Lift

Lift is the force keeping an airplane up in the air or in our case moving our boat forward. Generally lift is created when there is a pressure difference between two sides of an object, this difference in pressure can occur naturally or artificially generated like on the wing of an airplane.

According to Bernoulli’s principle (which is valid for any form of flow: gas and liquids) when flow speed is increased the pressure generated on the surface is decreased and vice versa.

Due to the shape of the wing the air on one side of the wing flows faster over the surface of the wing then the air on the other side (flat or concave side) of the wing.The pressure on the flat side of the wing is higher than the round side of the wing. In nature there is a tendency of evening up pressure and concentrations, gas or liquid will flow from high pressure to low pressure, in the case of the wing (or the sail) the air will try to move the body from the higher pressure towards the lower pressure resulting the wing being pulled upwards. Remember it has to be stronger then gravity in order to actually be able to lift the wing up in the air, the faster the airflow over the wing the bigger the pressure difference will be hence a stronger lift is created. Every bit of the wing will be pulled at exactly 90 degrees to its surface, so if there is a curve in the surface it will be pulled in various directions.

The same principle applies to our sail as we create a pressure difference between the windward side of the sail and the leeward side of the sail resulting in the fabric being pulled towards the leeward side.

The sail area is divided in to areas, the leech may create drag, drag is either a turbulence or a pull backwards resulting a reduction of speed in our case. Drag is reduced by setting the shape of the sail (or the design) to have a flat or open leech.

The center part of the sail will lift in the direction sideways to the boat direction of movement, this part is being eliminated with the center board preventing the boat from drifting sideways.

After minimizing the negative lift we are left with the front part of the sail who is pulling the boat in the desired direction, the depth of that part will determine how fast the boat will move if airflow remains attached.

In most classes the foils also have some sort of wing shape, round in the front and flat in the back. That also create some lift and influence the speed and height we will be sailing.

At TopLevel Sailing we provide the relevant overview of the science behind the feel of the boat to help our sailors better understand the interactions in their boat and allow them to maximize their speed and abilities