Sensational Subchaser

Nazi U-boat commanders have good cause this week to cry: Achtung! (Look out!) and crash-dive. Coming at them at hitherto unattainable speed is an improved type of U.S. subchaser powered by an extraordinary new engine and driven by an extraordinary new propeller.

The engine is a diesel only one-third as big and only a fraction of the weight of the best former diesels of the same horsepower. The propeller is the first marine screw whose blades can be adjusted to any angle or can be completely reversed while the boat is in motion. Results:

1) increased power, since the most efficient ratio between boat speed and engine speed can now be constantly maintained;

2) greater maneuverability, since the boat can be stopped on a dime, turned on a quarter. Hitherto, variable pitch propellers have not been practical for any boat larger than small launches.

For 25 years the U.S. Navy has been praying for just such an engine. Charles Kettering's General Motors engineers answered the prayer. The engine, a two-cycle diesel, is now being produced by an Illinois factory. Most diesels are four-cycled--three out of four piston strokes serve only to charge the cylinder for the power stroke. But in the two-cycle diesel, every downstroke has an explosive push behind it. Thus the weight of the two extra cylinders can be eliminated. Two of the lighter engines can now be installed where one was formerly used, and a lot more fuel can also be carried. Hence the new subchasers are not only twice as powerful as the old type, but have an increased cruising range.

The variable pitch propeller was necessary because the two-cycle diesel cannot be reversed, without heavy reverse gears. Like the light diesel engine, variable pitch marine propellers have baffled naval engineers for many years. Reason: the hub mechanism, which changes the pitch of the blades, broke down whenever the propeller churned the water vigorously. Design of the tough, new propeller, which is electrically controlled, is a military secret.

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