40 knots are about 75 km\h or 46 mph
Supposing Kawabata travel that fast for 30 mins to reach Rokkenjima the distance should be about 37 km or 23 miles away from Niijima.
the distance from Niijima to Shimoda is about 27 miles or 43 km.
That still doesn't sound right...
The red circle represents the various points where Rokkenjima would be if the data I used so far were correct.