New To Float Flying Question ?
 

On some float planes there is a downward vertical fin under the fuselage. What is the purpose or function of this fin? Thanks in advance.............

New To Float Flying Question ?
 

Known as a Ventral Fin, it makes up for the yaw instability created when floats are attached. The instability comes from more side area being added in front of the CG than aft of it, so the ventral fin makes up for that imbalance.

New To Float Flying Question ?
 

It is called a ventral fin. It enhances lateral stability due to the pendullem effect caused by the floats. If you look at the original Beaver configuration, it had a ventral fin. Kenmore Air Seattle STCd the the small finlets attached to horizontal stabilizer. These tend not to get smashed and bent around a high dock. Some aircraft such as the Cessna Caravan have finlets, and an enlarged dorsal fin and rudder.