A ships log can be used for a number of differing purposes. You may wish to record in it the times that you change the filters on the engine or when you replace the zincs. You may use a log as a diary to record events and impressions of places you visit. But the main use of a log is to record information concerning the vessels navigation.
Even if you are convinced of the reliability of GPS (Global Positioning System) navigation you still need a ships log to record navigational data and if, heavens forbid, your GPS should fail, the information in a written log can enable you to calculate a position and arrive safely at your destination.
Ships logs have been kept since the very beginnings of marine navigation and were used to help record positions and facilitate the calculation of a course to steer as the following excerpt from the log-book of the Phoenix indicates. The vessel was homebound from India in 1687 and had just sighted the Cape of Good Hope:
"Latt Obserd 32o 16'S Mer Dist 02o 55'Wt Morning Varia pr Ampld 08o 00'Wt SUNDAY ye 10h faire weathr & very little wind at SE & ESE & sometimes Callme. our Cors by Comps hath been NW runn by logg 24 miles wch dists I alow to be 30 miles because of a great swell out of ye So ward bord & what we have galed away when or sailes have been haled up wch upon or Cors steerd gives 18 miles Noward but by Observat we 6 miles more to ye Noward which I impute to somewhat of a streame we have had these 4 days although in trying to day we finde none I alow ye true Cors to be made NW?W dist 36 miles diffr lattd 24 depe 27"
The Captain of the Phoenix has recorded in the above excerpt, details about the weather, the ships latitude, the magnetic variation, a northward setting current and the true course that he ordered the vessel to steer. In fact, very similar information that we need to record when we are making a passage.
Although you can buy ready-made log books which have many columns already labelled (and a few for you to define) I have always chosen to use a plain notebook so that I can include the columns that I choose. One of the main reasons I choose to do this is that I often include different columns in my log book depending on whether the passage is across an ocean (Table 1), along the coast(Table 2) or in a bay (Table 3).
Note that all three log books pages begin with information concerning the starting point and a destination. But I was taught early on that it is prudent to write From ____ Towards ____ . The use of the word Towards being more realistic then To given the vagaries of wind, weather, crew and diesel engines! All three tables include columns for Time, Course, Distance, GPS position and Notes with other columns added as necessary. When making a coastal passage you may want to record bearings of lights so the notes column can be extended, but on an ocean passage this will be unnecessary though columns for date, barometer and watch error (for celestial navigation purposes) will perhaps be needed.
From the ships log it should be possible to reconstruct a course followed during a passage and at sea it will enable the navigator to calculate dead reckoning and estimated positions. These can then be compared with a GPS position to provide accurate information about the current, leeway or the combined effects of both (Figure 1).
Even if you use a GPS to give you a position, course to steer and distance to a waypoint, a ships log should be kept so that, at a very minimum, the time and position can be recorded at regular intervals. Then, should the unimaginable happen and your GPS fail, you will have a recent position recorded which can be plotted on a chart. Of course, prudent navigators will have done this at the same time as they recorded the position in the log book but not all navigators or skippers are so meticulous!
For your log book to be useful, it is essential that the instruments that you are using to record your information are accurate. The ships compass should be free of compass deviation (See Telltales Seamanship November 93) or the deviation should be a known quantity for all headings. The log should be calibrated to accurately measure distance (speed may be useful to help with sail trim but aboard a sailboat it is of much less navigational use than distance run). The ships clock should be reliable at sea.
Just as with all aspects of sailing, planning and preparation make a trip safe and enjoyable. Plan your navigation, check your instruments and draw up the columns in your ships log before you leave.