With a modern vessel it cannot be anchored when stationary; the anchor needs to be moving so that it ploughs into the sea bed. Perhaps the ancient stone anchors just depended on their weight, which would lead to many losses in stormy weather.
]]>Until the advent of containers in modern shipping, the turnaround could be a week or more. Now it can be a few hours. I guess the Roman rate would have been nearer the former.
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