Sailing Physics Simplified

Charles Stone

2025

Outline

• Types of Sails
• Square rig, Lanteen, Gaff rig, Bermuda.
• Basic Physics
• From wing to sail.
• Forces on the Boat
• How does all this work?
• Seeing the wind
• Telltales can tell you a lot.

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Types of sails

• Square Rig
• Around 3000 BCE in Egypt.
• Nile flows north, the wind is to the south.
• Great for sailing downwind.

Types of sails

• Lanteen Sail
• Around 200 CE
• A more cost effective sail than square rigs.
• Did not provide much improvement on performance over the square rig.

Types of sails

• Gaff Rig
• Around 1500 CE
• More efficient than square rigs or Lanteen sails.
• Large sail area in relation to mast height.

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Types of sails

• Bermuda Rig
• Around 1600 CE
• More efficient than square rigs or Lanteen sails.
• Simplified version of the gaff rig.
• Increased performance of sailboats.

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The physics

• Bernoulli's Principle
• Around 1738 CE
• Bernoulli deduced that pressure decreases when the fluid (air) flow speed increases.
• When fluid (air) is flowing horizontally from a region of high pressure to a region of low pressure, there is more pressure behind than in front.

The physics

• Air Foils
• Airplane wing dynamics.

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• The air above the wing has to speed up so that it can meet the air below.
• Creates low pressure above the wing.
• This causes a lifting force so that the wing is ‘pulled’ upward.

The physics

• From wing to sail
• A sail works like an airplane wing turned on end.

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• The air flows around the sail very much the same as a wing.

Sailing Points of Sail

• Adjusting the sails allows a boat to sail at almost any direction to the wind.
• The most difficult direction to sail well is close hauled.
• Requires more attention to trimming the sails.
• Requires more attention while sailing.
• Today we will focus on close hauled sailing.

Parts of the sailboat

• Hull
• Keeps the boat floating.
• Keel /Centerboard
• Provides stability and resistance in the water.
• Rudder
• Counteracts the twisting force of the sail. Directional stability.
• Sail
• Provides the driving force from the wind.

Sail

Rudder

Keel

Hull

The Balancing Act

• We need to balance the forces.
• Wind puts pressure on the sail causes heel.
• The boat weight and ballast counteract the heeling force of the wind.
• Ballast can be either a heavy keel, or crew weight on the rail.
• It is a constant battle.

Wind Pressure

Boat Weight

Ballast

The Balancing Act

• How do we create forward motion upwind?
• Wind on the sail creates a side force and lift.
• Keel counteracts sail side force and creates lift.
• Rudder counteracts the twisting and creates lift.
• Boat direction is always a little off center.
• The boat is forever fighting itself.

Boat direction

Rudder side force

Keel lift

Rudder lift

Keel side force

Sail side force

Sail lift

Wind

How do we ‘see’ the wind?

• Flags, wind vanes and telltales.
• A flag or wind vane can show you the direction of the wind.
• Telltales on the sail show you how the air is flowing across the sails.

Using Telltales

• Trimming to the telltales.
• When the air flow is attached to the sail, the telltales will stream back on both sides of the sail.
• If they dance, then there is some separation.
• If they fly up or forward, then they are stalled with no wind attached to that side of the sail.
• Why do we care?
• Provides better performance and it looks better.

Wrap Up

• Understanding the forces
• Once you have a better understanding of the basic forces that drive the boat, it gives you the knowledge to adjust for better performance.
• Feel the boat
• Feeling the rudder pressure lets you know if the forces are balanced.
• Keep an eye on the sails
• Don’t forget to watch the telltales. They can tell you a lot about what is happening with the wind.
• It will add to a more enjoyable day on the water.