Sailors learned to measure everything with almost nothing: a mark on the rail, a watch, a piece of wood that floats. These tricks work on land too.
UNDERSTAND
A constant bearing is pure geometry. Two moving things that are going to meet always see each other at the same angle, right up to the impact. If the angle changes, they won't reach the same point at the same time. What fools people is that nothing seems to move: a ship on a collision course looks still, it just gets bigger. The international rules of the sea say it in one sentence: if the compass bearing of an approaching vessel does not appreciably change, the risk of collision exists.
The horizon is close because the Earth is round: your line of sight grazes the surface at a distance that grows only with the square root of your height. Doubling your height only pushes the horizon 40% farther. The air bends light a little and adds a few percent more.
The tide follows roughly a wave-shaped curve: slow near low and high water, fast in the middle. The rule of twelfths is the approximation you can do in your head. It stops working where the coast distorts the tide (the Mediterranean, estuaries, some straits): there, trust the local tide tables.
The signs of land rest on three facts: an island heats up faster than the sea, which builds a cloud over it; coastal birds don't sleep at sea; and land stops or bounces back the swell.
ACT
Are we going to hit: the constant bearing
A boat, a car, a cyclist is coming at you at an angle. Will you cross or collide?
- Keep your course and your speed.
- Line the other one up with a fixed mark on your side: a post, a shroud, the corner of the windshield, a branch if you're on foot.
- Watch for a minute.
- It slides forward against the mark: it will pass ahead of you.
- It slides back: it will pass behind.
- It stays glued to the mark and gets bigger: you are going to hit. Change course or speed early and clearly, so the other one understands what you're doing.
How far the horizon is
Take the square root of the height of your eyes, in meters, and multiply by 3.6: that's the distance to the horizon in kilometers.
- standing on a beach, eyes at 1.7 meters: about 4.7 km (3 miles);
- on a roof or a rock at 10 meters: about 11 km;
- on top of a 30 meter cliff: about 20 km.
A lighthouse, an island, a ship can be seen from farther, because they have their own height: add their horizon to yours. A 100 meter hill (36 km) seen from your 1.7 meters (4.7 km) can appear about forty kilometers away.
Your speed with a piece of wood
- Throw something that floats and is easy to see into the water at the bow.
- Count the seconds it takes to pass from the bow to the stern.
- Boat length in meters ÷ seconds × 3.6 = speed in km/h. An 8 meter boat covered in 4 seconds: 7.2 km/h, about 4 knots.
It's the "Dutchman's log" of old navigators. On land, the same idea gives you the speed of a river: a stick, two marks on the bank, a stopwatch.
How high the water will be, hour by hour: the rule of twelfths
On coasts where the sea rises and falls twice a day, like the Atlantic and the Channel, it doesn't rise evenly. Split the tidal range, the difference between low and high water, into twelve parts:
- 1st hour: 1/12;
- 2nd hour: 2/12;
- 3rd hour: 3/12;
- 4th hour: 3/12;
- 5th hour: 2/12;
- 6th hour: 1/12.
In the middle of the tide, the water rises three times faster than at the start. That's when walkers get trapped on a sandbar or at the foot of a cliff. High water comes back about fifty minutes later each day. The biggest tides, spring tides, come one or two days after the full moon and the new moon.
Knowing that land is near
With no instruments, Pacific navigators found islands they couldn't see yet:
- a cloud that stays put while the others drift: it forms over an island the sun is heating;
- the underside of a cloud tinted green: it reflects a shallow lagoon;
- seabirds that head out to sea in the morning and, in the evening, all fly off in the same direction: they are going home to sleep on land, follow them;
- fresh plant debris, leaves, nuts, and a swell that gets distorted: something is bending it.
ADAPT
You're on foot by the sea. Before walking out on the flats, note the time of low water, and know that the sea comes back fast from the second hour on. Always keep a way back to the top of the beach. If the water carries you off: Caught in a rip current.
You're on land. The constant bearing works for crossing a road, meeting a cyclist, an animal charging, a car at a blind junction.
You're driving a boat. The basics of handling and engines are in Conduire un bateau and Boat engines — the complete guide. Holding a course without instruments, with the sun and the stars: Finding your way without instruments.
The sky is building up. Reading bad weather before it arrives: Reading tomorrow's weather in the sky.