This paper aims to investigate the heave-plate effects on the behavior of a multi-unit floating offshore wind turbine (MUFOWT). The global performance of the large-scale, square-shape, semi-submersible floating platform equipped with 4 wind turbines at each corner, and 24 wave energy converters on frames along near-waterline is numerically simulated in time domain by using the in-house turbine floater-mooring fully-coupled dynamic analysis program, MUFOWT. Cases with and without heave plates are systematically compared to identify the role of heave plates. For verification, an independent 1/50-scaled experiment was conducted by authors in the Korea Research Institute of Ships and Ocean Engineering's (KRISO) 3D wave basin. The numerical results with heave plates are extensively compared against the physical model-test results. Finally, the global performance results under survival-storm conditions, in which the WECs and blades of wind turbines are fixed, with and without heave plates are systematically investigated. The findings indicate that the heave plates are effective in reducing heave and pitch motions, and shift their natural frequencies with a minimal increase of mass. (C) 2018 Elsevier Ltd. All rights reserved.