I.S. Code Chapter 2.3 Severe wind and rolling criteria

Question 1

What direction of wind is the criteria specified for?

Answer 1

Beam on wind

Question 2

What is the steady wind lever

Answer 2

lw₁

Question 3

What is ϕ₀?
How is it measured?
What is the criteria?

Answer 3

• ϕ₀= Angle of heel under action of steady wind
• Measured from the first intersection of lw₁ and the GZ curve
• ϕ₀ < 16° or 80% of the angle of deck edge immersion (whichever is less)

Question 4

What is lw₂?
What is the criteria?

Answer 4

• Gust wind heeling lever
• area b shall be equal to or greater then area a

Question 5

What does area a represent?
What are the parameters?
How is measured?

Answer 5

• Represents the force required to the heel the vessel
• Parameters for area an are from ϕ₁ which starts at ϕ₀ and goes 15° backwards (or can be calculated). Then from the end of ϕ₁ directly up to be parallel with lw₂. From the first intersection of lw₂ and the GZ curve all the way down to ϕ₁ is area a.
• Measured using Simpsons rules

Question 6

What is ϕ₁?

Answer 6

Angle of roll to windward due to wave action

Question 7

What is area b?
What are its parameters?
How is it measured?

Answer 7

• Area b is the vessels residual dynamic stability
• Area under GZ curve where lw₂ acts as the x-axis and measured up to the angle of down-flooding, 50° or angle of second intercept between lw₂ and GZ curve
• Measured using Simpsons rules

Question 8

WHL Lw₁= P x A x Z / 1000 x g x △
Define each value in this equation

Answer 8

P = wind pressure (always 504Pa ≈ force 10)
A = Lateral windage area (m²)
Z = a distance from the centre of A to half the draft
1000 = 1000kg to convert from kg to Tonnes
g = gravitational acceleration (always 9.81m/s²
△ = displacement

Question 9

How do you calculate lw₂?

Answer 9

• 1.5 x lw₁
• drawn on GZ curve 50% above lw₁ not 150% above.