Interaction

Question 1

What are the 4 types of interaction?

Answer 1

1. Shallow water effect
2. Squat
3. Bank Effect
4. Interaction between vessels

Question 2

What are the effects of shallow water effect?

Answer 2

1. Reduces speed over the ground
2. Increases squat
3. Reduces steering control
4. Increases the size and shape of wake pattern
5. Alters acceleration/deceleration characteristics

Question 3

What is shallow water effect?

Answer 3

The restricted channel constrains the water and causes an increase in water pressure acting on the ship changing the hydrodynamics acting on the vessel

Question 4

When is shallow water effect felt?

Answer 4

When water depth is less than 1.5 x draft

Full SWE when depth is less than 1.2 x draft

Deep water between 2 and 3 x Draft when SWE are no longer felt

Question 5

What are the recommended UKC?

Answer 5

Open sea = Draft x 0.2
Channels = Draft x 0.15
Sheltered waters = Draft x 0.10
Berth = Draft x 0.07

Question 6

What is the MGN for Interaction?

Answer 6

MGN 199

Question 7

What are the 3 most critical examples of interaction?

Answer 7

1. Two vessels passing one another at very close range
2. Vessels manoeuvring in close proximity for operational reasons
3. Vessels with small UKC which stop rapidly approaching an enclosed basin resulting in unexpected shearing

Question 8

What is the relationship between vessel speed and interaction?

Answer 8

Interaction is relative to the squared of the speed.

(Halve the speed and the interaction is reduced by 75%)

Question 9

What happens when two vessels overtake each other?

Answer 9

When vessels are abeam, the bows push away by the bow pressure area and the sterns swing towards each other

When the bow of the overtaking vessel is abeam the stern of the vessel being overtaken, the bow pressure wave may shear the vessel being overtaken into its path

The smaller of the two vessels will feel the greater effect

Question 10

What happens when two vessels are head on regarding interaction?

Answer 10

The bows will repel towards the bank and will be subject to bank effect

The sterns may shear across each others path

Question 11

What dangerous interaction is felt by harbour tugs?

Answer 11

As the tug parallels the vessel, it has to increase power to push through the bow wave and has to apply helm towards the vessel to counter the bow pressure.

As the tug clears the bow pressure, its speed will rapidly increase and if it doesn’t alter to opposite helm and reduce speed, it will shear across the vessels bow.

The flow of water around the larger vessel acts on the underside of the smaller vessel and reduces its stability making it susceptible to capsize

Question 12

What is the danger of approaching shallow basins with regard to interaction?

Answer 12

A vessel in shallow water drags up to 40% displacement of water astern. When the vessel stops, this water continues ahead and can cause a sudden turning moment or push the vessel ahead.

Question 13

What is squat?

Answer 13

A decrease in under keel clearance when the ship is making way or alongside/anchored in moving water

Question 14

What causes squat?

Answer 14

An increased velocity of water passing under the ship and therefore a reduction in pressure, reducing the UKC

Question 15

What is Bank Effect?

Answer 15

Where negative pressure amidships creates bodily attraction to the bank

OR

Positive bow pressure creates a yawing moment away from the bank and an equal stern motion towards the bank.

Question 16

How is Bank effect counteracted?

Answer 16

By creating an angle to the bank with the bow slightly open so a wedge of water creates a cushion between the ship and bank. Helm towards the bank then counters the bow pressure creating the turning moment.