Ever since the paddlewheel gave way to the screw propeller, there has been little fundamental change in the way that ships and boats are propelled through the water. Over the last century or so, engines and hull forms have evolved substantially, but the means of converting horsepower into thrust has remained pretty much the same: a rotating propeller accelerates the surrounding water in one direction, and the resultant change in momentum produces an equal force (thrust) in the opposite direction.
This is not to suggest that propeller designs have not become more sophisticated throughout the years. On the contrary, an increased understanding of hydrodynamic flow has spawned numerous innovations, such as five-, six-, and seven-blade configurations, highly skewed blade shapes, and other variations on the theme. But their underlying principle of operation remains the same; the variants are simply different attempts to overcome problems inherent in the propeller’s basic concept.
The fact is, a rotating propeller operates in a nonuniform flow field, resulting from the movement of the hull through the water. As each propeller blade makes a full rotation, the inflow velocity varies according to the blade’s proximity to the hull. This inevitably creates a pressure pulse on each blade during each rotation, which often manifests itself as noise or vibration. High-tech propeller configurations can minimize the problem, but they can’t eliminate it; it’s inherent in the rotating nature of the screw propeller.
Viable alternatives to the venerable screw propeller are few and far between. One scheme that has gained a relative degree of notoriety is magnetohydrodynamic (MHD) propulsion. Not a household word, you say? Well, anyone who has read or seen The Hunt for Red October has heard of it by its more popularized name: caterpillar drive. It was the super-silent propulsion system installed on the fictional Russian submarine in the Tom Clancy story. But while Clancy’s caterpillar drive is imaginary, MHD propulsion is for real.
In concept, MHD is attractive because it offers a propulsion system that requires no moving parts. It works on a principle similar to that of an electric motor or generator; that is, passing an electrical current through a magnetic field creates a force. In an MHD propulsion system, a magnetic field is created within the hull, and sea water is used to conduct the electric current, as shown in the illustrations accompanying this story. The underlying principle is known as Fleming’s Left-Hand Rule, which is demonstrated by taking the thumb, index, and middle finger of the left hand and holding them perpendicular to each other. If a magnetic field is created in the direction of the index finger while electric current passes in the direction of the middle finger, an electromagnetic force (also known as Lorentz force) will be produced along the direction of the thumb. This force acts on the conductor (in this case sea water), so by constraining the sea water within a tube, the Lorentz force will push it through the tube, creating a viable propulsive force.
This article originally appeared in the November 2006 issue of Power & Motoryacht magazine.