Dangerous Phenomena in a seaway

Question 1

Define Synchronous Rolling

Answer 1

• Occurs in beam on seas
• When the ships still water roll period Tᴿ is equal to the apparent wave period Tᴱ

Question 2

Action to avoid/correct Synchronous rolling

Answer 2

Change Tᴱ (period of encounter)
- alter course and or speed

Change Tᴿ (vessel roll period)
- Vertical shift of weights (Changes GM)
- Horizontal shift of weights (winging out weights)
- Extend fins

Question 3

Dangers of Synchronous, Parametric Rolling and loss of stability in a seaway

Answer 3

• Larges angles of heel leading to excess loading cargo securing devices
• Increasing angles of heel leading to capsize
• Heavy weather damage from seas on deck
• Danger to personnel
• Cargo shift
• Loss of propulsion and or steerage

Question 4

Define Parametric Rolling

Answer 4

• Occurs mostly in head on/bow and following seas
  There are two situations it occurs;
  1. Tᴱ=Tᴿ (asymmetric rolling)
  The amplitude of the wave crest is greater then the amplitude the other side.
  2. Tᴱ= 0.5 Tᴿ (Symmetrical rolling)
  Vessel going from tender to stiff. Same explanation as loss of stability in a seaway.

Question 5

Define Surf-riding and broaching

Answer 5

• Occurs in large following or quartering seas
• The ship can accelerate down the wave (surf-riding)
• When surf-riding you loose flow over the rudder ∴ loose steerage ∴ turn sharply ∴ come to large angles of heel on turning

Question 6

Actions to avoid surf-riding and broaching

Answer 6

• alter speed and or course to take vessel out of the surf-riding region

Question 7

What factors effect stability in a seaway

Answer 7

• Changes in the underwater hull geometry
• Crest at midships, trough at ends BM decreases ∴ KM decreases (vessel is the tender)
• Trough at midships, crest at ends BM increases ∴ KM increases (vessel is stiffer)