Kayak Skeg
Principles and Techniques of Use
Use of the Skeg by Robert Finlay of Kayak Lake Mead

Many paddlers have have asked me, "exactly what is the skeg for" and "how can I use it", hence this article. It's use is simple but often misunderstood.

The kayak skeg is not a rudder. It is not a steering device. It is a trimming device. With a skeg you can add variable amounts of lateral (sideways) resistance to the stern of your boat. That is a good thing, here's why...

The skeg is used to trim your kayak in winds and to correct for unbalanced loading. Its thoughtful use allows the paddler to track (go in a straight line) in the variable conditions of wind and in various loading configurations of your kayak.

A kayak will tend to turn into the wind, this is called weathercocking. This is caused by the difference in pressure between the bow and stern of your kayak, see the diagram at the top of the page. 

As you paddle forward, water flows past your kayak. At the bow, this flow is laminar as it follows the shape of your hull, gradually this flow becomes turbulant, and near the stern the flow separates from your hull completely and forms eddies.

Because of the laminar flow near the bow, the pressure is the highest, and the kayak 's ability to move laterally is the least. Because of the turbulance near the stern, the pressue is the lowest, and the kayak's ability to move laterally is the greatest.
Differential in pressures as your kayak moves forward through the water...
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skeg down
Rudder Deployed
Skeg Down
Water Flow Past a Kayak as it Moves Forward - Credit for this diagram belongs to PaddlingLight.com
Weathercocking in the Wind
Thus, the wind's effect will be greatest at the stern.

The wind will blow your stern downwind, pointing your bow upwind, this is weathercocking.
The skeg, when lowered into the water, adds resistance to lateral movement at the stern.

Since the kayak will want to turn upwind (given a neatrally trimmed boat, see discussion below) no influence is needed when paddling upwind, so skeg up.

When paddling downwind, you will need full use of the skeg, so skeg down.

In various other directions of wind, you can vary the amount of skeg for best effect.
The wind's ability to move the stern downwind is also affected by how you have your boat loaded.

A kayak loaded evenly front and back is a neutrally trimmed boat. The effects of wind will be as described above. 

However, a kayak with more of a load in the stern is stern heavy trimmed. This will help reduce the wind's effect at the stern. However, you might find your bow being blown downwind. 

In summary, with a skeg you can add variable amounts of lateral resistance to the stern of your boat to compensate for effects of wind and loading.