03. EFFECT OF WIND
General
The ship handler faces many problems but there is none more frequently experienced and less understood than the effect of wind. All too often when slowing down after a river passage, whilst entering locks and during berthing, it can create a major difficulty. With or without tugs, if the problem has not been thought out in advance, or if it is not understood how the ship will behave in the wind, the operation can get out of control extremely quickly.
Needless to say, with no tug assistance, it is wise to get this area of ship handling right first time and also appreciate what the limits are.
Vessel Stopped
Looking at figure 1 we have a ship on even keel, stopped dead in the water. It has the familiar all aft accommodation and we will assume, at this stage, that the wind is roughly on the beam.
Whilst the large area of superstructure and funnel offer a considerable cross-section to the wind, it is also necessary to take into account the area of freeboard from forward of the bridge to the bow. On a VLCC this could be an area as long as 280 x 10 meters.
The center of effort of the wind (W) is thus acting upon the combination of these two areas and is much further forward than is sometimes expected. This now needs to be compared with the underwater profile of the ship and the position of the pivot point (P).
With the ship initially stopped in the water this was seen to be close to amidships. The center of effort of the wind (W) and the pivot point (P) are thus quite close together and therefore do not create a turning influence upon the ship. Although it will vary slightly from ship to ship, generally speaking, most will lay stopped with the wind just forward or just abaft the beam.
Vessel Making Headway
When the same ship is making headway, the shift of the pivot point upsets the previous balance attained whilst stopped, figure 2. With the wind on the beam, the center of effort of the wind remains where it is but the pivot point moves forward. This creates a substantial turning lever between P and W and, depending on wind strength, the ship will develop a swing of the bow into the wind.
At lower speeds the pivot point shifts even further forward, thereby improving the wind's turning lever and effect. When approaching a berth with the wind upon or abaft the beam that as speed is reduced the effect of the wind gets progressively greater and requires considerable corrective action.
When approaching a berth or a buoy with the wind dead ahead and the ship on an even keel such an approach should be easily controlled. Even at very low speeds the ship is stable and will wish to stay with the wind ahead until stopped.
Vessel Making Sternway
The effect of the wind on a ship making sternway is generally more complex and less predictable. In part this is due to the additional complication of transverse thrust when associated with single screw ships.
Figure 3, we have already seen that with sternway the pivot point moves aft to a position approximately 1/4 L from the stern. Assuming that the centre of effort (W) remains in the same position, with the wind still on the beam, the shift of pivot point (P) has now created a totally different turning lever (WP). This will now cause the stern to swing into the wind.
Some caution is necessary, however, as the turning lever can be quite small and the effect disappointing, particularly on even keel. In such cases, the stern may only partially seek the wind, with the ship making sternway 'flopped' across the wind. This situation is not helped by the center of effort (W) moving aft as the wind comes round onto the quarter. This, in turn, tends to reduce the magnitude of the turning lever WP.
The other complicating factor is transverse thrust. If the wind is on the port beam, there is every likelihood that the transverse thrust and effect of wind will combine and indeed take the stern smartly into the wind. If, however, the wind is on the starboard beam, it can be seen that transverse thrust and effect of wind oppose each other. Which force wins the day is therefore very much dependent upon wind strength versus stern power, unless you know the ship exceptionally well, there may be no guarantee as to which way the stern will swing when backing.
Trim and Headway
So far we have only considered a ship on even keel. A large trim by the stern may change the ship's wind handling characteristics quite substantially.
Figure 4 shows the same ship, but this time in ballast and trimmed by the stern. The increase in freeboard forward has moved W forward and very close to P. With the turning lever thus reduced the ship is not so inclined to run up into the wind with headway, preferring instead to fall off, or lay across the wind. Because the ship is difficult to keep head to wind, some pilots will not accept a ship that has an excessive trim by the stern, particularly with regards SBM operations.
Trim and Sternway
The performance when going astern is also seriously altered. With the wind on the beam and W well forward, the turning lever WP is consequently increased (figure 5). Once the ship is stopped and particularly when going astern, the bow will immediately want to fall off the wind, often with great rapidity while the stern quickly seeks the wind.
When berthing with strong crosswinds, or attempting to stop and hold in a narrow channel, it is best to plan well ahead as such a ship can prove very difficult to hold in position. However, as long as we have some prior knowledge as to how the ship will react to the influence of the wind it can be turned to advantage and readily employed to aid rather than hinder ship handling.
Vessel Head to Wind with Headway
The middle diagram in Figure 6 shows a vessel making Headway through the water, and Heading directly into the Wind. W is now well forward of amidships, and in fact very close to P; the wind is exerting no turning moment, or sideways force, on the vessel. A comparatively small change in relative wind direction (either by alteration of course, or wind fluctuation), will place the wind on the vessel's bow; the whole of one side of the vessel will now be exposed to the wind, and W will move aft as shown in the side diagrams of Figure 6. The following effects will now be experienced:-
a) The Turning Force will now develop a turning moment about P, tending to turn the vessel into the wind again.
b) The Wind Force will also develop a sideways force on the vessel, away from the exposed side.
Head to Wind therefore, the vessel is "course stable", provided that she maintains Headway through the water.
If the ship has a large Trim by the stern W will be further forward, with a reduction, or even loss, of "course stability". This can sometimes result in a rapid and violent loss of control.
Vessel Head to Wind with Sternway.
Consider the situation when our vessel remains Head to Wind, but now starts to make Sternway through the water. W remains forward, whilst P has moved aft, as shown in the middle diagram of figure 7: the wind is exerting no turning moment or sideways force.
A comparatively small change in the relative direction of the wind will move W aft, as shown in the side diagrams of Figure 7: however P remains aft of W. The following effects will now be experienced:-
a) The Wind Force will develop a strong turning moment about P, tending to turn the vessel's bow further away from the wind.
b) The Wind Force will develop a sideways force on the vessel, away from the exposed side.
Head to Wind, as soon as the vessel starts to make Sternway through the water, she loses "course stability" and the bow will pay off away from the wind, sometimes quite rapidly.
If the ship has a large Trim by the stern W may move further forward, perhaps quickly, and the loss of "courses stability" is even more pronounced. This can sometimes result in a rapid and violent loss of control.
Vessel Stern to Wind with Headway
The middle diagram of figure 8 shows a vessel making Headway through the water, and with the Wind directly Astern. P is forward, a long distance from W, which is well aft. A comparatively small change in relative wind direction will move W forwards as shown in the side diagrams of Figure 8: however W is still some distance abaft P. The following effects will now be experienced:-
a) The Wind Force will develop a strong turning moment about P, tending to turn the vessel's Stern further away from the Wind.
b) The Wind Force will develop a sideways force on the vessel, away from the exposed side.
Making Headway with Stern to Wind, the vessel loses "course stability" and is difficult to steer, this effect is greater when there is also a following Sea or Swell.
If the ship has a large Trim by the Stern, W may move further forward, and loss of "course stability" may be generally less pronounced, but still a potential danger.
Vessel Stern to Wind making Sternway
The middle diagram of Figure 9 shows a vessel making Sternway through the water, and with the Wind directly Astern. P has moved aft, fairly close to W, which remains even further aft. A change in relative wind direction will eventually move W forward of P, as shown in the side diagrams of Figure 9, with the following effects:-
a) The Wind Force will develop a turning moment about P, tending to turn the vessel's Stern back into the Wind.
b) The Wind Force will develop a sideways force on the vessel, away from the exposed side.
Making Sternway through the water, with Stern to Wind, the vessel is again "course stable".
If the ship has a large Trim by the Stern W may move further forward, generally improving "course stability"; however with such a Trim, there is always the possibility of an unpredictable loss of control.
Calculations
It is very useful to have a quantitative understanding of the actual force that ship experiences whilst influenced by the wind. This may be of considerable benefit to pilots/masters when endeavouring to estimate the wind limitations of a particular class of ship, establishing the size of tugs.
When confronted by the harbor authorities it is perhaps better, in the interests of professionalism, to be armed with concrete facts rather than simply say "we do not think it can be done". Worse is, to be forced to attempt a movement with unacceptable risks.
Whilst complicated formulae do exist, for calculating the force of wind upon a ship, it would be more practical to have at hand a relatively simple method of achieving a working figure. The first requirement is to obtain the best available
estimation of the area of the ship presented to the wind in square meters. This can be as simple as:
Length overall(m) x max. freeboard(m) = windage area(m2)
An approximate wind force in tonnes per 1,000 m2 can then be calculated using:
If V = Wind Speed (metres / second) = Wind Speed (knots ) ÷2, then Force (tonnes) per 1000m2 = V2 ÷ 18
It should be noted that the wind force varies as the square of the wind speed. Small increases in wind speed can mean large increases in wind strength, especially in stronger winds, when gusting can place an enormous strain on the ship.
Wind force
Wind force depends on- windage, wind velocity (wind pressure), the angle between apparent wind, and heading. Wind pressure is proportional to wind velocity squared.
The Centre of wind pressure depends on the distribution of windage alongside the ship.
Ship in a beam wind
Ship stopped
The wind force is large.
There is no longitudinal component.
The behavior of the ship depends on the center of wind pressure, which could be in front of or behind the point of application of transverse resistance force (pivot point). This point is approximately at midship.
Ship is drifting and turning either way, depending on the relative position of these points.
Ship with headway
Point of application of wind force is behind the pivot point.
Ship has tendency to swing towards the wind line.
Ship with sternway
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Point of application of wind force is in front of the pivot point.
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Ship has tendency to swing out of the wind line.
Wind from bow quarter
Ship with headway
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The point of application of wind force is behind the pivot point.
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The ship has a tendency to swing towards the wind line.
Ship with sternway
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Point of application of wind force is behind the pivot point.
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Ship has tendency to swing towards the wind line.
