Distance, speed and time

Nautical miles, cables and knots

At sea, distance is measured in nautical miles. A nautical mile is the length of one minute of latitude, fixed at exactly 1,852 m: a little longer than a land mile (1,609 m), and nearly twice a kilometre. Sixty minutes make a degree, so a degree of latitude is 60 miles. That link is what makes the chart’s latitude scale a ruler.

A knot is a speed: one nautical mile an hour. A yacht making six knots covers six miles in an hour, three in half an hour. Because the hour is built in, there is no such thing as “knots per hour”.

Measuring distance with dividers

To measure a distance on a paper chart you need a pair of dividers and the latitude scale down the side of the chart:

1. Open the dividers so that one point sits on each end of the line you want to measure.
2. Without changing the spread, carry them to the latitude scale at the side of the chart, level with where you measured.
3. Read the minutes and tenths between the two points. Each minute is a mile.

On a chart that covers a big area, the latitude scale itself stretches a little towards the poles, so always measure against the part of the scale level with your line. On the large-scale charts you use for a charter week the difference is tiny, but the habit costs nothing.

Longer passages and routes

For a line longer than the dividers will comfortably open, set them to a round figure on the latitude scale (one, two or five miles) and walk them along the line, swinging them over at each step and counting. When less than a full step is left, close them onto the gap, measure it on the scale and add it on.

Most days have more than one leg: out of the bay, round a headland, across to the next island. Measure each leg on its own, write its distance beside it on the chart, and add them up.

60D = ST: distance, speed and time

Distance, speed and time are linked by one rule. Time at sea is counted in minutes, so the rule is written with a 60 in it:

Cover the one you want and the other two tell you how to find it:

T = 60 × D ÷ S
  = 60 × 7.5 ÷ 5
  = 450 ÷ 5
  = 90 min

Two shortcuts are worth learning by heart:

• Minutes per mile. Sixty divided by your speed is how long a mile takes: 15 minutes at 4 knots, 12 at 5, 10 at 6.
• The six-minute rule. Six minutes is a tenth of an hour, so in six minutes you cover a tenth of your speed: 0.6 M at 6 knots, 0.5 M at 5.

Sizing a charter day

A good charter day is 15 to 20 miles. At five to six knots that is three to four hours of sailing, which leaves room for a swim stop, a long lunch and an unhurried arrival. Most of the legs in our cruising guides sit in that range on purpose:

Three things make a day longer than the sum suggests:

• Average speed. A charter yacht may touch seven knots, but light patches, tacking, sail changes and the last mile into harbour pull the average down. Plan on five knots, and treat anything better as a bonus.
• Upwind legs. A yacht can’t sail straight into the wind; she zigzags at roughly 45° either side of it, so the distance through the water can be half as far again as the line on the chart. Allow more time, or plan to motor.
• Stops. A swim, lunch at anchor, a wait for fuel: write them into the plan, because they always take longer than you think.

Fifteen to twenty miles, planned at five knots, with the stops written in. Save the 25 to 30 mile days for a settled forecast.

Arrive in good time

Plan the day backwards, from when you want to arrive, because three things on a charter happen at the end of the day:

• The afternoon breeze. In much of the Mediterranean the wind builds through the afternoon: the maestral in Croatia from late morning, the maistro in the Ionian from around midday. It is lovely to sail in, and much harder to berth in. Mooring stern-to in a crosswind is the trickiest job of the day.
• Full harbours. In summer, popular town quays and buoy fields fill by mid-afternoon. Aim to be in by about three.
• Darkness. Charter contracts usually forbid sailing after dark, and harbours, rocks and unlit pot buoys are far harder to see at dusk. Keep at least a couple of hours of daylight in hand. Sunset in the Mediterranean is between about half past eight and half past nine in midsummer, depending on where you are, and much earlier in spring and autumn; in the Seychelles it is dark by about half past six all year.