(Than open propellers)? I have seen some new ship designs, where the propellers are enclosed within a duct-does this make the prop more efficient? Or is is a control thing? What is the maximum efficiency of a propeller?
A pic would be helpful, as it could be one of a few different designs. For instance, some propellers are enclosed in a ‘Kort Nozzle’, which replaces the standard rudder system. The nozzle moves around a fixed prop, thereby directing thrust in the desired direction. Here’s a picture. Or it could be a Z-drive system in which the propeller and shroud rotate as a unit for directional thrust. Picture.
Some props are shrouded to protect them from underwater hazards.
To answer some of your OP - Z-drives systems have much more control over the standard prop/rudder set up. I don’t know much about Kort nozzles, nor prop efficiency.
No, the ducts make the beanies too heavy to wear.
Kort nozzles are more efficient than open props at low speeds, and under high loadings. As speed increases, the drag caused by the nozzle itself tends to outweigh the efficiency gained. Kort nozzles also provide some protection against grounding damage for obvious reasons.
Kort nozzles are widely used where high efficiency at low speed and high load is required. Trawlers and tugs particuularly. I also know of them being used on smaller ships operating in remote locations without tug assistance, to assist with low speed manouevering into berth.