Question 1

The log measures speed through the WATER; GPS gives speed over the GROUND. The difference between them is…

Accepted Answer

The tidal stream (and leeway) — the water itself moving — Log 5.0 kn with SOG 6.5 kn means about 1.5 kn of fair stream under you. Two instruments disagreeing is not a fault: it is the stream made visible.

Question 2

An echo sounder reading depends on where its zero is set — commonly…

Accepted Answer

Below the transducer, below the keel, or below the waterline — you must know which YOURS shows — Three conventions, a metre or more apart. Check the offset before you trust a shallow reading — and sanity-check against charted depth + height of tide.

Question 3

The deck log earns its keep because…

Accepted Answer

A row of times, logs and courses is exactly what you need to work up an EP when the electronics die — Position sources fail; a kept log means your last good fix plus course and distance run — an EP — is always available. It is also your record if anything is ever questioned.

Question 4

Walking a distance off the chart with dividers, you measure against…

Accepted Answer

The side (latitude) scale, level with where you are working — One minute of latitude is one nautical mile everywhere; the side scale is the chart’s built-in distance ruler — read it at the latitude of your working, since the scale stretches on a Mercator chart.

The navigator’s instruments

This lesson is part of a paid course