There have been several questions about prop nozzles since we posted the photos of this Norwegian trawler. So, to get the most up-to-date information we asked naval architect and propeller expert, Ed Hagemen, for his experience with these.

Ed’s comments follow.

The propeller “shroud” is probably a “ducted propeller”, aka “Kort Nozzle”
and a few other similar names. Trawlers, Mississippi River tow boats (more
accurately “push” boats, and other tugs intended for heavy duty, relatively
low speed towing commonly use them. Harbor tugs occasionally use them but
they will sacrifice maneuverability.

At low vessel speeds, (well below 10 knots, like say 5), the nozzle itself
will develop quite a bit of thrust that adds to the prop thrust. Ideally,
you might get a 30% augment. As speed builds, the parasitic drag of the
nozzle eventually becomes a handicap rather than an assist.

A section through the ring is a heavily cambered aircraft wing section, with
the cambered side pointed INWARDS. The opening (front) is an appreciably
larger diameter than the exit. The prop diameter is basically set at the throat (smallest D) and sees water speeds that are significantly faster than
the boat speed. I did a pair once with 15 foot nozzle openings swinging 12
foot props. With 136 Shaft RPM and 2800 HP/side, I want to tell you that
thing moved a LOT of water when at bollard.

Total Bollard thrust was in the order of 170,000 pounds, it could have been over 190,000 were it not for the compromises necessary to meet the
requirement to be as equal as possible ahead and astern. “Normal” nozzles
back down very poorly.

There are a couple “patent” type articulated rudders commonly seen in Europe. One is the “Becker Rudder” which is pretty rare out here in the
“Colonies”. We have a lot more room in our harbors.

Ed H