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'Squat' is the general name for a number of hydrodynamic phenomena caused by a buoyant form such as a boat, ship or barge moving through the water.

Contents

What you see

Why does this happen?

How does this affect Ships & Boats?

See also

What you see

The ship or boat may change trim or slightly pitch and/or bodily sink slightly deeper into the water from its static draft.

Why does this happen?

As the water is pushed downward (and outward) by the passing hull it accelerates and its pressure falls (see Bernoulli's principle). In the example of a boat, the reduced pressure on one side is counteracted by the reduced pressure on the other side and they cancel each other out. However, the reduced pressure on the bottom of the boat ''sucks'' the boat slightly downward until the increased displacement counteracts the force generated by the reduced pressure.

How does this affect Ships & Boats?

The change in draft and trim increases the resistance of the hull so more power is required than if this did not occur.
This phenomenon becomes more pronounced in shallow water (Shallow Water Effect). In extreme cases as when a boat exceeds the critical speed in very shallow water it can be sucked down until it strikes the bottom (common in river barges).

See also

Society of Naval Architects and Marine Engineers (SNAME), "Principles of Naval Architecture", 1989, Vol. II "Resistance and Propulsion"
Squat does not cause an increase in the draft of a vessel, but decreases the UKC (under keel clearance) of the vessel in question.