Comment by nwallin

They're actually highly efficient. More efficient than screw propellers. They have very large surface areas, and move at uniform speed through the water. Screws spin very quickly at their tips, very slowly at their base, and the sweet spot in the middle is the only prime area for converting energy to propulsion. The fast moving parts generate significant drag, the slow moving parts generate insufficient forward propulsion for the drag they generate. VSPs do not have this disadvantage.

They are, of course, as you pointed out, a goddamned nightmare for maintenance. Lots of moving parts. Lots of seals. Lots of mechanical parts in contact with saltwater.

They're generally only used on tugboats, because tugboats must be able to point in one direction, but apply force in a different direction. And they must be able to change which direction they're applying force on a dime. And if and when tugboats break down -- they're already in a port. If a tugboat breaks down they've by definition not stranded in the middle of nowhere. Which mitigates the maintenance nightmare significantly.