⚖️ Rolling Period Calculator
Calculate rolling period or Metacentric Height (GM). Assess vessel stability, compare formulas, and verify safety thresholds with integrated tools.
Calculation Mode
📚 Understanding Rolling Period
What is Rolling Period?
The rolling period is the time it takes for a ship to complete one full roll cycle. That is, moving from the maximum angle on the port side, across to the starboard side, and back to the maximum angle on the port side. It is a critical dynamic indicator of a vessel's stability and Metacentric Height (GM).
Conducting a Roll Test
- Conditions: Calm water, wind < 5 knots, wave height < 0.5m.
- Freedom: Vessel must roll freely without touching the pier or bottom.
- Procedure: Induce a roll (e.g., pulling a line, shifting weights). Time one complete cycle (Port → Stbd → Port).
- Averaging: Record 4-6 rolls. Discard the first roll and average the rest.
Metacentric Height (GM)
GM is the vertical distance between the vessel's Center of Gravity (G) and its Metacenter (M). It dictates the initial stability.
- Stiff Vessel (Large GM): Very short rolling period. Will snap back quickly. Can be uncomfortable and subject cargo to massive lashing forces.
- Tender Vessel (Small GM): Long, slow rolling period. More comfortable for passengers, but dangerously close to instability (negative GM).
Formula Details
Simplified (Meters): T = 0.797 × Beam / √GM
Simplified (Feet): T = 0.44 × Beam / √GM
Safety Limits & Errors
Accuracy: ±15-20%. Only valid for small angle rolls (< 15°).
Factors reducing GM: Ice accumulation, free surface effect (slack tanks or fish holds full of water), and heavy deck cargo. This tool is NOT a substitute for a formal approved stability booklet.