We model the incompressible, inviscid flow in a hydraulic jump in a closed horizontal tube. Downstream of the jump the flow fills the cross‐section of the tube, and there the pressure is hydrostatic plus an additional pressure caused by the confines of the tube. We calculate the rate of energy loss by the jump and use this to account for the dramatic damping seen when an enclosed tube, partly‐filled with water, swings freely like a pendulum. ©1996 American Institute of Physics.