The weather criterion (IS Code 2.3) at a glance
1The lesson in plain words
A beam wind heels the ship, waves roll her to windward, then a gust strikes. The weather criterion checks she has energy to spare.
The scenario
Part A 2.3 pictures your ship beam-on to wind and waves. A steady wind heels her to an angle θ0. Waves then roll her to windward by an angle θ1 from there. As she rolls back, a gust strikes. The criterion compares the energy the gust puts in with the righting energy she still has.
The two levers
The steady wind gives lw1 = P × A × Z ÷ (1000 × g × Δ) in metres. P = 504 Pa; A is the above-water side area exposed to wind, including deck cargo, in m²; Δ is displacement in tonnes; g = 9.81 m/s². Z is the vertical separation in metres of the above-water and underwater lateral-area centres. Approximate the latter at half the mean draught below the waterline. The gust lever is lw2 = 1.5 × lw1. Draw both as horizontal lines.
The two tests
First, θ0 must not exceed 16° or 80 % of the deck edge immersion angle, whichever is less. Second, area b must be at least area a. Area a lies between lw2 and GZ from θ0 − θ1 to their first crossing. Area b lies above lw2 from there to θ2: the least of θf, 50° and θc, the second crossing of lw2 with GZ.
Finding the steady heel
Read θ0 at the first stable intersection of lw1 with the GZ curve. Wind and righting moments balance there. Use this angle for the steady-heel limit. The approved booklet supplies θ1; the windward extreme is θ0 − θ1. Bilge keels generally reduce θ1. A shorter rolling period tends to increase the estimate when the other factors stay fixed. GM alone does not determine θ1; use the approved assessment.
What you do on board
The approved booklet shows the result for each condition, or a curve of maximum KG or minimum GM against draught that covers all the criteria. Follow the booklet's KG-limit and free-surface convention at your actual draught and trim. Compare fluid KG only with a limit defined for fluid KG. Changed deck stowage requires a new check using A, Z, Δ and KG; recheck the approved deck-cargo limits.
2Key data
- Wind pressure
- P = 504 Pa (about 26 m/s); less only if approved
- Steady wind lever
- lw1 = P × A × Z ÷ (1000 × g × Δ) (m)
- Gust lever
- lw2 = 1.5 × lw1
- Steady heel θ0
- ≤ 16° or 80 % of deck edge immersion angle, if less
- Energy check
- Area b ≥ area a
- θ2
- Least of θf, 50° and θc (second GZ–lw2 crossing)
3Key points
- 1
Steady wind lever lw1, gust lever lw2 = 1.5 × lw1, roll to windward θ1.
- 2
θ0 ≤ 16° or 80 % of deck edge immersion, whichever is less.
- 3
Area b ≥ area a, with b ending at θf, 50° or θc, whichever is least.
- 4
Follow the booklet's KG-limit and free-surface convention at your actual draught and trim.
- 5
Changed deck stowage requires a fresh windage and stability check using A, Z, Δ and KG.
4Frequent mistakes
Wrong: Passing 2.2 means the weather criterion is passed too.
Right: The weather criterion is a separate mandatory check. A high-windage ship can pass 2.2 and fail 2.3.
Wrong: The steady heel limit is always 16°.
Right: It is 16° or 80 % of the deck edge immersion angle, whichever is less.
Wrong: Every maximum KG curve uses the same free-surface convention.
Right: Read the curve's stated free-surface allowance. Compare fluid KG only when the limit is defined for fluid KG.
5Practice case & exam keys
A container feeder has Δ 15,000 t, lateral windage area 2,400 m² and Z 9.0 m. The deck edge immerses at 18°. The planner wants an extra tier on deck. What do the numbers tell you, and what do you check?
Show the answer
lw1 = 504 × 2,400 × 9.0 ÷ (1000 × 9.81 × 15,000) ≈ 0.074 m; lw2 ≈ 0.111 m. The steady heel limit is 80 % of 18° = 14.4°, less than 16°. An extra tier changes windage, displacement and KG; recompute the lever and criterion. Check the KG and deck-cargo limits at actual draught and trim, following the booklet's free-surface convention. Outside those limits, the condition is not approved.
Exam keys
- lw1 = P A Z ÷ (1000 g Δ), P = 504 Pa; lw2 = 1.5 lw1.
- θ0 ≤ 16° or 0.8 × deck edge immersion angle, whichever is less.
- Area b ≥ area a; θ2 = least of θf, 50°, θc.
- On board: follow the KG limit's free-surface convention at actual draught and trim.
Finish this unit
When you've read it, answer three questions. Get all three right to finish the unit; opening the page alone doesn't count.
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