Antonio’s toy boat is bobbing in the water under a dock. The vertical distance ${H}$ (in ${cm}$ ) between the dock and the top of the boat’s mast $t$ seconds after its first peak is modeled by the following function. Here, $t$ is entered in radians. ${H}({t})=5 \cos (\frac{2 \pi}{3} t)-35.5$

How long does it take the toy boat to bob down from its peak to a height of $-35 {~cm}$ ? Round your final answer to the nearest tenth of a second.