Ducted Propellers Could Boost Marine Wildlife Safety

manatee with scars
manatee with scars

Published Jan 13, 2019 4:04 PM by Harry Valentine

Environmentalists and animal rights activists have for long sounded the alarm about boat propellers inflicting injuries on aquatic mammals and marine wildlife. To protect people, life guards at public beaches use powered watercraft built with inboard propellers and water-jet propulsion, while recent advances have occurred in boat propulsion system layouts that install the propeller inside a duct. Canada is one of the nations to have recently enacted legislation aimed at protecting marine wildlife from injuries inflicted by powered watercraft.

Legal Action

In recent years, some jurisdictions urged by animal rights activists, have expanded the scope of the authority of the Society for the Prevention of Cruelty to Animals. Their influence has expanded into governments that have enacted various forms of animal protection laws.  

In many regions of the world, collisions occur between the bows of ships and certain varieties of whales, the result of too high a ship speed and whales not quickly responding to approaching ships. Canada’s national department of transportation has set a maximum sailing speed for ships sailing in Eastern Canadian waters including across the Gulf of St. Lawrence and into the eastern end of the Lower St. Lawrence River where whales swim. Ship captains face steep fines should their ship bow collide with a whale. Canada has set the legal precedent for other governments internationally.

The Hidden Propeller

The design profile of small personal watercraft are heavily based on the design layout of snow mobiles, with the same manufacturers producing both types of vehicles during different times of the year using on the same production lines in the same factories. Both personal vehicles share common engines, common seat layout and the same basic profile. Perhaps realizing the potential for such small watercraft to be used at public beaches, the manufactures hid the propeller inside the lower portion of the watercraft to eliminate possible contact between people and propellers. A water jet provides propulsive thrust. 

In North America’s modern legal environment, any threat of injurious interaction between propeller and beach goer in the water would have attracted the attention of litigation lawyers who would have made the manufacturer liable. Many product manufacturers have faced lawsuits and class-action lawsuits over perceived defects in the design of their products that resulted in people being injured. 

Manufacturers of small personal powered watercraft protect themselves from lawsuits by eliminating any chance of contact between watercraft propellers and people who are in the water. Over time, the threat of lawsuits has the potential to influence the future of boat propulsion systems.


Tests have been undertaken comparing the thrust performance of open propellers to propellers enclosed inside ducts at rotational speeds up to 1,000 RPM. At 500 RPM, no measurable difference occurred while at speeds below 500 RPM, the enclosed propeller delivered slightly higher thrust. However at high rotational speeds well in excess of 500 RPM, the open propeller delivered higher measurable thrust. There are two configurations of enclosed propeller with one layout including a rotating rim to which propeller blade tips were secured and another layout that involved a stationary duct. A third configuration combined rotating rim housed inside a stationary duct. 

The idea of the enclosed propeller is gaining support from numerous people who are researching and designing various designs of enclosed propeller, including a design that applied power to a rotating rim to which tips of propeller blades are attached. Some tug boats use enclosed propellers that achieve peak thrust efficiency at below 250 RPM, while some ship propellers achieve peak thrust efficiency at 75 to 90 RPM. Given the weight and size of ship propellers, a design that delivers higher thrust efficiency while combining rotating rim secured to propeller blade tips with stationary duct could gain market acceptance.

Efficiency Gain Versus Legislation

An enclosed design of propeller that delivers higher thrust and higher thrust efficiency could offer savings in ship fuel consumption. Shipowners would also want ease of maintenance and repair should they choose to switch to an enclosed propeller. A design that includes roller bearings between rotating rim and stationary duct could offer greater structural strength in the stern area, perhaps requiring a lighter weight of drive shaft to transmit power from engine to propeller. The design of the ship stern area would need to prevent large marine wildlife from coming into contact with the propeller.

Legislation represents an alternative method by which to eliminate contact between marine wildlife and ship/boat propellers. Canada’s national department of transportation has already established the legal precedent of government acting to protect the well-being of marine wildlife by enforcing severe speed restrictions for vessels sailing in waters where whales are likely to be present. Animal rights activists now have the option to use that legal precedent as a stepping stone to have legislation enacted that further protects marine wildlife, including protecting such wildlife from injury inflicted by ship or boat propellers through the use of enclosed propellers.


In the future and as a result of Canada enacting legislation on the maritime sector to protect whales from being injured as a result of collisions with ships, it is perhaps a forgone conclusion that in the future, legislation will be enacted seeking to protect marine wildlife from injury inflicted by boat and ship propellers. 

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.