The fin mounts to the anti-ventilation plate... That plate should be out of the water at planing speed... If it's out of the water it's doing absolutely nothing....
He most likely needs to lower the motor.
He may need to lower it, but the fin can help depending speed and if you are turning.
The faster you go the less of an issue ventilion may become. At high speed the top of the prop can come out of the water and still work well. At lower speeds it may ventilate without a little help.
In a turn the flow over the prop changes and again a fin can help.
As I said before, there is no guaranty it will help, it's very dependent on the exact combination you're working with.
My experience is from doing this many times to fine tune the performance.
