The Finnsailer 35 is equipped with a pair of Bilge Keels as shown in the photo.
But, what is the purpose of Bilge Keels?
The explanation is below the photo.
A bilge keel is used to reduce a ship's tendency to roll.
Bilge keels are employed in pairs - one for each side of the ship. A ship may have more than one bilge keel per side, but this is rare. Bilge keels increase hydrodynamic resistance to rolling, making the ship roll less.
Bilge keels are passive stability systems.