Supercavitation projectiles potentially have 200,000 less drag than a regular object in water

Here is a summary of supercavitation an article from Caltech written in 2001.

For ships traveling faster than 60 miles per hour, propeller-induced cavitation is unavoidable. Supercavitation offers a solution.

In supercavitation, the small gas bubbles produced by cavitation expand and combine to for mone large, stable, and predictable bubble around the supercavitating object. The bubble is longer than the object, so only the leading edge of the object actually contacts liquid water. The rest of the object is surrounded by low-pressure water vapor, significantly lowering the drag on the super-cavitating object. Modern propellers intentionally induce supercavitation to reap the benefits of lower drag.

A super cavity can also form around a specially designed projectile. The key is creating a zone of low pressure around the entire object by carefully shaping the nose and firing the projectile at a sufficiently high velocity. At high velocity , water flows off the edge of the nose with a speed and angle that prevent it from wrapping around the surface of the projectile, producing a low-pressure bubble around the object. With an appropriate nose shape and a speed over 110 miles per hour, the entire projectile may reside in a vapor cavity.

Since drag is proportional to the density of the surrounding fluid, the drag on a super-cavitating projectile is dramatically reduced, allowing supercavitating projectiles to attain higher speeds than conventional projectiles. In water , a rough approximation predicts that a supercavitating projectile has 200,000 times less skin friction than a normal projectile. The potential applications are impressive.

Water has 1000 times more drag than air. Supercaviation has the potential for an enclosed object in water to attain higher speed. The speed of sound is 5 times higher in water than in air.

Read more »