Three types of wind
Real wind: A flag on a flagpole, the branches of a tree, or smoke particles from a chimney are moved by only one type of wind, real wind.
Draft wind: As soon as you move, draft wind is added. You can feel this when riding a bike, for example. The faster you ride, the stronger it is. Its direction is always exactly opposite to the direction of travel. And its speed is exactly the speed of the moving vehicle.
Apparent wind: This is composed of the real wind and the wind resistance. Let's assume that a cyclist has wind at a speed of 15 km/h coming from the side and is traveling at the same speed. Then he feels the apparent wind from an angle of 45 degrees from the front side. If the real wind is blowing directly towards them, they feel the apparent wind as a headwind of 30 km/h. If, on the other hand, the real wind is blowing directly from behind, the real wind and the airflow cancel each other out. The cyclist has the impression that there is no wind.
Sometimes faster than the wind
Sailing is always done with the apparent wind. If a wind-powered vehicle—an iceboat or sandyacht or a ship—has the wind directly behind it, then the theoretically maximum achievable speed is half the real wind speed, because the airflow would reduce the usable apparent wind to half the real wind.
The situation is completely different when the real wind blows diagonally from behind or directly from the side: then the airflow adds to the real wind. This then hits slightly further from the front and is stronger than the real wind. In other words, the vehicle itself generates part of the wind used for propulsion. Even classic sailing ships, which include sailing cargo boats, prefer side winds to direct tailwinds.
Sandyachts and ice sailboats, which have very little resistance, can easily reach speeds that are significantly faster than the actual wind. Modern racing boats, which lift themselves out of the water with hydrofoils, greatly reducing water resistance, can sail faster than the actual wind. The “Vestas Sailrocket” – which can hardly be called a boat, as the sail can only be used on one side and it only works in flat water – covered the 500-meter measuring distance off Namibia in 2014 at wind speeds of around 25 knots (46 km/h) at 65.45 knots (121.21 km/h) at wind speeds of around 25 knots (46 km/h) off the coast of Namibia in 2014, reaching a top speed of 68.01 knots (125.95 km/h).
How does it work?
Aerodynamic lift
When the wind blows directly from behind, it simply pushes the sail area and thus the ship. However, as soon as the wind can flow around the sails, other forces come into play, known as “aerodynamic lift.”
Sails have been around much longer than airplanes. However, the effect of dynamic lift can be explained most easily using the example of an airplane wing: the wing is more curved at the top than at the bottom. And it has an angle of attack relative to the direction of the airflow. Due to the curvature, the airflow must be faster on the upper side, which, according to the laws of physics, creates negative pressure. On the underside, there is positive pressure due to the angle of attack. This pressure difference between the top and bottom keeps the aircraft in the air. The decisive factor here is that the flow layers on the top run nicely parallel – i.e., laminar – to the curvature of the wing. If the angle of attack is too steep, only vortices are created and the flow breaks off. Then the aircraft crashes.
The same principle applies to sails. Classic textile sails are very thin profiles. However, the fabric panels from which the sail is sewn together are cut in such a way that they create a curve. This is referred to as the sail having a belly. If the flow on the leeward side is optimal, the ship is sucked forward. If the angle of attack is too steep or too flat, or if the belly is poorly adjusted, the ship loses speed.
The lift generated by the laminar flow does not pull directly in the direction of travel. Part of the force therefore acts on the heeling and drift of the ship.
Physicists believe that the dynamic processes at work on the sails are somewhat more complex. However, the explanations presented here are sufficient for practical understanding on the ship.
A special case are the so-called “crab claw sails” developed by the Polynesians for their outrigger canoes. Here, lift is not generated by the belly of the profile around which the air flows, but by special vortexes that form at the edges when the air flows over them. Measured in terms of sail area, this shape is said to be more than one and a half times as efficient as Western sails.
Outrigger canoe with crabclaw sail in the Ethnological Museum in Berlin
The most important parts of a rig
Here we will only discuss the elements of traditional rigging. Modern rigs will be covered in a separate article.
The mast is either a round timber or a metal tube that stands more or less vertically and supports the rest of the rig. On large ships, it is often extended upwards. Traditionally, the mast stands freely only on small boats. As a rule, it is supported by standing rigging. This is usually steel cables. If they support the mast laterally, they are called shrouds. If they support the mast longitudinally, they are called stays. Many ships have several masts. Depending on the number and how they are arranged, each type of rigging has its own name. Masts and standing rigging are not operated during sailing; they are rigidly connected to the ship.
There are basically two types of sails.
Squaresails are positioned transversely to the longitudinal axis of the ship when not in use. They are attached at the top to the yard – a round piece of wood that is suspended in the middle of the mast so that it can move. Winking ships and some ancient galleys each had a single large square sail. Over time, square sails were divided so that on newer ships, several are hoisted one above the other on the masts. Square sails show their strength on broad reach and downwind courses.
The replica of a Viking ship with a large, undivided square sail.
Fore-and-aft sails are positioned lengthwise along the ship when not in use. Staysails are attached at their front edge—the luff—to a stay, i.e., the part of the standing rigging that supports the masts lengthwise. Staysails, which are usually triangular, are flown at the very front and sometimes between the masts.
From this perspective, you can see the triangular staysails of the Tres Hombres.
There are countless shapes of sails that are rigged with horizontal or upward-sloping timbers. They are often attached to the mast at the luff. A horizontal bar at the foot of a sail is called a boom. An upward-sloping spar at the upper edge of the sail is called a gaff. The boom and gaff are movably mounted on the mast at the front end. However, there are also fore-and-aft-sail shapes in which the upper spar protrudes beyond the mast at the front, such as latin and lugger sails. These are also referred to as yards. Today, triangular Bermuda sails are widely used. These are attached to the front of the mast and controlled at the bottom by a boom, but do not have a gaff at the top.
The sails are operated using the running rigging. These are lines made of various materials, nowadays usually syntetic fibre. Halyards are used to pull something up the mast, especially sails with their associated spars, such as the gaff. Sheets are used to control the position of the sails when sailing. With square sails, the sheets are used to pull the lower corners of the sail to the yard below. The position of the sail is then adjusted with the braces. There are also a number of other lines, for example for setting and dowsing sails, controlling the gaff, etc.
How do you sail against the wind?
Sailing directly against the wind is physically impossible. The sails would not be filled by the wind, but would flap like a flag. No belly – no propulsion... So you need an angle to the wind so that it fills the sails and flows around them, sucking the boat forward. Although there are modern yachts that can sail more steeply against the wind, for many boats this angle is plus or minus 45 degrees.
However, the wind then also pushes the ship to the side. The ship therefore does not sail exactly in the direction in which the bow is pointing, but slightly diagonally away from it in the direction of leeward. This is called leeway. This means that you lose some distance in the direction from which the wind is coming. In sailing terminology, this is called “making less height".
The amount of leeway depends on the shape of the hull. To reduce leeway, yachts have a keel, which increases the lateral surface area. Since the boat does not sail exactly straight ahead, but slightly off course, this creates an angle of attack between the flowing water and the hull and keel. Similar processes take place in the water as above the water in the sails. Small, light dinghies have a centerboard for this purpose, a board that is inserted into the water through the hull to reduce drift. Dutch flat-bottomed boats have large leeboards for this purpose. In multihull boats, some of the hulls are asymmetrically constructed so that the hull is flowed around like an airplane wing. Over the centuries, sailors and boat builders have come up with various solutions to reduce drift.
When the Tres Hombres was converted from a war fishing cutter to a sailing freighter, a keel was installed along its entire length. This means that the drift on a close-hauled course is around 15 degrees. The actual angle to the wind is therefore around 60 degrees. The Tres Hombres therefore runs less high than a yacht. The Avontuur, originally built in Holland, has a flat bottom. A sailor who sailed on it as a bosun therefore estimates the Avontuur's actual course to be 80 degrees to the wind direction. As a result, it makes almost no headway. It is not alone in this: the large sailing freighters of the past could only make little headway. Sailing away from the coast in an onshore wind was and is challenging. That is why it is better to wait for wind from a more favorable direction.
The different courses
- If the wind comes from behind, you sail “downwind.”
- If the wind comes diagonally from aft, this is called a “broad reach.”
- If the wind comes from the side, you sail “beam reach.”
- If you sail diagonally against the wind, you are on a “close-hauled course.” If the ship sails at the maximum possible angle against the wind, this is referred to as a “close-hauled” course. Sailing too close to the wind is called “pinching.” This is usually not worthwhile because you then travel slower and thus the drift becomes greater.
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Image
D. Haller
A distinction is also made depending on which side the sails are on. If the wind is coming from port, you are sailing on portside tack. In the opposite case, you are on starboard tack.
Maneuvers
- Saililng into the wind: One of the key aspects of any voyage is being able to stop. The easiest way to stop a sailing ship is to point the bow directly into the wind. The sails then no longer work, but flutter like flags. And the wind helps to slow down the ship.
- Heading up: You change course so that the wind comes more strongly from the front.
- Bearing away: You change course so that the wind comes more strongly from behind.
- Tacking: If, for example, you are sailing close to the wind on the portside tack and want to change to the starboard tack, in most cases you will tack. To do this, you luff up until the ship is pointing in the direction of the wind and then heel down again on the other side. In other words, you turn the bow through the wind. After tacking, the sails are on the other side of the ship.
- Jibing: Here, too, the sails change sides. However, the boat goes through the wind with its stern. You therefore bear away until you are running downwind and continue turning until the sails are on the other side. The involuntary jibe is feared, when, due to a wind shift or carelessness on the part of the helmsman, the mainsail changes sides uncontrollably with great force. To avoid this, on larger ships, the boom is secured forward on a course before the wind with a boom preventer.
- In a controlled jibe, the mainsail is pulled in as close as possible just before the course change so that the path of the boom and the sail remains as short as possible when they change sides. After the sail has changed sides, the mainsheet is eased again and the boom preventer is set on the new side.
- When tacking and jibing, unexpected things can happen. For example, when tacking, the boat can get stuck in the waves and fall back to its old tack. Then you go for plan B instead – a jibe. Or in strong winds, you want to avoid the risks of a jibe on a yacht and tack instead.
- Heaving by: This could be described as “parking at sea.” With minimal sail area, you set one sail for propulsion and the others “back,” i.e., to brake. Ideally, this positions the boat relative to the waves so that it drifts calmly and slowly. For example, to take the dinghy on board. Or to wait so that you don't arrive at the harbor too early before you have permission to enter.