“DECEMBER 2, 2016 — Wärtsilä EnergoProFin is now available for controllable pitch propellers (CPP) as well as fixed pitch propellers (FPP).”
“The specially designed propeller cap has hydrofoil-section fins on the downstream side of the propeller, creating fuel savings of up to five percent.”
They claim the addition pays for itself in one to two years.
From our perspective, it should also increase the range of the ship, and may very slightly increase maximum speed.
Chuck Hill's CG Blog 
MAN claim their Kappel designed propellers will increase propulsion efficiency by 3-6% and lower noise.
The tip of propeller is turned up sharply to reduce flow and generated vortices coming off end of propeller tip by reducing the difference in pressure between the pressure and suction sides of the propeller. The pressure on both sides near the tip will therefore be equalised reducing the cavitation, pressure impulses, vibration and noise. Applicable to both FPP and CPP.
Marketed it as suitable for submarines and navy ships.
http://primeserv.man.eu/docs/librariesprovider5/propeller-aftship-brochures/propeller-upgrade/kappel-propellers_2016-06-27_screen-1510-0229-00ppr_.pdf?sfvrsn=6