An electro-mechanical, semi-analytic, reduced-order (RO) model of a fluid-loaded transmitting capacitive-micromachined ultrasound transducer (CMUT) operated in collapse mode is developed. Simulation of static deflections, approximated by a linear combination of six mode shapes, are benchmarked toward state-of-the-art models and validated with digital holography microscope measurements of a fabricated CMUT device. The dynamic response of a detached single CMUT cell and an array of CMUT cells is predicted and analyzed for the effect of mutual radiation. The step-wise validation shows that our model predicts the static response including hysteresis behavior of a collapse-mode CMUT with a high accuracy. The dynamic response and frequency-tunability are modeled with a satisfactory accuracy. The developed RO model is computationally efficient and in general faster than finite element methods. It is concluded that the presented RO model allows fast parameter analysis and is a powerful tool for CMUT pre-design.