How does a Boat Propeller Work?

Dale Marshall

A boat propeller works by pushing water, which propels the boat in the opposite direction. More precisely, it transforms rotational energy into thrust; thus, the turning propeller increases the water pressure to its rear, and moves into the area of reduced pressure in front of the propeller, taking the boat with it. The forces involved are consistent with Newton’s third law — for every action, there’s an equal and opposite reaction. That is, when the boat propeller pushes the water backward, it propels the boat forward.

Boat propellers transform rotational energy into thrust.
Boat propellers transform rotational energy into thrust.

Before the development of powered engines, marine propulsion was limited to wind power and manual rowing or paddling. Rowing and paddling generally uses blades to drag a boat through the water, but one use of a blade, called sculling, acts most like a propeller because the sculler pushes the water to the rear of the boat. A sculler stands in the rear of a small boat and, using a single oar, will sweep it through the water in an arc more or less perpendicular to the boat’s direction of travel, with each sweep of the arc twisting the oar so that the blade is at an angle of 30 to 60 degrees to the direction of travel.

The blades of a propeller are twisted from a parallel plan to one nearly perpendicular to the propeller's shaft.
The blades of a propeller are twisted from a parallel plan to one nearly perpendicular to the propeller's shaft.

The concept underlying the boat propeller was developed by Archimedes in the third century BC. He developed a device called the Archimedes Screw, which is still used today, that transports water from lower to higher elevations. Very similar to a modern auger in design and operation, it is still in extensive use worldwide for a wide variety of tasks, including irrigation and waste management. Similar screws were experimented with for marine propulsion. The famous one-man submarine Turtle, which attempted to sink British ships in the New York harbor in 1776, employed manually-operated screws for propulsion.

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Powered propulsion became available for maritime craft with the development of the steam engine in the 18th century. The trend was initially to use large paddles to provide thrust, as on the stern-wheel and side-wheel steam paddleboats. Inventors continued to experiment with screw-type propellers, which were literally very large, very long screws. In 1835, part of such a screw broke off during testing, leaving a portion that looked much like a modern boat propeller blade. This broken screw proved to propel the boat faster than the conventional screw, leading to the development of the modern boat propeller.

The dynamics involved in marine propulsion are very similar to those employed for aviation. For example, like the blades of an airplane, the blades on a boat propeller aren’t flat, but instead appear to have been twisted from a parallel plane to one nearly perpendicular to the propeller’s shaft. This phenomenon is based on research by the Wright brothers in the early 20th century, in which they determined that the optimal angles for thrust are different at various parts of the propeller’s blade. To enhance the propeller’s efficiency, the blade is twisted relative to the shaft.

Mankind has been traveling on the water for thousands of years, most of that time relying either on wind or muscle for propulsion. Propellers are a very recent development in the field of maritime propulsion, but in that small fraction of man’s maritime history have risen to a level of unchallenged predominance in the field, with no real challenger in sight.

A boat's propeller pushes water, which creates pressure.
A boat's propeller pushes water, which creates pressure.

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