The Physics of Sailing

The vessel moves because the sails catch part of the wind, send the energy down to the hull and push the vessel forward. We plan to explore the physics of sailing in detail by analyzing the forces that are exerted upon the vessel that make it move.

The Driving Force

This is caused by the wind flowing across the sail, mostly the main sail and the spinnaker. To make good use of this force, it is important to keep the sail at a small angle to the wind – this is why constant trimming is crucial for optimizing the driving force. The angle requires simple adjustments.

The forces around the mask are both the driving force and the sideways force. These two forces are vectors that sum up to the total force – the force that moves the boat. This movement occurs in the forward direction mostly, but should also move a bit sideways.


The Sideways Force

The sideways force is also generated by the wind when it does not hit the sail head on and pushes the vessel sideways. As the force that the wind presses into the sail does spread not only into the forward-direction (driving force), but partly also off the forward-direction, sideways forces are generated. The faster the boat is sailing, the smaller the sideways force. It becomes more apparent if the sail is pulled in too far. If you are sailing on upwind courses, you will always have to consider the effects of the sideways force – the drift off your steered course due to the sideways force is called leeway.

When the wind is coming from the side

It is easy to understand how Sails work when the Wind is coming from the back. When the Sails are let out, the Wind pushes them, driving the boat forward. If this is the case, then the Sailboat will move sideways if the Wind is coming from the side. But this can be avoided and the boat can move forward through the use of the Centerboard or Keel. It resists the sideways force, therefore preventing the Sailboat from moving sideways. Keep in mind that the Keel should be well-shaped to resist the sideways force.

external image sideways-force.gif

The Heeling Force The heeling force is the force of the wind that approaches the boat parallel to the direction of course. This causes the boat to tip sideways, and often the passengers must lean to the high side in order to keep the boat from capsizing.


This is one of the most dangerous forms of sailing, and it is important to know how to counteract heeling. Some of these procedures are listed below:
  • Raising the centerboard to increase leeway
  • Have a weighted keel if you know you may be experiencing the heeling force on a frequent basis.
  • The crew should move to the upwind side of the boat to balance the weight. This action is known as hiking.
  • Turn the boat upwind.
  • Loosen the sail so that wind can be "spilled out".
  • Reduce the sail area. This is known as "reefing".

Conclusion Now you should understand how the three forces we discussed above come into play – driving, sideways and heeling force. Every boat has a center of ffort, a spot at which all forces act. This spot is most likely somewhere in the mast. Note that lots of very large boats can sail faster than the wind; they use relative wind speeds (relative to the moving boat, that is) that occur when sailing at an angle to the wind. In that case, the friction between the hull and the water limits the speed of the boat.

That much about the theory of sailing. However, the more important part is the practice – once you have routine in your moves, twists and turns, you will notice that you won’t think a lot about vectors, forces and angles anymore.

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