Along with dihedral, sweepback, and weight distribution, keel effect is one of the four main design considerations in aircraft lateral stability.
[2] Examples of sideforce-generating surfaces are the vertical stabilizer, rudder, and parts of the fuselage.
Keel effect is the contribution of these side forces to rolling moment as sideslip increases.
It reduces the tendency to diverge to a greater bank angle when the aircraft starts wings-level.
The effect is an important consideration in seaplane design where pontoon floats generate strong sideforces with a long moment arm.