The ADDARUDDER is not a duel blade rudder.
Addarudders were single ruders.
they had a fitting that went on the front of the outboard or outdrive and then two rods that went over the cavitation plate to the rudder assembly.
Do they work ? yes very very well.
Do they lower performance ? not unless your looking at microscopic differences.
Acceleration if anything is better with one, as more directional thrust.
I had one fitted on a boat that done 42 mph without it.
when fitted it still done 40 to 41 mph and the steering improved ten fold.
The boat became much safer and more usable at slower speeds.
At slow speeds it was fantastic, even when not under power i had steering.
You do not get any wander at slow speeds like all outboards and outdrives give.
At high speed the steering is also much more responsive.
Many people had these fitted to boats on the thames during to 70's.
The twin rudder set ups were not as good and a lot of them fell off or got damaged too easy.
they were very weak compared to the original addarudders.