Sail and trim
Most people can easily understand that a sailboat can sail downwind. Any artefact that can float is able to do this. In simple terms, the wind pushes the sail and drives the boat forwards.
But how can sailing boats sail forwards with the wind coming from the side or even obliquely from the front? In order to understand this we have to look at the form of the sail.
A sail is sewn with a certain curvature so that seen from above it resembles an aircraft wing – and it also functions in the same way. When the wind hits the luff, the front edge of the sail, the stream of air is forced to change direction and speed.
According to Newton's third law, an object A (the sail), which influences an object B (the air) with a force, will be subject to an equally great force in the opposite direction.
This force acts on the sail, and therefore the boat, at right angles to the sail in the effective centre of the sail, which is a point about ¼ of the distance from the front of the sail and, accordingly, obliquely forward relative to the direction of the boat.
If this force is resolved into a forward and a transverse component it can be seen that it makes a contribution to the forward motion but predominantly pushes the sail in a sideways direction, i.e. heel over the boat and causes it to drift sideways.
The fact that the boat does not simply move in the direction of the force is due to the boat extending down into the water where it "stands fast" in the water with the aid of, depending on the type of boat, a more-or-less prominent keel. When the boat begins to move through the water with the aid of the wind's force, it will do so diagonally so that the keel causes the water flowing past to change direction in the same way as the sail influences the wind but in the opposite direction. In this way most of the thwartships component of the sail's force is cancelled out; that which remains is termed the leeway or driftage. The faster the boat sails, the smaller the leeway.
Trim and balance
On a longship such as the Sea Stallion, it is of crucial importance to be aware of this longitudinal trim due to the long leverage effect and the 65 free souls (and wills) on board. Even if only a few crew members make an unplanned movement from their places this can have a dramatic effect on the trim of the ship. This applies of course also in the transverse direction where the living ballast can be used to counteract heeling over and keep a particular trim in increased wind strength. Therefore the rule at that time was that the crew on a longship must never be less that ¾ of the full crew.
So that the boat cannot turn up against or away from the wind it is important that the sail centre and the lateral centre, the centre for the influence of force on the underwater hull, lie vertically above one another. Otherwise the two forces will attempt to turn the boat about an axis and the boat becomes "hard on the helm" – it is necessary to compensate constantly for the boat's tendency to turn. If the cargo or the crew are moved too far forward in the boat the transverce force of the water is also moved in the same direction and the boat turns up against the wind. It is said to have become luvgirig. Conversely, if the weight is moved towards the stern the boat becomes lægirig.
The tack is always the forward lower corner of the square sail and when tacking or changing tack it is shifted quite literally from one side of the ship to the other. Port tack and starboard tackmeans that the tack has been secured to the side in question (which is the side the wind is coming from). Even on modern vessels, where use is not made of a square sail with a loose tack, the term starboard and port tack are used to denote how the vessel is sailing relative to the direction of the wind.
When a ship is turned relative to the wind to such an extent that the sail has to change sides, this is referred to as a turning. With a square sail this can be done in two ways: With the wind from behind the term is veering. In this, the sail is moved round at a leisurely pace as the ship turns. The sail remains full and pulls during the whole manoeuvre but if the boat is in the process of beating up against the wind, it costs a full downwind turn before the boat is in place on the new course.
1. The boat beats on starboard tack (with the wind coming from the right in the direction of travel).
2. The boat pays off to port, still with the wind from starboard.
3. The wind is now coming from astern, the sail is slowly turned and the boat continues turning towards port (on the left in the direction of travel).
4. Sail and boat have now been turned and the boat is able to beat once more, this time with the wind on the port tack.
If, however, the boat turns up against the wind, this is termed tacking. Here, the ship is turned into the eye of the wind, i.e. when the wind comes directly from the front. The sail then begins to fill from the "wrong" side and while the ship turns further round to its new course, the sail is swung round. Tacking requires speed in the ship and was not always successful with the long-keeled Viking ships. The manoeuvre cost, as a rule, some metres of backwards sailing while the sail backs and the ship passes through the eye of the wind.
Tacking and beating
1. The boat beats on the starboard tack.
2. The boat lies head to the wind and the sail is set aback by the wind coming from dead ahead.
3. The sail backs the boat until the wind is on the port side. At this point the sail must be quickly swung round so that the wind is able to drive the boat forward again.
4. The sail has been swung round and the boat is brought about, now with the wind on the port tack.