We execute the counting sequence step-by-step:
\begin{equation*}
30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330, 360, 390
\end{equation*}
Stop here! If we attempt to add another full 30-foot line, our count hits 420 feet, which exceeds the total amount of rope we actually have on our boat.
Let us count the full segments completed: we have exactly 13 full lines (reaching 390 feet). To find our final remaining fragment, we subtract our progress from our initial total:
\begin{equation*}
400 - 390 = 10 \text{ feet}
\end{equation*}
Answer: The layout produces 13 full zig-zags and 1 short leftover line of 10 feet at the end.