Shallow Water Effects

Question 1

How do you know if the ship is in shallow water?

Answer 1

By sight:
- Increased width of the propeller wash.
- Mud stir up.
- Bigger bow waves.
- Bigger stern wake.

By data:
- Echo sounder display.
- Change in draft indicators.
- RPM reduction.
- Speed reduction.

Question 2

Manoeuvrability and ship handling in shallow water.

Answer 2

By ship-handling:
- Sluggish.
- Sudden vibration
- Reduced in motion.
- Reduction in the helm response.
- Increase turning circle
- Increased stopping distance.

Question 3

Block coefficient

Answer 3

Cb = Volume of ship / Volume of rectangular block.

Volume of rectangular block = L x B x d.

Question 4

Squat definition and calculation

Answer 4

Squat is the decrease in UKC of the ship when moving through shallow water, there is low pressure created under the ship because of Bernoulli’s theorem.

The 3 main factors that affect squat are speed, Cb and blockage factor.

Open water:
Squat = Cb Vk2 / 100

Confined water:
Squat = Cb Vk2 / 50

Cb = Block coefficient
Vk = Velocity in kts.

Question 5

Blockage factors

Answer 5

Blockage factor = Immersed cross-section of the ships’ midship / cross-section of water with the canal or river.
b x h / B x H.

The factor is <0.100 in open sea and 0.265 for narrow channel.

Question 6

Influencing factors

Answer 6

• STW
• Cb
• Bllockage factor
• Shallow water or low UKC
• Presence of other ships in a canal or channel within crossing or passing.