[solididiot]

>> Keeping an eye on the weather, studying charts carefully, picking good anchorages, and having a good rode and anchor was our best possible insurance, but there are dangers that are beyond anyone's control, dangers that even the most skilled sailor cannot avoid, e.g., a shipping container adrift, dead heads at night, whales, etc.

dead heads at night. What is that?

[capableweb]

> A deadhead is a log or large tree in a waterway submerged just below the surface and difficult to see. If a fiberglass boat strikes a deadhead, the boat could be damaged and even sunk. The lower unit could be ripped off causing a hole in the transom and/or render the boat inoperable. A diligent forward watch will help prevent this potential danger.

[totony]

How difficult would using sonar be to protect against that?