Skysail Training
	CONTACT	RYA CEVNI TEST ONLINE	BUY NAVIGATION SKILLS CHARTS	BUY CEVNI SIGNS CHART	RADAR COURSE  1:1 ON YOUR DATE

The Log

1) Log Instrument - indicates Speed at present and distance run 

2 ) Log Book - to record position, course, speed, events - see below

Boats have two kinds of log - 

1) Distance and Speed Log    A paddle wheel (or other measuring device) pictured above.  It is mounted through the hull of the boat, usually ahead of the keel where the water flow is smooth. It acts like the speedometer and mileometer (odometer) in a car:  The rim of the wheel contains a small magnet and with each rotation it acts on a sensor to measure distance run and speed.  The cockpit digital display shows speed in knots though the water, and can be switched to show distance run in nautical miles.  It may be reset to zero at the start of a passage.

The term 'log' comes from old ships where a wooden chip (or log) was thrown overboard at the bow of the boat, and the time for the stern to pass the chip was measured, presumably with an hourglass or by counting!  With experience this is still a usable method for speed but is difficult to apply for distance. (Also known as a 'Dutchman's log').

Later a small triangular plank attached to a reel of cord was used for the same purpose; the cord had knots at regular intervals and as the cord unwound the crew counted the knots in a given time period shown by a small hourglass - usually 15 seconds - hence the term 'knot' for boat speed.  Later still a 'Walker Log' was a rotating impeller towed over the stern and driving a rev counter to measure distance run  - a very accurate method but susceptible to loss, tangle or sharks!

The modern impeller or electronic log can be very accurate, but must be checked regularly for fouling by weed or barnacles.  This involves removing the log tube from the hull and quickly stopping the resultant inflow of water.  An under reading log is dangerous as the boat may have travelled further than indicated and away from safe water.

2) Log Book

'LOG' is also used for the vessel's record of the passage. This can be a bound book or loose leaf, or even electronic with the right chart plotting software.

Log entries will include Time, log distance run, course, speed, wind, weather, barometer, events, fuel state, position - see below for example.  Enter your destination as 'towards'; it was tempting fate to say you were going to Valparaiso when you could often end up somewhere else.... though it is less of a problem today.  The log book is a legal document, and should contain all relevant details of the passage including VHF radio messages. It might be inspected by Customs officers.

Log of Skyva II 2003   Date: 10 June 2003
From:  Hythe      Towards:  St Vaast
Crew: Keith & Celia + Marcus, Sandra       Weather: Fair, SW 4
High Water {.Portsmouth .} : ....0800 BST.........and..................... Engine hrs    Start:     Finish:        Fuel: Full

 Posted 27 May 2011 boat log speed entries course