On your setup there Armody I don't think a washer is necessary. However, out of experience [:@], I found out why I will always use a washer. One day after taking of the cowl to clean the engine, I forgot to put back the washer when putting the prop on. As I tightened it, I found that it still goes and goes - when I looked down for inspection on what was going on - I was starting the turn the nut right into the 'plastic' of the prop. The washer stops this from happening aswell.