Technical Notes for the Archives
By Bruce Rule - August 24, 2013
Preface: Some who read this posting may conclude the information provided is only ancillary to the analysis of acoustic data and has small practical application. Not so!! The more extensive the technical background analysts have, the higher the probability they will fully exploit the data under review in terms of the detail of the recovered information. It is of paramount importance to understand as much as possible about submarines, i.e., how they operate and under what constraints those operations are executed. It is never possible to know enough; there is always more to learn, and curiosity should not be age-limited.
- When nuclear submarines that employ geared-turbine propulsion systems - which can be up to 97-percent efficient - execute evolutions known as “crash backs,” which are about the noisiest evolution possible because of heavy cavitation, steam is redirected from the ahead stages of the turbine to the astern stages. The resulting maximum propeller shaft speed produced by the astern stages is approximately 50-percent of the maximum propeller shaft speed that can be produced using the ahead-stages.
- Crash-backs are used to “kill” forward speed and are almost never allowed to continue long enough to achieve “sternway,” i.e., forward speed is reduced to near zero knots. If forward speed is 30 knots, two to three minutes are required for a crash-back to kill all forward speed.
- Crash-backs usually are performed only during the initial acceptance trials of a submarine to determine if the propulsion shaft line can withstand the most extreme forces to which it is likely to be subjected during the service life of the submarine. ((Read SILENT STEEL (about the loss of SCORPION) by Stephen Johnson which discusses sea trials in PAC during which a 585/589 Class submarine operating at a depth of 300-feet lost its propeller (shaft broke external to the pressure-hull) while executing a crash-back in Dec 1961. The shaft had been altered to fit strain gauge wiring. The submarine (SNOOK/SCULPIN?) had to be towed in. Had the shaft broken internal to the pressure-hull, the submarine probably would have been lost.))
- When nuclear submarines execute course-reversals at speeds in excess of 30 knots using rudder angles of about five degrees, speed can be reduced by as much as 20-percent during the turn while propeller rpm is reduced by only about five-percent. This apparent anomaly occurs because of the increased drag produced when the rudder is not aligned with the longitudinal axis of the submarine and that axis is no longer aligned with the direction of motion because of side-slip; hence, the effective turns-per-knot value can increase by as much as about 20-percent during tight course reversals. Note following paragraph for a transient rpm exception.
- When nuclear submarines execute major course changes at high speed, the propeller rpm makes an extremely rapid transient speed increase of as much as two-percent for less than five-seconds before showing the usual drop in speed. Such very short duration speed increases may occur because, for a very brief period, the flow – and the resulting load – on the propeller is decreased. It is during the initial phase of such turns that side-slip velocity and roll angle are maximum. Maximum loss of speed occurs near the mid-point of the course change. The following article provides a useful discussion of these forces and the phenomenon known as “snap-roll:” http://archive.org/stream/investigationofs00libe#page/n25/mode/2up.
- Nuclear submarine propellers are designed to be most efficient at maximum speed. Accordingly, turns-per-knot values are lowest at flank speed and can be as much as 2-3 percent higher at half that speed. Note; however, that the horsepower requirements increase dramatically with speed as discussed elsewhere in these articles.
- Only when propeller cavitation becomes so advanced as to occur along a significant section of the leading edges of the blades is there a breakdown (loss) of thrust.
- The extreme forces associated with the hydrostatic collapse of submarine pressure-hulls are sufficient to cancel all pre-existing directions of motion. For example, if a submarine is operating at 20-plus knots when collapse occurs, the fragmented pressure-hull will sink nearly vertically from the point (position) of collapse; none of the wreckage will follow a trajectory in the direction of motion extant before collapse. All submarine collapse events for which information is available occurred as a result of the loss of propulsion power; hence, collapse while power is available is improbable.
- When nuclear submarines execute turns, a gyroscopic force is exerted on the propeller shaft. The longer the shaft and the greater the course change, the greater the force exerted on the shaft and the greater the resulting hull excitation. Had the writer known this on 20 June 1964, he would not, after two days of cogitation, have misclassified a Project BRIDGE target.
And finally, ZUI (you attention is invited to) the following website which discusses What is known about the character of noise created by submarines written circa 1995 by E. V. Miasnikov, a Russian expert in Arms Control and Submarine Strategic Nuclear Weapons: http://www.fas.org/spp/eprint/snf03221.htm