IMO Weather Criteria Calculator

Introduction

Updated 2024-02-22 to fix a bug in the calculation of the angle of roll to windward due to wave action and area A.

The IMO Weather Criteria Calculator is a tool designed to ascertain whether a ship conforms to the International Maritime Organization's (IMO) weather criteria for a given loading condition. This calculation involves inputting specific information related to the vessel's loading condition, geometry, and righting arm calculation. By accurately determining factors such as draught, displacement, vertical center of gravity corrected for free surface effect, waterline length, moulded breadth, block coefficient, downflooding angle, bilge keel area, and Cross Curves of Stability, this tool aids in evaluating a ship's stability under various weather conditions.

Click here for an example

Weather Criteria

Except from The International Code on Instact Stability, 2008 amended in 2016 and 2018.

The ability of a ship to withstand the combined effects of beam wind and rolling shall be demonstrated, with reference to the figure below as follows:

  • the ship is subjected to a steady wind pressure acting perpendicular to the ship’s enterline which results in a steady wind heeling lever (lw1);
  • from the resultant angle of equilibrium (φ0), the ship is assumed to roll owing to wave action to an angle of roll (φ1) to windward. The angle of heel under action of steady wind (φ0) should not exceed 16° or 80% of the angle of deck edge immersion, whichever is less;
  • the ship is then subjected to a gust wind pressure which results in a gust wind heeling lever (lw2); and
  • under these circumstances, area b shall be equal to or greater than area a, as indicated in the figure below:
Severe wind and rolling
Severe wind and rolling

Abbreviations

  • KGcorrected - Vertical center of gravity measured from the baseline (top of keel plate) and corrected for the free surface effect. If unknown, you can compute it using the Loading Condition Calculator.
  • θf - Downflooding angle in cases of unprotected openings. Not mandatory.
  • KN - Cross Curves of Stability. Use the cubic interpolation calculator if some values are missing
  • θ - Angle of heel
  • GZ - Righting arm
  • Z - Vertical distance from the centre of the lateral wind area to the centre of the underwater lateral area or approximately to a point at one half the main draught

Calculator

Loading condition data

Description Symbol Value Unit
Mean moulded draught T m
Waterline length Lwl m
Moulded breadth B m
Block coeffiecient CB -
Bilge keel area Ak m2
Wind pressure p t/m2
Projected wind area A m2
Wind arm Z m
Displacement Δ t
Vertical center of gravity corrected for free surface effect KGcorrected m
Downflooding angle. Will only affect the results if less than 40 degrees. Keep empty if not known. θf deg

Cross Curves of Stability

θ [deg] KN [m]
5
10
15
20
25
30
35
40
45
50