Duoprop

Duoprops are just what their name would suggest--counter-rotating twin-propellers used in place of single-screw installations. Studies by Volvo Penta and others have indicated that two propellers are indeed better than one. Why, however, is this the case? Many of the major advantages of dual-propeller systems come from 1) a reduction in "propeller slip," and 2) the reduction in cavitation-related inefficiencies.

Propeller slipping occurs when a single propeller churns the water. A propellor moves in much the same way a screw does. However, while a screw has a solid environment (such as wood) in which to work, a propellor must deal with a less-resistant world of water and, sometimes, air. The inefficiency created by non-uniform resistance to each propeller turn decreases both fuel efficiency and speed. The degree of propeller slipping is directly proportional to the pitch of the propeller.

When a propeller moves through the water, the pressure on the reverse side of each blade becomes less. At some point, the pressure becomes so low that the water begins to "boil" and bubbles of steam are formed. This cavitation, and the air and bubbles from it, makes the blades of the propeller act as if they are thicker and increases the resistance of the water to the propeller blades. This not only decreases efficiency, but over time can cause damage to the actual blades as the bubbles travel to where the pressure is higher and implode.

Duoprops solve the problems of propeller slipping and cavitation through the presence of a second counter-rotating propeller that allows for decreased pitch in each of the two propellers and helps to break up the formation of the air and steam bubbles. Two propellers working in tandem are able to deliver better fuel efficiency (by 15 percent) and increased accuracy when moving in reverse. Thrust and acceleration will increase at slower speeds (by 15 and 20 percent, respectively), and top speeds will also increase thanks to reduced propeller slipping. Duoprops also suffer less cavitation-related damage.

Installing Duoprops
Here at Marine Parts Express, we specialize in helping our customers to choose and install duo-props--many of which cost only a few hundred dollars more than single props. Many sterndrives can be converted to dual-propeller systems with the use of the right equipment and components. (Keep in mind, however, that larger engines are needed when using dual propellers, 4.3L being the bare minimum.) We also offer a number of complete sterndrive setups that feature dual propellers. Our customer service team here at Marine Parts Express can give you thoughtful advice about brands, models and maintenance for marine propulsion systems. To find out more about the properties of different propeller configurations, contact us via phone (877-621-2628) or e-mail (info@marinepartsexpress.com).