Pinch & Jet Ship

Ship propellers, churning out their wakes with magnificent forcefulness. look pretty efficient. But John H. MacMillan Jr., president of Cargill, Inc. of Minneapolis, suspected them of churning a little too turbulently. Since Cargill is a grain firm, deeply involved in water transportation, MacMillan decided to save his company some money by improving its ships' propellers. Last week he described a system that he believes gets more propulsive effect out of a ship's engine.

Key to the system is two deep grooves built into the hull near the ship's stern (see diagram). They converge toward the propeller, and when the ship is in motion, the propeller sucks streams of water from both grooves. As the streams move together, they exert pressure on the wedge-shaped section of hull between them and "pinch" it forward, rather as a watermelon seed is pinched from between two fingers.

Then the streams pass through the propeller, and since they are still converging, they combine into a well-defined jet that has a superior propulsive effect. A steel plate below the propeller keeps it from drawing water inefficiently from below. MacMillan says that his system has been tested on two Cargill towboats and that careful comparison with conventional equipment proves it 20% more efficient.

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