The possibility of providing over-pressure protection to a submarine hull from an underwater explosion is well worth exploring. A finned design, it is observed, causes an incoming transient pressure wave to strike a submarine pressure hull at two different times due to the different speeds of sound in the fin and in seawater. The pressure wave interferes with itself, thus reducing the peak intensity of the incoming pressure wave when it impacts the submarine pressure bull. By causing a 50 microsecond “shifting” of the incoming pressure wave, the finned design proposed here causes a peak pressure of 61% of the original impacting peak pressure – resulting in a reduction of 39%. This result hinges on the fins being of such close spacing that the effect of an impacting pressure transient is spread over the fin’s base area and half of each adjacent water channel. Thus, a submarine bull covered with .1 meter long fins of steel can provide a passive means of reducing pressure wave transients on submarine pressure hulls due to underwater explosions. The reduced peak pressure results in increased survivability of the submarine, reduced lethal radius of enemy weapons, (reduced active sonar returns) and increased difficulty in conducting anti-submarine warfare against a submarine equipped with such a finned surface. (If not practical for large submarines, midgets might benefit.)
When a submarine is subjected to a pressure transient due to a nearby underwater explosion, it is this peak pressure which collapses the hull. If the pressure transient is spread over a longer time, then the peak pressure experienced would be reduced and the chances of survival for the submarine and crew improved.
In tests conducted by Woods Hole Oceanographic Institution for the U.S. Navy, the pressure profile due to an underwater explosion was recorded, the result of which is magnified in Figure 1.
The main points to note in the pressure profile are the short duration of the pressure transient (100 microseconds) and the even shorter duration of the main pressure excursion (50 microseconds). The reduced pressure wave (seen as a pressure reduction below zero) that occurs at 200 microseconds after the initial transient response is the reflection of the pressure wave from the surface of the ocean and thus cannot be used to destructively interfere with the initial pressure transient.
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If the initial pressure transient wavefront were to impact a submarine pressure hull at two times that are 50 microseconds apart, the effect of the transient is spread out. How does the pressure transient become split into two waves 50 microseconds apart? The different velocities of sound in different materials allows this to be done.
Figure 2 shows a very finely spaced fin arrangement (like the fins on an automobile radiator, only much more closely spaced). If this spacing is sufficiently small, a pressure transient in a fin impacting the submarine hull will affect an area of the hull immediately around the base of the area of the fin. If the base area of all the fins is equal to the base area of all the seawater channels, then the local intensity of the pressure transient will be halved due to the effect being spread over twice the area of the fin base (the base area of the fin and half the area of the two adjacent water channels.)
The speed of a pressure wave in a metal fin is much greater than the speed of the same pressure wave in seawater! This results in the pressure wave in the fin “racing ahead” of the. same pressure wave in the seawater channel.
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If the length of the fins were such that the pressure wavefront in the fins impacted the hull 50 microseconds before the same pressure wavefront in the seawater channel impacted the hull, the pressure felt by the hull would be a superposition of the two pressure wavefronts, as shown in Figure 3. The local intensity of the pressure transient will be halved due to the effect being spread over twice the area of the fin base.
Figure 3 shows that by splitting the pressure transient into two waves 50 microseconds apart, the peak pressure experienced by the averaged acoustic wave was 8×10″3 dynes/cm2, whereas the initial peak pressure was 13xl0″3 dynes/cm2″ Thus the peak pressure is reduced 39% by splitting up the pressure transient!
How long should the fins be in order to cause a 50 microsecond separation of the wavefronts? The velocities of sound in pertinent materials shows:
seawater 1531 (m/sec)
mild steel 5960 (m/sec)
Distance sound travels in seawater in 50 microseconds is .0766 meters.
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The velocity difference between steel and seawater is 4429 m/sec. Time for wavefront in steel fin to get .0766m ahead of the same wavefront in seawater is 17.3 microseconds, calling for a steel fin .1 meters long to obtain a 50 microsecond separation between incoming pressure waves.
Summarizing, by using a steel finned arrangement of modest height (.1 meter) we can cause an incoming pressure wave to interfere with itself and produce a 39% reduction in the peak pressure experienced by the impacted surface. This result hinges on the fins being of such close spacing that the effect of an impacting pressure transient is spread over the fin’s base area and half of each adjacent water channel.
The above concept can also be applied to the reduction of a submarine hull’s echo return due to an active sonar search by an opposing platform — by tailoring the length of the fins to cause destructive interference at the pressure hull surface causing an active sonar “ping” to be attenuated.
Jamie Hogan
[Ed. Note: This concept won an award for “technical excellence” in a student competition at San Diego State University.]