Squat Effect

The squat effect is a phenomenon which causes an increase in ship’s draught, trim and reduces UKC. It is caused by the relative movement of the ship’s hull through the surrounding body of water.
Squat occurs when a vessel is making way through the water or is not making way but laying in flowing water (current).
What causes squat?
– A ship making way pushes a mass of water in front of her bow. This water has to flow back under and at the sides of the ship to replace the water that the ship’s hull displaced.
– In shallow or narrow water the water particles’ velocity of flow increases. This creates a pressure drop (Bernoulli’s Law)
– The drop in pressure result’s in a vertical sinking of the ship’s hull.
– Depending on the ship’s block coefficient (Cb) a change in trim can also occur.
The main factors affecting the size of the squat are:
– Present water depth
– Ship’s peed
– Ship’s block coefficient (Cb)
– Whether the ship is sailing in restricted (canal, river, channel) or unrestricted water (large estuary, open sea)
How to prevent squat?

When looking at what produces squat, the only factor that we can actively control is the ship’s speed which is the only effective measure to minimise or eliminate the effect.
By reducing the speed through the water by about a half, the Squat reduces to about a quarter.

 

Sources/further reading

http://www.vht-online.de/PDF/Practical_Advice/SQUAT-uk.pdf

https://www.youtube.com/watch?v=89Nitr-MUxI#t=38.9579297

http://www.tsb.gc.ca/en/reports/marine/2000/M00L0039/M00L0039.asp

https://en.wikipedia.org/wiki/Squat_effect