Good morning oricine.eu, unfortunately I can't answer your questions, but for those interested in the matter of counter-rotating propellers here is a link that could give some explanations to you http://www.nauticexpo.com/prod/veth-propulsion/product-31774-475121.html

Best regards Hans

Why does the one propeller have 5 blades, while the other one only four? One or going forward the other for going aft, or a question of the whirl of the 5 blader?