Notes on Bank, Cushion, and Squat

This post is just the annotated graphics that we used in our video about the grounding of the Ever Given in the Suez Canal.

To understand bank and cushion:

• Think of ship stationary and river of water running by it.

• Bow shape at the waterline funnels water into the near shore side, which piles up as it cannot compress.

• This hydraulic head forces water aft and bow out. 

• Since it cannot compress, the water must move faster toward the narrowing gap aft. 

Area 1 x Speed 1 = Area 2 x Speed 2 

Then we have new physics entering into the situation:

• The Bernoulli principle (based on conservation of energy) dictates that the relative pressure in the water will decrease when the fluid speeds up (see lab demo below). This is relative to the pressure on the other side of the vessel. Arrows shown are the differentials.

• The net effect is the bow is pushed out as the stern is sucked in toward the bank.

• Helmsman must turn toward the bank to track straight. 

Reference: https://towmasters.files.wordpress.com/2009/05/squat-interaction-maneuvering.pdf