To realize numerical towing-tank tests by substantially shortening the time to the solution, a general-purpose finite-element flow solver, named FrontFlow/Blue (FFB), has been fully optimized so as to achieve the maximum possible sustained memory throughputs with three of its four hot kernels. As a result, a single-node sustained performance of 179.0 GFLOPS, which corresponds to 5.3% of the peak performance, has been achieved on Fugaku, the next flagship computer of Japan. A weak-scale benchmark test has confirmed FFB runs with a parallel efficiency of over 85% up to 5,505,024 compute cores, and a sustained performance of 16.7 PFLOPS has been achieved. By these, the time needed for large-eddy simulation using 32 billion grids has been significantly reduced from almost two days to only 37 min., or by a factor of 71. This has clearly indicated that a numerical towing-tank could actually be built for ship hydrodynamics in a few years.