# Strip Theory

Discussion in 'Hydrodynamics and Aerodynamics' started by tea_floss, Feb 1, 2017.

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### tea_flossJunior Member

Can anyone here explain to me strip theory in very simplified manner? Thank you in advance.

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If you take a slice through a vessel, let’s say a box shaped vessel, the shape becomes a thin strip and thus changes from a 3D shape to a 2D shape. If you place a wave profile onto the box (3D) and then transfer the location of the wave profile onto that 2D strip, you can work out the bouncy/weight to ascertain where that strip should float. From that you can see how far from the equilibrium – at rest – the strip has moved. This displacement then allows you to calculate the range of displacement which = motions and thus over a time base the accelerations and hence forces. Then you summate all the thin strips together which makes the 2D strip simulate a 3D shape by adding together all the thin 2D strips, but in reality it is merely a 2 ½ D analysis of the box, or vessel and that simulates the 3D.

In an over simplified nut-shell

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### upchurchmrSenior Member

Thanks

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### b1ck0Senior Member

Let me also try. This will be bit more technical.

You slice the ships in frames (strips). Then for each frame (strip) your aim is to find the hydrodynamic loads which act on it given some wave frequency. The hydrodynamic loads are classified in the following manner:
• Radiation Loads - these are caused by the fluid on our frame when it is oscillating in still water. Important forces here are the added mass and the radiation damping, which more or less depend on our frame geometry.
• Diffraction Loads - these are caused by the fluid on our frame when it is being fixed in waves.
• Incident Wave Loads - also known as Froude-Krylov Force or Ambient Pressure Force. These are caused by the undisturbed incident wave

The whole problem of finding these forces is caused by the geometry of our frames which is not quite "regular" or "simple". Thus lots of methods were developed which can "map" the hydrodynamic loads (calculated for a semi-circle) onto our actual frames based on mathematical technique called "conformal mapping". The reason for this is that for the semi-circle there is an analytical solution for the mathematical expressions for the hydrodynamic loads.

Then in the last part of the strip theory all these loads calculated for each frame needs to be integrated over the length of the vessel in order to get the whole picture for the ship (same principle as if you were integrating the buoyancy or the weight along the vessel).

Hope this helps, and if you have another doubts let us know.

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