It's a prop protector and in my actual experience the added drag will reduce performance to a major degree. So much that my dinghy would no longer get on plane with that device fitted on its 9.8 engine. The fitting was done with guidance and advice from the maker (photos were sent etc.) Read my write-up in the link below.

I'm just trying to help Blye avoid wasting his money like I did. The only possible value to that particular device is to avoid catching lines in your prop, and possibly to avoid cutting up a swimmer or marine life -- but the latter benefit is dubious at best because 1) it's not a complete cage and at anything above trolling speed the sharp edges and protruding bolts would still cause deep rips/cuts and 2) blunt force trauma to a swimmer or marine animal can be just as deadly as laceration.