Knots, nautical miles, fathoms. All at once.

One knot is 1.15077944802 mph — a nautical mile per hour, where a nautical mile is 1,852 metres exactly and a statute mile is 1,609.344 metres exactly. The rule of thumb at sea is 'add about 15 per cent', which is close enough to talk with but not to work with: at 30 knots the shortcut is off by about half a mile per hour. In the other direction, 1 mph is 0.868976 knots.

One nautical mile is 1.15078 statute miles, or 1.852 kilometres exactly. It is longer than the land mile because it is not an arbitrary length at all: it is one minute of latitude, one sixtieth of a degree. That is the whole point of it — you can measure a distance straight off a chart against the latitude scale without needing a separate distance scale.

Because a minute of latitude is a nautical mile everywhere, while a minute of longitude shrinks as you move away from the equator — at 45° N it is only about 0.7 of a nautical mile, and near the poles it collapses towards nothing. The latitude scale runs up the sides of the chart, so that is the one you step off distances against. Using the longitude scale along the top is a classic way to get a plot wrong by a margin that grows with your latitude.

Six feet, or 1.8288 metres exactly. Fathoms come from the span of a man's outstretched arms and survive because that is how a lead line was marked. Older US charts sound in fathoms, newer ones in feet or metres, and which you are looking at is printed in the chart's title block. Reading a metric chart as though it were in fathoms would have you believe there is roughly three times the water there actually is — check the block before you trust a sounding.

Nautical miles and knots throughout, with time in minutes on the 60D = ST relationship. Depths appear in whatever the chart extract uses, so read the title block. If a question hands you statute miles or km/h, converting is part of the question — the answer options will usually include the value you get by forgetting to.