EFFECT OF WIND & CURRENT ON SHIP HANDLING

pivot  point  is  a  point  on  the  centerline  about  which  theship  turns  when  the  rudder  is  put  over.  The  pivot  point  scribes  theship’s  turning  circle.

A ship of 160 metres It is stopped in the water and two tugs are secured fore and aft. Tugs apply the same bollard pull of say 15 tonnes (t) each. It is to a position 80m fore and aft of the pivot point. Thus two equal turning levers and moments of 80m x 15t

(1200tm) are created resulting in even lateral motion and no rate of turn.

A ship of 160 metres 2 tugs both applying 15 T each ? Turning lever F & A

A ship of 160 metres 2 tugs both applying 15 T each ? Turning lever F & A

Effect of Wind in Ship HandlingCenter of effort of the wind (W) – the center of the area on the wind

force, but there are factors to be taken into account when dealing with wind in ship handling.

These are:1. Ratio of draft / freeboard2. Angle between the ship’s heading and wind direction.

How much will your vessel feel the wind?As a rule of thumb, based on tests with various types of ships, at very

low maneuvering speeds a high-sided ship such as a passenger ship or a containership will feel the wind speed of three times the ship’s speed of at least five times the ship’s speed before being affected to the same degree.

How will the ship react as you reduce speed?Ship’s of most configurations will normally head up into the wind at

increasingly larger angles as the ship loses headway. When finally dead in the water, the ship will usually want to lie beam to the wind. With sternway, the ship will want to back into the wind.

“Air is about 900 times less dense than water and thus by comparison, water is capable of generating forces of enormous magnitude”

(6.2) stopped…. …. No trim ….

WooP

oW

oP

Negligible turning lever

Wind on the port beam

(6.2) making headway…. No trim ….

o oP

WooP

Turning lever

(6.3) making sternway…. …. No trim ….

o o P

Woo P

Turning lever

W

Wind on the port beam

W ooP

oW oP

Negligible turning lever

Wind on the port beam

(6.2) stopped…. With a trim ….

(6.2) making headway…. With a trim ….

W ooP

o W oP

Weak turning lever

Wind on the port beam

W oo

P

o W o P

Strong turning lever

Wind on the port beam

(6.3) making sternway…. With a trim ….

SWINGING TO AN ANCHOR – used when there is not enough space to turn around. It is a technique which uses the force of tide in turning the vessel to stem the tide. It keeps the pivot point well forward even the vessel is almost stopped or dead in the water.

 

This maneuver is open to individual interpretation but should consider:

1.   depth of the water

2.   under keel clearance

3.   strength of the current

4.   type of bottom (soft mud)

5.   type engine power available

6.   size of ship

7.   amount of room available for turn

USEFUL POINTS WHEN SWINGING TO AN ANCHOR

1. Plan to conduct the swing in a direction that favors transverse thrust when going astern, if that is practicable.

2. Endeavor to get the speed down to the minimum for steerage way, when approaching the swinging area.

3. Ensure there is ample space for the stern to swing around in during the turn.

4. Before letting go make certain that the ship is positioned the right way, so that the tide is on the correct quarter to assist the turn.

- at the instant of letting go anchor the speed over the ground should be as low as possible and before the brake is applied , the engines should already be going astern, to ease the weight on the windlass.

DREDGING (Drag) ANCHOR OR ANCHORS – an old technique using anchor or anchors used in berthing in confined areas, often in difficult tidal and wind conditions without the aid of tug boats.

Things to consider prior conducting the dredge (drag) using anchor.1. Local knowledge - the area, room to maneuver - sea bed ( soft mud, free of obstructions such as power lines, pipes etc )2. At least 20% of max. loaded draft for the under keel clearance to avoid damaging the hull.3. Windlass to be able to stand the weight of the anchor and cables. -  min. designed lifting weight for the windlass is the weight of the anchor plus four shackles of the cable hanging dead in the water, or as specified by the classification society.4. Safety parameters - amount of cable in the water should not exceed 1.5 cable x UKC. - 1 shackle = 15 fathoms / 90 ft / 27.43 meters - designed speed of a windlass gypsy in gear is about 30 ft / min, 9.144 meters/minute is approx 3 minutes a shackle. This is equal to a ship speed of 0.3 knots over the ground. - The 1.5 shackles x under keel clearance is a safety factor which guards against speeds in excess of 0.3 knots, the anchor will not dig rather drag (some companies uses 0.5 knots) it prevents the flukes to dig in and hold. - Letting go anchor by brake or walking-out

ADVANTAGES OF DREDGING (DRAG) ANCHOR

1.   Pivot point is brought right in forward between the two windlass.

2.   Improved steering lever.

3.   Control of heading

4.   Control over the speed of approach.

  

WHY DREDGE (DRAG) USING TWO ANCHORS?

- Dredging (drag) a single anchor only achieves 50% of the effect which otherwise gained by dredging two anchors.

- Using two anchors eliminates two major problems in berthing without tugs

a. control of heading

b. control over speed

IMPORTANT POINTS WHEN DREDGING1.    Maintain normal speed of approach prior to letting go or walking-out the anchors.2.    Walk the anchors out as early as possible.3.    Do not let the ship stop too early.4.    Do not let the speed build-up.5.    Do not exceed the 1.5 shackles x depth ratio.6.    Keep the weight on the anchors.7.    Avoid rushing the operation.8.    Slack back the tension once the vessel is in position. HOLDING POWER OF THE ANCHOR ITSELF = about four times its own weight. Thus, a 15 tons anchor will have a holding power of 60 tons load, depending on the nature of the bottom, weight, and shape of the anchor.

Effect of Tide (current) Upon ShiphandlingTidal current can be used to advantage:- Improve slow speed control- Create lateral motion

Working in the Tide:1. with the tide from ahead - stemming the tide – when the vessel is facing the tide, balancing the ship’s headway against the tide and keep the pivot point forward by using low revolutions or short kicks ahead in order to have a positive control of the ship.2. with the tide from astern. - most unsatisfactory situation and one where it is extremely difficult to maintain a positive control.3. working across a tide - can be used to advantage when a lateral motion is intended. a. size of shipb. amount of room available.