How to use stern fins
1. To mount or not to mount?
If you have a windvane with its own auxiliary rudder (Fig. 1), you do not have to mount stern fins. Simply block the main rudder and leave the steering to the self-steering and auxiliary rudder. A boat with the main rudder blocked is strongly course-stabilized. Now the blocked rudder acts as stern fins.
If your windvane steers the boat using the main rudder (Fig. 2), mount stern fins. It does not matter what type of self-steering it is. The fins will help the self-steering and allow it to carry large sails for longer. The yacht will hold its course better. Most importantly, when sailing with the wind and a large wave, the fins will minimize the risk of the boat being sideways to the wave.
2. When we raise the fins up?
a. When we steer manually and we have no problem with it.
b. When sailing at low speeds (this is in regattas, tourists can always have the fins down when sailing with the wind).
c. When sailing upwind, you not use the two fins. We use lower the leeward fin partially or completely.
d. When we perform tight maneuvers. The fins interfere with maneuvers. They cause the radius of circulation to increase significantly. For example, when entering a port, the fins should be raised.
3. When we lower the fins down?
a. When we sail with moderate or strong winds guided by a wind self-steering. The fins will help to hold the course. This is important when we want to carry large sails.
b. When the wave is high and there is a risk of the yacht being set broadside to the approaching breakers. Fins prevent you from going off course. In a storm, they can save you from a yacht capsizes.
c. When you steer manually for a long time and are tired. Fins will help you keep the course.
If you want to see how course stabilization works, you can conduct a simple experiment.Take a motorboat or a yacht with a large engine. Hook a small centerboard boat (e.g. Optimist) on a long tow. Raise the centerboard and rudder.
When you sail at a low speed, e.g. two knots, the towed boat sails evenly behind you. When you increase the speed, the boat starts to S-bend. Once to the left, once to the right. When you continue to accelerate, the boat goes crazy. You tow it by the bow and it positions itself perpendicularly to the towing rope.
Now the second tow.
The centerboard is still raised. The rudder is put on, lowered down and fixed rigidly in the axis of the boat. We tow at a low speed - the boat sails straight behind you. We increase the speed - the same thing. The boat sails like it's on rails. The locked rudder acts as a fin that stabilizes the course.
This is how course stabilization works. It is beneficial and effective. The higher the speed, the more it is needed.
You can read more about course stability on blog https://setkaatlantyk.blogspot.com/ entry from February 27, 2013.
