How do sailing ships work?

Question

If the wind is blowing in one direction, how does a sailing ship travel in any other direction? It would seem that it could only move with the wind.

Answer 1

Bigpathome, there's definitely a pressure difference across a sail, just as with an aircraft wing! Air flows faster round the outside due to the convex shape of the sail, and slows due to the concave inner surface of the sail.  Faster air is at lower pressure than slower air (along a streamline, Bernoulli's Principle - overly simplified, but it works) and the pressure imbalance 'sucks' the sail downwind. One of the differences though is the angle of attack - an aircraft wing is moving with high velocity relative to the air, so the flow is more or less parallel to the wing. A sail will have an AoA of 45 degrees or greater (I'd love to find a boat that could sail at 15 degrees to the wind! Funnily enough high performance dinghies, which will get upwind very fast, actually point lower, but the additional speed they have more than compensates for the extra distance travelled) Another key issue is the centreboard or keel - this is a foil that is below the boat and counteracts leeway (the boat moving sideways). This foil is symmetrical, but due to the AoA of the water it generates lift to windward. The effort to try and heel the boat (i.e. the mast towards the water) is counteracted either by a weighted keel or the weight of the crew. With these forces acting together the boat will have a net force driving it forwards - the bar of soap analogy. Ideally the Centre of Effort of the rig and the Centre of Resistance of the hull/keel should be close to each other - otherwise the tendency is for the boat to turn about the keel, which would require corrective rudder input, creating extra drag (this is known as lee or weather helm). I have seen a different approach - I think it was a 40ft catamaran, with a wind turbine mechanically linked to a large propellor. The turbine was mounted at the top of a mast and could be rotated so it was at the optimum angle to the wind whilst sailing in any direction. I think one of the major limitations (other than cost) was that the large diameter of the prop restricted where it could sail, in terms of marinas etc. I understand I've probably not explained this very well, but there's some good animated diagrams on how a sail works around on the internet which are far easier to understand. It's a very complex area and some theories which had been accepted for a long time have shown to be without basis - especially when multiple sails are involved (slot effect etc.). I don't think there's many people who would claim to entirely understand all the factors, though Bethwaite's High Performance Sailing is meant to be one of the most in depth books on the subject (I've not had a chance to read it, but it's written by a prolific Australian boat designer, who's son designed the Olympic 49er skiff, along with many other influential boats).

Answer 2

The previous answerers have listed some of the forces that power a sailboat.

Previous answerers were quite close, but you really need to forget the analogy with an airplanes wing.

If you think about it, an airplanes wing works because, in cross section, it's curved on the top and flat underneath. This keeps two airflows apart (one on top -one below) and makes a pressure difference that gives lift to the wing.

A sailboat's sail in cross section is curved on the top and curved underneath (because the fabric of the sail is only millemetres thick) - so nothing like an airplanes wing!!

A sailboat sails because it has a sail which catches the wind and pushes the boat to one side. It also has a keel in the water that provides an opposite force. So it's better to describe the action as if you hold a wet piece of soap between two fingers, if you squeeze your fingers together the soap shoots forwards.

Technically, the sail is attached to the mast, the mast and sail's centre of effort (CE) is ahead of the hull and keel's centre of resistance (CR), therefore the boat sails forward within 45 degrees of the direction of the wind (it obviously can't sail directly into the wind, but 12-15 degrees either side of it (on a good day)).

All the best bigpathome.

Answer 3

Except when you're sailing directly down wind your sail acts just like an airplane wing and generates lift. Your angle of attack between the sail and the wind doesn't change much but the hull is more or less rotated about the mast. The keel or center board do the same thing in the water and help redirect the thrust from the sail to the direction that the boat is pointing. Without the fin in the water you would only be able to sail pretty much down wind.

Answer 4

Adding to the above answer, we tack if the wind isn't in our favor (gotta love when it's coming strait at you from your destination). Basically that means we travel at angles back and forth to reach our destination like so (just an illustration to give you the general idea):

      /

      \

      /

      \

This allows us to catch that little bit of wind (as described by the first answer) in order to keep moving and yet still get to where we want to go.

Answer 5

The sails generate two forces. Drag which takes you downwind, and lift from the flow of air over the curved surface of the sail. Balancing the two, and the resistence of the boat to them, allows you to sail to within about 45% either side of the direction the wind is blowing from.