, it is preferable to berth with head to wind with headway and to berth with stern to wind with sternway. In addition, knowledge of the location of W, compared with P, makes it possible to predict whether the ship’s head or stern will ‘go to wind’ as a ship is stopped. The ship will want to settle with P in alignment with and to windward of W.
High-sided ships may suffer more from leeway than from heading change.
Berthing in wind
A ship is most vulnerable when presenting its broadside, the area of greatest windage, to the wind. In strong winds, it may be difficult to counteract the effect without tug assistance or the use of a thruster. If close to a berth, it is essential that mooring lines are set as quickly as possible. Ideally, plan the manoeuvring so as to present the minimum profile to the wind, i.e. head to wind, or at least reduce to a minimum the time the wind is at a broad angle to the ship.
Points to remember:
• Ensure that conditions are safe and suitable for the envisaged manoeuvre. It will be cheaper to delay the ship until the wind moderates than to deal with the aftermath of an accident.
• Wind force acting on a ship increases with the square of the wind speed. Doubling the wind speed gives four times the force. Gusts of wind are dangerous.
• If berthing in high winds, take evasive/corrective action early. Attach tugs early and before they are needed.
• Tugs should be of sufficient strength not only to counteract the effects of wind but to get the ship to the required destination.
• The berthing plan should be devised to minimise the adverse effect of wind and to maximise its assistance.
• A ship is more vulnerable to wind at slow speed. As speed reduces hydrodynamic forces reduce, and the effect of wind on heading and leeway increases.
• Take corrective action as soon as it becomes obvious that it is needed. The earlier that action is taken, the less that needs to be done. The longer things are left, the more drastic will be the action needed to correct the situation.
• Kicks ahead are effective in controlling a ship in windy conditions.
• Consider any special circumstances where wind may affect ship handling. Trim, freeboard and deck cargo can vary the position of W and the force of the wind on the ship, and change the ship’s natural tendency in wind. For example, significant trim by the stern can cause W to move ahead of P. In these circumstances the bow will have increased windage. Consequently, if the ship is heading into wind, the bow may show a tendency to blow downwind, even if the ship has headway.
• Enclosed bridges can lead to a false impression of wind strength, as opposed to open bridge wings where the wind strength will be obvious.
• The windage area, and hence the force of the wind on the ship, will vary with the heading relative to the wind. The maximum force on the ship is when the ship is broadside to the wind.
• Good control is easy to achieve when the ship’s head is to wind and the ship has headway. Control is difficult when wind is following and strong turning forces are created.
• High freeboard ships are more difficult to berth. When berthing high freeboard ships such as car carriers, it is essential to pay extra attention in windy conditions.
• Apply large passing distances when it is windy. Always pass any obstructions well upwind. Gusts and squalls can arrive very rapidly and with little warning. When wind has caused a ship to move rapidly to leeward, it can be difficult to overcome the motion and return to a position of safety.
• Allow plenty of distance from the berth when wind is onshore. If berthing in an onshore wind, it is best practice to stop half a ship’s length from the berth and then come alongside in a controlled manner. An uncontrolled landing on a downwind berth can result in damage to both the ship and the berth.