Here is what John Chamberlain of San Francisco, California relates on the movement of large ships in his paddling waters of San Francisco Bay. A friend and I were going from Alcatraz back to San Francisco. We saw a large tanker was just moving under the Golden Gate Bridge. Beating it across seemed to be little problem. But as we set off we realized that we had greatly underestimated the speed of the tanker and wisely decide to do a U-turn out of its way.
Tankers move very fast, although their size makes their speed deceiving.
Many such vessels can hit 20 knots. Distances are very hard to judge unless you have an idea of a commercial craft s size. And even then you can never absolutely be sure. An incoming tide in some places can give a ship 3 or 4 knots of added speed. It may be lightly loaded and not drawing as much water.