Seakeeping Strip Theory
Seakeeping Analysis Using Strip Theory · Also known as: strip theory, 2D strip method, seakeeping prediction
Seakeeping strip theory is a method for predicting the dynamic motion of a ship in regular and irregular waves by decomposing the hull into two-dimensional transverse sections (strips) and computing the hydrodynamic forces on each strip. Developed by Salvesen, Tuck, and Faltinsen in 1970, the method efficiently estimates ship heave, pitch, and roll motions, accelerations, and loads without resorting to expensive three-dimensional computational fluid dynamics. Seakeeping analysis using strip theory is standard in ship design and operational planning.
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When to use it
Use strip theory for seakeeping analysis during ship design when you need estimates of motions and loads in realistic sea states. Ideal for preliminary and contract design; fast enough for parametric studies. Deploy for operational planning (speed/heading optimization, stability assessment). Suitable for conventional monohull ships; specialized methods needed for multihulls or unusual geometries. Use when computational cost is a concern.
Strengths & limitations
- Computationally efficient; solves the problem in seconds to minutes on a desktop; allows rapid parametric studies during design.
- Physically meaningful; decomposes the problem into understandable 2D slices; results align with hydrodynamic intuition.
- Well-validated against experiments and full 3D analyses; particularly accurate for heave and pitch in head seas.
- Handles irregular waves via spectral analysis; provides probabilistic estimates of extreme motions and accelerations.
- 2D assumption; neglects interaction effects between distant ship sections and 3D flow phenomena (e.g., edge effects near bow/stern).
- Accuracy degrades for high-speed ships and very short waves; frequencies near hull resonances may be over/underestimated.
- Roll motion less accurate than heave/pitch; cross-coupling effects and non-linear roll damping are heuristic.
- Requires empirical calibration for some parameters (e.g., roll damping, slamming); tuning needed for best results.
Frequently asked
What is a Response Amplitude Operator (RAO)?
An RAO is the amplitude of ship motion (e.g., heave, pitch) per unit wave amplitude at each frequency. Plot RAO vs. frequency to see resonance peaks and phase relationships. An RAO of 0.5 in heave means the ship heaves at 0.5 m per 1 m wave. RAOs are used to predict actual motions in irregular seas by convolving with the wave spectrum.
How many strips do I need for accurate results?
Typically 20–50 strips are sufficient for monohulls. More strips improve accuracy but increase cost (often negligibly with modern computers). A rule of thumb: use at least one strip per 5% of ship length. Fewer strips for preliminary design; more for final design.
Can strip theory predict slamming loads?
Pure strip theory is linear and does not handle slamming. Slamming is a non-linear phenomenon (impact of bow on water). Some software adds slamming loads empirically or uses specialized non-linear extensions. For ships prone to slamming, combined with detailed CFD analysis.
How does wave direction affect seakeeping?
Seakeeping varies with wave direction. Head seas (waves from ahead) cause pitch and heave; beam seas (waves from side) cause roll; following seas (waves from behind) cause pitch. Strip theory can be extended to handle arbitrary wave directions by computing directional transfer functions.
Sources
- Salvesen, N., Tuck, E. O., & Faltinsen, O. (1970). Ship motions and sea loads. Journal of the Society of Naval Architects and Marine Engineers, 78(4), 250–287. link ↗
- Journée, J. M. J. (1992). Prediction of speed-dependent ship motions and capsizing in irregular head seas. Ph.D. thesis, Delft University of Technology. link ↗
- Faltinsen, O. M. (1990). Sea Loads on Ships and Offshore Structures. Cambridge University Press. link ↗
How to cite this page
ScholarGate. (2026, June 3). Seakeeping Analysis Using Strip Theory. ScholarGate. https://scholargate.app/en/aerospace/seakeeping-strip-theory
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