SHIP HANDLING & INTERACTION Flashcards
What is interaction?
Interaction refers to hydrodynamic interaction between ships. As the vessel moves through the water, pressure zones are formed. In the case of a vessel moving ahead, at the bow and stern a high pressure zone is created, along the sides of the vessel a low pressure zone is created. High flow of water causes low pressure.
The following are examples of interaction: Vessels which are attempting to pass one another at very close range, vessels manoeuvring at close quarters for operational reasons, effects in a narrow channel (Squat).
General precautions: Moderate speed. Keep distance.
MGN199 highlights dangers of hydrodynamic interaction between ships.
Describe how interaction would affect 2 vessels approaching head on in a narrow channel. How to prevent it?
As the vessels approach one another. The positive pressures at the bows meet and causes the bows to repel. This causes the vessels to move past eachother at an angle. This could cause a cushioning effect on the bank of the narrow channel. As the vessels pass each other’s beam, the low pressure down the side of the ship draws the vessels toward eachother. There is the danger of the sterns touching.
To avoid it. Moderate our speed. But not too low that there is no steerage. Put helm towards the other vessel. To counteract the repelling of the bow, and to keep the stern away from the other vessel
Describe how interaction would affect an overtaking situation. How to prevent it?
As the bow of the overtaking vessel approaches the stern of the vessel being overtaken, the positive pressure on the bow of the overtaking vessel meets the positive pressure of the stern of the vessel being overtaken. This can cause the stern of the vessel being overtaken to sheer towards the overtaking vessel and cause the bow to come across. To prevent this, the vessel being overtaken must apply helm away from the overtaking vessel.
As the bow of the overtaking vessel reaches level with the midships of the vessel being overtaken, the bow can get drawn in to the low pressure at the midships of the vessel being overtaken. To prevent this, the overtaking vessel must apply some helm away from the vessel being overtaken.
As the stern of the overtaking vessel reaches level with the bow of the vessel being overtaken. The positive pressure of the bow of the vessel being overtaken and the positive pressure of the stern of the overtaking vessel can cause the bow and stern to repel, and therefore can cause the overtaking vessel to veer in front of the other vessel. To prevent this, the overtaking vessel must apply helm away from the vessel being overtaken.
What is cushioning effect?
The vessel approaches a bank at an oblique angle. The positive pressure at the bow of the vessel repels the bow from the bank and assists the vessel in turning. It is unpredictable and should not be relied upon.
What is Bank effect?
TBD
Tug interaction
When a tug approaches from the stern, it will approach directly from astern.
The danger is when the tug is approaching the bow of the other vessel. The positive pressure of the ships bow will interact the positive pressure at the stern of the tug. This can cause the stern of the tug to be repelled and cause the tug to sheer in front of the ship. To prevent this, the speed must be reduced (6-7 Knots).
If there is wind, the tug will approach from the leeward side. Less chances of being set down towards the path of the ship. More range on the heaving line of the ship´s crew.
How would you organise a towing procedure?
• Establish communications with tug
• Communicate SWL of equipment to the tug
• Carry out an assessment and establish procedures.
• Setup equipment as per the Emergency Towing Plan
• Carry out Risk assessment and toolbox talk with personnel involved.
• Tug will slowly increase RPM to take up slack in the lines.
Lights and shapes
What is an emergency towing booklet? What is contained in it?
A document required onboard. supporting the crew in establishing the safest and most efficient course of action to be taken when confronted with an emergency that requires towing
• Ship-specific data
• Emergency towing procedure
• a decision matrix that summarizes options under various emergency scenarios
• organization of deck crew and tasks
• diagrams for assembling and rigging bridles, tow lines, etc., showing possible emergency towing arrangements for both fore and aft
• a communications plan for contacting the salvage/towing ship
• inventory and location of equipment on board that can be used during an emergency towing situation
What is squat? What are the signs of squat?
Squat is a Reduction of the under keel clearance caused by increased flow of water between the hull and the seabed resulting in a low pressure which causes the vessel to sink. Squat has greatest effect when the depth of water is less than twice the draft. The amount of squat is directly proportional to the vessel´s speed. To prevent squat, reduce speed.
Signs of squat
- increased wake. (Increased bow and stern wave).
- Reduction of UKC.
- Steering difficulties due to air pockets and disturbed flow of water across the rudder
- Reduction of speed caused by the build up of water in front of the ship
- Vibration. Caused by the cavitation.
Ref: MGN199
What are the main principles of ship handling?
Know your limitations. If the wind and tide is unfavourable. Wait until the conditions change until it is safe to manoeuvre.
Manoeuvre a ship at no faster than walking pace. (2-3 Knots)
Make use of transverse thrust and pivot point.
Explain transverse thrust
Transverse thrust (otherwise known as prop walk or paddlewheel effect) is the tendency of the vessel´s stern to walk in the direction that the propeller is turning when the vessel is going astern. A right-handed propeller, when engaged in astern propulsion, will turn anti clockwise. The downward sweep of the propeller blades passes through deeper water of higher pressure than the top sweep of the blades at the top of the propeller. Causes wash to be thrown out of the side of the ship. Causes the stern to walk to port and the bow to walk to starboard. A right-handed propeller, when viewed from astern, turns clockwise when the vessel is going ahead. They are the most common on single screw ships.
Explain the pivot point.
The point around which the vessel is considered to rotate. When the vessel is stationary, the pivot point is at midships. When the vessel starts moving ahead, the pivot point moves to the bow. Once the vessel has increased speed, the pivot point moves to 1 third of the length of the ship from the bow. If going astern, the pivot point initially will be at the stern, then once there is speed going astern, the pivot points move to 1 quarter forward of the stern. The pivot point affects the lever.
How would you turn short around on a right hand, single screw propeller.
- Keep the pivot point at the forward part, use the transverse thrust and lever.
- Check your quarters that clear of traffic. Check oncoming traffic.
- Favour the portside of the channel, be aware to not get too close.
- Approach as slow a speed as possible. Hard to SB and dead slow ahead.
- Once the vessel starts gaining headway. Stop engine. Put the rudder amidships and engage dead slow astern.
- Once the vessel starts going astern. Stop engine. Hard to starboard. Dead slow ahead.
- Carry out this manoeuvre as much as possible until the vessel is head to the correct direction.
Describe how you would manoeuvre the vessel in order to come PS alongside the quay?
- Head to tide
- Approach at Safe speed
- Stop main engine to reduce vessels headway.
- Approach at 30 degrees to the dock
- When the vessel is approximately 2 ship lengths from the berth, put the engine in astern. (Dead slow astern)
- The transverse thrust will bring the stern to port. The pivot point will be 1/3 from the bow.
- The vessel will come parallel to the dock.
- ## Cast lines ashore. AFT spring first.
Describe your process for letting go the ship when alongside. Tide coming from ahead.
- Pre-departure checks completed
- Carry out a risk assessment. Conduct a briefing/toolbox talk (Highlight the weight on the spring)
- Setup additional fenders on the PS Quarter. Double up AFT spring
- Check clear of traffic in the channel
- Let go head and stern lines. Leaving just the springs.
- Weight should be on the AFT spring. Let go the forward spring. Engine dead slow astern
- The transverse thrust combined with the AFT spring will cause the bow to come off the dock
- The tidal flow will cause a wedge of water to assist the vessel to lift away from the quay.
- Once at 20-30 degrees to the quay. Stop the engine. Let go the AFT spring
- Put the engine dead slow ahead.
- If the Port quarter is close to the quayside, apply some port helm.
