What if the "tube shroud enclosed the spinning blades" gets stuck with wildlife? I think the Idea is good but do they realy kill so much plants and fish? (Just asking, I don't know)

Of course everyone that has a jet-boat or a PWC already knows the advantages of the enclosed prop. I suppose in a low-speed situation where you want less turbulance/noise (like trolling) it's worse to duct or jet the motor, but generally, a jetprop is better anyway. But just like a ducted propeller in the sky, they increase efficiency to a point, but after the sweet-spot, they are less efficient. And jets (not ducts) are easily damaged by sand or gravel.

I'm sort-of surprised that normally not-political sand is saying 'there ought to be a law'--a State Law? A US law? A Finland law? a UN law? Does it just apply to new boats, or are we going to make poor Otis with his 1956 Evinrude retrofit? I prefer voluntary movement toward conservation based on knowledge--not laws.