Under Regulation V/28 of the International Convention for the Safety of Life as Sea (SOLAS) 1974 all vessels are required to keep a navigational log by law. British registered vessels must comply with this regulation as defined by the Maritime and Coastguard Agency.
Keeping a log is, therefore, not only a sensible thing to do, it is also a legal requirement.
A log may be kept on paper or electronically. If on paper it should be kept in a book with numbered pages so that pages cannot be removed. If errors are made they must be crossed out, not erased. Furthermore, a log should be kept when alongside and also when underway or at anchor.
The primary reason for maintaining a ship’s log is for navigation and safe passage. The log is a legal document that is used to record fixes, courses steered, weather forecasts, wind speed and direction, periodic readings of the ship’s heading and speed and it’s barometer. In addition, the log should record watch changes, numbers of persons on board, changes and sail plan, regular maintenance checks whilst on passage and a record of battery voltage, engine running hours and water and diesel consumed.
Nowadays, many skippers will use the log to record when they deliver a safety brief to their crew and also include all or part of their passage plan in the first entry. This may all seem overkill, but as the person legally responsible for your vessel and crew maintaining an efficient logbook acts as an excellent memory jogger and records what you did and when, in the unfortunate event that an incident occurs.
Maintaining a defects book, a maintenance schedule and a medical log is usual on larger yachts and commercial sail training vessels, especially when on passage.
However, the main reason we keep a log is for the purposes of navigation. In today’s world of chartplotters and GPS transponders, the log can sometimes be neglected. But it shouldn’t be. If you were to lose power or GPS signal, how will you navigate? You need a starting point and if the last position entry in your log was 10 hours ago, you have a problem! Maintaining a log entry on the hour every hour, including a time, log mileage reading, heading, wind speed and direction, sea state, cloud cover, barometric reading, sail plan and a position fix will mean that even if you lose all navigation equipment, a compass, or even your watch and the position of the sun on the horizon, will enable you to set a decent course for safety from a known starting point.
An added benefit of the hourly watch is that it offers one lucky crew mate brief respite and a place to shelter during a wet night watch and whilst down there, he may as well get the kettle on.