Good morning Arthur, 

Scroll down the pictures of the Bow Thruster illustration: 

http://nikimat.com/bow_thruster_overhaul.html

I once scuba dive to replace the propeller, sadly to discover later that the hub holding the propeller was wobbly… because of elongated holes and end up having to overhaul it

http://nikimat.com/bow_thruster_lost_propeller.html

Other informations:

http://nikimat.com/bow_thruster_spin.html