Chapter 5 : Effect of the WindChapter 5 : Effect of the Wind

Ch5. Effect of wind

• mostly experienced when proceeding at slow speeds• can create major problems when:

-During river passages-When entering locks-while berthing

• behaviour of the ship in the wind

Ch5. Effect of wind

Ch5. Effect of wind

• Real wind• Relative wind :

a) influences the shipb) With speed ahead: closer to the bow then the real wind

• Windmeter measures the relative wind• Two effects on a ship: a) drift b) turning moment

• Wind is usually not a problem under 4 Beaufort unlessa) On a ship in ballastb) When coming from astern

• Relative wind resistance is biggest for an angle of 25-30°

Ch5. Effect of wind

To find the vector of real wind (Vv):a) Determine Va (relative wind)b) Bring Vn (vector of ship’s speed in opposite direction of ship’s course/ not opposite to ship’s heading) a) Join extremity of Vn to extremity of Va

Ch5. Effect of wind / Center of windage area

• Center of wind pressure (W) depends on the area of the ship exposed• Windage area to be compared with:

a) the underwater profile b) Position of the pivot point

• W is not always at the center of gravity of the windage area : it depends of the angle between wind direction and ship’s heading.

Ch5. Effect of wind / Position of W

• Position of Center of wind pressure depends: a) on the form of the exposed surface b) on the angle of attack

• C1 and C2 are the extreme positions of W for different forms.

Ch5. Effect of wind / Wind components

Ahead wind: • ship’s speed decreases• ship is drifting• turning moment acts on the ship

Wind force F is not parallel to direction of Va

Ch5. Effect of wind / Wind components

Wind from astern:• ship’s speed diminishes• vessel is drifting• turning moment is acting on the vessel

Wind force F is not parallel to direction of Va

Ch5. Effect of wind / Heeling moment

Inclining moment FF’’ is producing an heeling angle

Ch5. Effect of wind Beam wind force in tons bollard pull

Ch5. Effect of wind Beam wind force in tons bollard pull

• Previous diagram applies to wind coming from the Previous diagram applies to wind coming from the beambeam

• Under 10 knots: winds are negligible for ships in Under 10 knots: winds are negligible for ships in ballastballast

• Under 15 knots: winds negligible for all other shipsUnder 15 knots: winds negligible for all other ships• Even a weak following wind may influence the conduct Even a weak following wind may influence the conduct

of a shipof a ship• Loaded ship less influenced by wind due to the bigger Loaded ship less influenced by wind due to the bigger

underwater areaunderwater area• Little ships more influenced than big shipsLittle ships more influenced than big ships• Wind pressure is proportional to square of wind speedWind pressure is proportional to square of wind speed

Ch5. Effect of wind

Ch5. Effect of wind/ Drifting moment

Drifting moment = Wind force x distance in meter between Center of water pressure and wind force point

•The drifting moment turnsthe vessel untill it reachesher drifting heading

•A this moment there is anequilibrium between windforce on the windage areaand water pressure on theunderwater area

Ch5. Effect of wind/ Ship even keel and stopped Beam wind

•« The right funnel aft acts like huge sail »: large area of superstructure + funnel

• area of freeboard from forward of bridge to bow

• Center of wind pressure close to the pivot point.

• Result: ship not turning and stopped with wind on the beam or just abaft the beam.

Ch5. Effect of wind / Headway / Beam wind

Pivot point moves forward: turning lever between P and W and the shipswings to port.When approaching the berth the vessel’s speed decreases and the wind Effect gets greater: it require considerable corrective action.

Ch5. Effect of wind / Sternway / Beam wind

•With sternway is wind effect less predictable and more complex•Pivot point moves aft creating a different turning lever: the bow fall off the wind or with other words the stern seeks the wind.

Ch5. Effect of wind / Vessel stopped &trimmed Beam wind

•The pivot point of a stopped vessel coincides with the center of gravity of the underwater profile: when trimmed by the stern, it moves astern (from A to B)• the windage area is bigger at the bow and the center of wind pressure moves forward Turning moment = wind force x lever W2B (Bow falls off)

Ch5. Effect of wind / Headway & trimmed Vessel Beam wind

When the same vessel gets headway: point B moves forward, ahead of W2 - the turning moment changes from direction and the bow turns into the wind.

Ch5. Effect of wind / Trimmed vessel / HeadwayBeam wind

Trimmed vessel: a) W moves forward and very close to P b) the turning lever is reduced and the vessel has tendency to fall off c) ship difficult to keep head to wind (SBM mooring operations)

Ch5. Effect of wind / Trimmed vessel / SternwayBeam wind

• Performance when backing is seriously altered• The bow wants to fall off rapidly• The stern seeks the wind• When berthing with beam wind: ship difficult to hold in position• Always try to use the wind to your advantage :  »poor man’s tug »

Ch5. Effect of Wind / Water resistance / Headway

Water resistance R has two components:• the drag, opposed to Vn (ship’s course)• the lift, perpendicular to Vn.

With headway, center of drift forward of center of gravity: the vesselturns into the wind.

Ch5. Effect of Wind / Water resistance / Sternway

With sternway: center of water pressure applies behind the center of gravity:The bow is falling off the wind.

Several factors modify the water resistance: angle of heel , speed of the ship,effect of waves and current.

Ch5. Effect of wind / Position of Equilibrium Ship stopped

Position of equilibrium between wind and water forcesFunction of the windage area:

•Passenger ship: wind on beam

•Tanker in ballast: forward of beam

•Loaded tanker: from bow quarter

•Tugboat: wind from stern quarter

No water pressure when vessel isstopped

Ch5. Effect of wind / Position of Equilibrium Ship with headway

•The arrow of the wind represents thereal wind.

•The wind action diminishes as soon as the vessel gets some speed

• Water pressure influences the ship: all ships with headway will come into the wind (between one and two points)as compared with the ships stopped.

•With sternway:a) nearly all ships take aan equilibriumposition with stern windb) Loaded tankers take the wind in thestern quarters

Ch5. Effect of wind / Various forces acting on the ship

Balance of Forces

Rudder(30 )

Wind(30 KTS)

Current(3 KTS)

Propeller(15 KTS)

Ch5. Effect of wind / Behaviour of ship (7000T tanker)

Head wind

• with head wind: no transverse force

• with headway: a) good steering / pivot point near bow b) after course deviation: wind brings ship back on course

• when backing: a) pivot point near stern and bad steering b) after course deviation: windtends to put ship off initial course.

Ch5. Effect of wind / Behaviour of ship (7000T tanker)

Wind in bow quarter (30°)

• for same wind speed ( 25knots): wind force is 15 tons bollard pull compared to 6T for stopped ship

• At rest or at slow speed: wind force swing ship off the wind/ center of windage ahead of pivot point

• At speed: pivot point near bow / wind force brings ship to the wind.

Ch5. Effect of wind / Behaviour of ship (7000T tanker)

Wind in bow quarter (60°)For the same wind speed 25 knots) the wind force amounts nowto 27 tons bollard pull.

Ch5. Effect of wind / Behaviour of ship (7000T tanker)

Beam wind / ship dead in water

•Large wind force (36 tons)

•No longitudinal component

•Ship’s behaviour depends of relative positions of pivot point and centre of windage

Ch5. Effect of wind / Behaviour of ship (7000T tanker)

Beam wind / Speed ahead

•Pivot point shifts forward: ship tends to swing to the wind•Exception: hips with very large windage area in forward part.

Ch5. Effect of wind / Behaviour of ship (7000T tanker)

Ship is backing• The pivot point is near the stern• The wind swings the ship off the wind

Ch5. Behaviour of ship in following wind

7000 tons tanker/ speed: 6kn/ 30 knots wind Ship to be stopped1. Engine stopped / speed 6 knots2. Full astern: transverse thrust* of propeller pushes stern to port3. Wind on Sb quarter: wind + transverse thrust = ship swings to Sb4. Ship comes beam to wind and wind force increases5. Ship stopped / transverse wind force max / ship drifts to port6. Ship gets sternway / pivot point closes stern7. Due to position of pivot point: wind force is bigger than transvers thrust = swing to port8. The more sternway the faster swing to port.

*Transverse thrust could be 10% of applied stern power