When you back up a boat that has a single propeller, the propeller not only pulls the boat backward, but it also pulls the boat in the direction the propeller turns. When you reverse a propeller that turns toward the right side of the boat, the boat's stern is somewhat pulled to the right. Volvo claims the Duoprop system solves this problem and called its 1982 introduction, "the biggest event in boating since the propeller was invented."
Shaft Within a Shaft
The Duoprop system uses two propeller shafts -- the first hollow, with the other shaft inside it. The two propeller shafts are geared to turn in opposite directions, counter-rotating the propellers which are situated one in front of the other. The idea behind the Duoprop is that two counter-rotating propellers cancel out each others' lateral motion and transfer more of the engine's power directly astern or directly ahead.
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