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laminations which cover different umts as the course progresses. Course standards are established to assure a high rate of achievement for each sepa- tate unit area. This testing program Permits identification of specific SWO PQS items accomplished by each student. Upon successful completion of the course, a graduate certification report is sent to the prospective surface Warfare officer and the commanding officer of his first ship. This report numerically lists those SWO PQS items achieved or certified by the graduate through the SWOS testing program including both objective and “hands-on” Performance tests. Certification at the SWosc basic course requires considerable student effort and is indicative of a degree of preparedness based on the sWo pqs requirements when the graduate reports on board his ship. However, final qualification remains w,th the ship’s commanding officer based on observed performance on board the actual ship.
An equally significant result of SWO basic training involves the degree to which the graduate will become read- dy employable and effective on board che ship within his first SWO assignments. To assess this and other aspects °f course effectiveness a refined train- |ng appraisal program is used. Part of lt: ls done by an internal evaluation Process within the school itself. The external portion of the program provides for continuing course content improvement to meet the needs of the fleet and depends on a very close interaction between the fleet and the school. The 70% return rate of commanding officer and graduate appraisal forms over the past three years indicates the continuing interest the fleet maintains in surface warfare officer training. The appraisal trend appears to be of general satisfaction with the overall SWO basic course training plan. However, as an increasing number of graduates move to the fleet under the qualification standards of the SWO PQS, a confident and knowledgeable ensign—once an exceptional individual—will become the general rule, and a more critical eye will be turned toward the training appraisal. The aggregate results of the training appraisal program are coordinated by the school staff, translated to specific curriculum revision recommendations, and forwarded to higher authority for approval. In the past, training appraisal results have provided significant input for two major approved curriculum revisions and with the processed results of the last fiscal year appraisal information, a third recommended revision is being constructed.
More precise refinements are planned in the not too distant future with the installation of a revised SWO
PQS which has been in development for the past two years. This revision builds upon the April 1975 publication to more clearly define the task qualifications throughout all areas. The Chief of Naval Operations has promulgated transition guidelines to preclude disruption of ongoing SWO qualification procedures. The SWOSC basic course will align to the revised SWO PQS requirements commencing with classes convening 5 June 1978. Officers who will commence their SWO qualification after this date will use the revised PQS.
The implementation of the Surface Warfare Officers School Command basic course has established a new dimension in the practical learning achievements for newly commissioned officers based upon realistic and standardized performance requirements. Tomorrow’s surface warfare leaders are acquiring their fundamental skills today through a progression of systematic and functional training. As basic course graduates return to SWOSC to attend the intermediate department head course and prospective executive and commanding officer courses, they will be building on a firmer foundation than have their predecessors. The building begins for the Navy’s future surface leaders with the first step after commissioning— the surface warfare officer basic course.
Seakeeping—and the SWATH Design
by Lieutenant Commander Stephen R. Olson, U.S. Navy, formerly the Principal Systems Analyst for the Center for Naval Analyses’ Assessment of SWATH; now, special Project Officer for the Naval Personnel Program Support Activity.
• ■ . seakeeping ability has affected our sbips. On a fleet exercise conducted several m,jnths ago, our ships were simply no match against the sea and winds for which the North Atlantic is notorious. Our commanders and commanding officers were f°rced to forgo many of the objectives of the eXercise in order to accommodate to the leather. In some cases:
^ Our ships were forced to slow to prevent or lessen the impact of damage,
► Exercises were cancelled,
► We could not refuel our ships,
► Equipment was damaged, and
► Personnel were injured.
... the ships we introduce for the future must have every technological edge possible in order to ensure the success of that ship's mission.” (Vice Admiral R.E. Adamson, Jr., U.S. Navy, Commander Naval Surface Force, Atlantic Fleet, June 1975.)
Admiral Adamson’s comments provide an excellent qualitative description of the importance of a naval ship's seakeeping ability. The comments do not, however, provide any specific insight on the quantitative worth or utility of seakeeping attributes.
The seakeeping capabilities of naval ships became a major concern for the Center for Naval Analyses (CNA) when it was directed by the CNO in May 1976 to identify those Navy missions that could be more readily accomplished by small waterplane area twin hull (SWATH) ships. Although the SWATH offers other design improvements, such as greater deck area available for aircraft operations, its greatest contribution to warship design is based on claims of vastly improved seakeeping qualities vis-a-vis the conventional monohull. In keeping with a systems analysis viewpoint, CNA decided to investigate and quantify these claims.
A complete assessment of the seakeeping qualities of a naval combatant should attempt to identify every significant aspect of the ship’s mission that might be affected by platform motion. Therefore, the first step in our seakeeping analysis was to attempt to identify how motion affected ship subsystems and personnel. We soon learned that the Navy’s understanding of the ship motion environment and its effects is far from complete.
Perhaps the most obvious area of concern is the impact of ship motion on the effectiveness of shipboard weapon systems. The specifications that relate to motion usually are determined by target motion considerations, rather than ship motion. For example, the operating limits for the 5-inch/54-caliber lightweight gun system specify train and elevation rates of 30°/second and 207second, respectively, and accelerations of 60°/second2 and 40°/second2, respectively. These limits on velocity and acceleration are constraints on the gun’s capability against high-speed air targets. However, no information on the impact of ship motion on gun performance was available for these systems. As a result, we reviewed ship gunnery exercises to determine whether sea conditions and, implicitly, platform motion, had any impact on the recorded results. Analysis of Pacific Fleet gunnery exercise results, collected over a ten-year period, and weather and wave height observations for the same period indicated that over 95% of the exercises were conducted in observed wave heights of less than six feet. This fact and inadequacies in the data base itself precluded drawing any conclusions on the relationship between gunnery exercise performance and local sea conditions. Interestingly, the Naval Gun Accuracy Improvement Program (NGAIP), which has been in existence since 1968, reported that it was not investigating the part platform motion played in causing gunnery inaccuracies. This information also was not available at the Naval Surface Weapons Center, Dahlgren Laboratory.
Analysis of 528 missile firing exercises revealed that only 18 exercises were reported to have occurred in Sea State 5 or higher. Again, the lack of data made it difficult to determine if the success of an exercise was dependent on local sea conditions. In fact, there was no evidence to suggest a trend of decreased missile exercise performance through Sea State 5. There was evidence, however, though not strong, that on a seasonal basis the likelihood of achieving a “successful” exercise evaluation diminished when higher waves of longer periods are normally present. As a general observation, it was noted that there appeared to be a significant bias against conducting missile exercises in Sea State 5 or higher. While there arc- numerous explanations for the few exercises conducted in Sea State 5 or higher, they do not mitigate the observation that such sea states may be dominant in many parts of the world where the Navy desires to operate. (Sea States of 5 and higher can be expected more than 60% of the time in a North Atlantic winter.) Thus, the question of our ability to effectively employ a weapon system under such conditions is highly appropriate.
The High Energy Laser Program indicated to us that it did not consider gross platform motion a major concern. Lasers are affected by motions measured in milliradians, and the opinion of the program office was that stabilization for roll and pitch of the ship would be subsumed by the requirements for extremely fine accuracy for target acquisition and tracking. The potential for a less complex system on an inherently stable platform is not being considered.
Specifications for equipment designed for use in ships usually state- motion limits within which the equipment is expected to operate normally. Limits frequently used are 30° roll and 10° pitch, and/or satisfactory operation through Sea State 5- However there is no evidence that any equipment is tested in Sea State 5 to verify this capability. Indeed, many system specifications treat the subject in a cursory manner. For example, the specifications for the tactical towed array sonar state that the system shall operate normally when experiencing continuous pitching of 10° (20° peak to peak) every six seconds for four hours. Such requirements demonstrate a lack of appreciation for the nature of ship motion and are quite unrealistic.
Although there are many sea stories that imply that heavy seas cause an increase in electronics equipment corrective maintenance, few exercises or operational reports actually document the problem. The staffs of the Surface Force Atlantic Fleet and the Fleet Maintenance Support Office indicated that their maintenance records did not contain data that could be used to identify adverse weather or seakeeping as a cause of system failure. Thus, it was not possible to use actual fleet experience to substantiate claims of system failures caused by heavy seas. Ic was possible, however, to make a general appraisal of standards and specifications for electronics systems and to evaluate their adequacy in mitigating against ship-motion and slam-induced failures. The David W. Taylor Naval Ship Research and Development Center concluded that environmental standards for shock appear to be fully adequate to protect against slam- induced system failures for ship lengths greater than 350 feet. Thus, if electronic reliability does degrade in a severe ship motion environment, the problem should not be attributable to the design specifications. Nothing can be said about fatigue failures, however, because no attempt was made to evaluate the cumulative effect of slamming over a ship’s lifetime.
An extensive review of numerous fleet exercise reports revealed only one case in which the seakeeping qualities of the exercise ships (of destroyer size and greater) were believed to have significantly affected the outcome of the
exercise. This report contained the following comments:
“Once at sea, materiel readiness deteriorated. This trend, to some degree, can be attributed to the lower level of expertise aboard the ships compared to that found in the technical assist teams ashore. Two other contributing factors to the decline *n materiel readiness were adverse weather (Sea States 3 to 6) and demanding operational requirements (which prevented normal preventive maintenance) for the equipment, particularly in electronics.”
,c is apparent, from the absence of comments in other exercise reports, that the seakeeping qualities of ships are not evaluated as factors that could affect the overall exercise results.
Although we realized that direct mputs from fleet units would be most desirable, we believed that a formal survey of fleet operators on the subject °f seakeeping was well beyond the scope of the CNA effort. As a compromise, a letter soliciting specific observations on the impact of seakeeping °n fleet operations was sent to all CNA representatives who are stationed at major fleet commands. The following response was received from the Third Fleet representative.
“Systems that suffer degradation, of course, are the various radars. They are stabilized to ship’s roll (from about 15° to 30°) but in excess of 30° roll their operational functions cease since they no longer can stay locked onto the target. . . . Most skippers feel that the unstable platform has a deleterious effect on ship’s personnel and this is the more significant problem. In this regard, they consider it a great advantage to function from a stable platform in higher sea states in which a seaborne enemy suffers the effects of roll and pitch. The increased platform stability of the friendly unit would have a distinct advantage. In general, senior surface officers highly endorse the prospects of a platform of improved stability in heavy sea state.”
The response from the CNA representative on the Sixth Fleet staff contained the following observations:
“In destroyer-type ships, short
periods of up to 48 hours of heavy sea state have very little effect on normal operations; however, periods in excess of 48 hours cause serious degradation of personnel and their ability to effectively utilize ship’s equipment. The constant need to physically restrain the motion of an individual coupled with the loss of sleep and reduction in food causes a general weariness in all hands that impacts on all normal operations.”
Because the British have adopted the practice of installing fin stabilization systems in all their destroyer-type ships, we believed that they must have had a substantive reason for doing so. We were informed by the British Naval Staff Office, however, that no documented rationale existed for the use of stabilization systems. The practice had simply evolved from “common sense, tradition, and the insistence of the air people.” The U.S. Navy’s Naval Sea Systems Command representative at the Royal Navy’s counterpart facility at Bath reported that the British consider platform stabilization to be worthwhile for helicopter operations, and for no other purpose.
Because personnel safety was believed to be an obvious area of interest on ships experiencing excessive motion, a complete listing of all property damage accidents and personal injury reports associated with heavy weather or the unexpected seaway movement of ships was reviewed. This data listing, supplied by the Naval Safety Center, included all Navy ships’ reports for calendar years 1973, 1974, and 1975 and identified only 14 property damage accidents, one of w'hich involved personnel injury. (The injury occurred w hen cans of toxic fluid overturned in a CV, and four sailors were temporarily incapacitated by the fumes.) This data base does not seem consistent with the practical experiences of many naval officers, and it can only be assumed that it is due to either a faulty data collection system or inadequate reporting by the fleet. A similar query to the staff of the Surface- Force, Atlantic Fleet, revealed that it had no data that could document personnel injuries associated with ship motion in a seaway.
Despite the fact that the significance of motion on subsystem and personnel performance is not well understood, we did have some success in quantifying many seakeeping factors. For example, it is possible to estimate- limiting ship speeds as a function of w-ave heights based on the consideration of avoiding permanent deflection of a ship’s hull plating that could be caused by intemperate seamanship. Reports by M.K. Ochi and L.E. Mutter, of the David W. Taylor Naval Ship Research and Development Center (NSRDC), and G. Aertssen, a Belgian researcher, suggest a seakeeping criterion that limits the rate of severe slams to an acceptable number. (Slam is used to describe the event that occurs when the bow of a ship comes out of the water and reenters with an appreciable impact, frequently causing a shudder felt throughout the ship.) Aertssen derived this criterion based on numerous voyages in commercial hulls in which he observed that a ship’s master would, on the average, slow his ship or alter course if severe slams occurred at a rate of more than three per hour. Thus, if we equate the economic incentives of the commercial master with the mission incentives of the naval captain, Aertssen’s slam criterion appears reasonable.
Deck wetness, which we define as submergence of the bow to a point where water comes over the bulwarks, is another seakeeping consideration we judged worth evaluating. Traditionally, deck wetness has not been assigned much significance as a major design consideration probably because it can be controlled with relative ease once other design considerations have been satisfied by the addition of bulwarks at the bows and along the ship’s sides. However, the treatment of deck wetness has rather strong implications for naval combatants. The placement of bulwarks in a combatant acts to reduce the firing arcs and rates of fire of guns and trainable missile launchers, interferes with ammo and stores handling during underway replenishment, and can be a significant nuisance if the ship’s mission calls for extensive over- the-side handling operations. It also appears that the preponderance of heavy weather damage suffered by naval combatants has been caused by taking water on the main deck.
Although Captain James W. Kehoe used a wetness criterion of no more than one wetness per minute to compare U.S. with Soviet warships in his November 1973 Proceedings article, "Destroyer Seakeeping: Ours and Theirs,” his selection of that threshold was quite arbitrary. The choice of one minute was made as a simple means of comparing the design practices of the two navies. Unfortunately, we discovered no other discussion of a specific wetness criterion. Based on my own experience, I believe that ships’ captains rarely choose to take water over the bow more than once every two to five minutes, especially if gun mounts, missile launchers, or major deck equipment are located forward. It is emphasized that the consideration in question is the occurrence of actual waves coming over the bow, not simply spray. If waves come over the bow every two minutes, there is considerable risk that an occasional wetness will be extreme to the point that a wave may cascade the length of the forecastle.
The effect of ship motion on a ship's hull-mounted active sonar is also worthy of consideration. If the sonar dome emerges from the water at a rate that precludes a reasonable number of detection opportunities, a significant degradation in the total system effectiveness will occur. There is, of course, the important question of the degraded acoustic environment in heavier seas, and no amount of ship stabilization can ameliorate this situation. However, a detailed discussion of ship sonars and the ambient ocean medium is beyond the scope of this article. However, passive sonar systems are significantly less affected by a rough sea condition because of their lower operating frequencies.
Returning to the subject of personnel effectiveness, the Navy’s Surface Effect Ship Program Office sponsored research on the effects of ship motion on humans in the at-sea environment. The research attempted to quantify the incidence of actual emesis (vomiting) of individuals subjected to vertical sinusoidal motion. The experiments showed that the occurrence of emesis was correlated with both acceleration and frequency and, using empirical data, a motion sickness incidence (MSI) relationship was derived as depicted in Figure 1. The MSI value indicates the percentage of subjects that experienced emesis in a two-hour test period. The MSI is of interest because the frequencies of high susceptibility to motion sickness coincide very closely with the natural periods of pitch and heave of naval warships, as shown in Table 1.
In the actual experiments conducted by Human Factors Research, Inc., 30% of the test subjects experienced vomiting under accelerations of 0.1 g at approximately 0.2 Hz. This figure rose to almost 60% when the acceleration was increased to 0.2 g. The research also evaluated the impact of roll and pitch added to the original vertical sinusoidal motion and revealed that the onset of emesis was relatively insensitive to these motions. It was also determined that, while the number of laboratory subjects who ex-
Table 1 Natural Periods of Pitch and Heave for Four Monohull Designs
%> Class | Natural Pitch Period | Natural Heave Period |
CGN-38 | 5.9 sec. (. 169 Hz) | 5.9 sec. (.169 Hz) |
DD-963 | 5.8 sec. (.172 Hz) | 5.5 sec. (. 182 Hz) |
FF-1052 | 5.3 sec. (.189 Hz) | 5.0 sec. (.200 Hz) |
FFG-7 | 4.7 sec. (.213 Hz) | 4.7 sec. (.213 Hz) |
Pwienced emesis rose steadily in the first two hours of exposure to vertical Motion, those who survived the first two hours rarely became sick.
Two personnel surveys—one based °n interviews with crew members of the USS Glover (ex-AGDE-1, now AGFF- ’)> the other based on interviews with Slx commanding officers of destroyers and their squadron commodore— further indicated that personnel effectiveness was significantly degraded as average rolls exceeded 10°.
The effect of sea-state-induced motion on destroyer/helicopter operations ls yet another important seakeeping consideration because of the increasing r°le helicopters play as embarked ""capon and sensor systems. We attempted to identify limiting ship mo- t!on envelopes for helicopter opera- t>ons based on aviation safety reports °f ship motion-related mishaps. The Naval Safety Center provided a data fisting of all helicopter mishaps that °ccurred during the takeoff, landing, 0r on-deck phase of operations in Navy ships from 1969 through August 1976. (A mishap generally was defined as any event in which an unSafe condition occurred that resulted, 0r could have resulted, in material damage or personnel injury.) Of the 184 events reported, 19 contained explicit or implied comments indicat- lng that ship motion had been a major °r contributing cause of the mishap. AH occurred during the landing phase 0r shortly after the helicopter had touched down on deck, and no per- s°nnel injuries were associated with the mishaps. Landing gear failures caused by hard landings and attributed to pilot error were not considered although the vertical motion at the flight deck may be presumed to increase the likelihood of a hard land- lng- Unfortunately, the nature and content of the data reported to the
Safety Center were inadequate to quantify a particular motion threshold at which ship/helicopter problems begin to occur. Where roll and pitch amplitudes were recorded, the values were relatively small when compared with expected ship motions in truly rough weather (e.g., Sea State 5). The reason for the small number of mishaps reported over the seven-year period is very likely attributable to Navy doctrine, which generally restricts helicopter operations on most destroyer-type ships to Sea State 4 or lower. Therefore, an examination of this doctrine is required.
Motion and wind envelopes for various ship-helicopter combinations— other than CV/LHA/LPH classes—are contained in NWP-42, “Shipboard Helicopter Operating Procedures.” These envelopes, determined by the Naval Air Test Center (NATC), art- based on actual test flights of a specific helicopter on a ship of a specified class and limited by the actual wind and sea conditions available at the time of the scheduled NATC evaluation. Thus, it can only be assumed that the motion limits are “single amplitude” (e.g., vertical-to-out roll, versus side-to-side), and there is no indication of the allowable frequency of occurrence of the roll amplitudes before the threshold is considered to be exceeded.
A more detailed report on the evaluation of the FF-1052 class and the SH-2F helicopter gets around some of the deficiencies in NWP-42. This report, prepared by the Naval Ship Research and Development Center (NSRDC), states that:
^ “Difficulties which produce waveoffs can be expected, (a) when significant double amplitudes of pitch reach values from 2.2 to 4.0 degrees and vertical accelerations attain values ranging from 0.12 to 0.20 g, and (b) when significant double amplitudes or roll reach values of 4.4 to 11 degrees. . . . These levels do not represent the highest safe operating values. More- trials in high sea conditions are required to establish the highest acceptable motion limits.
► "The most practical and efficient way to extend the flight envelope for unassisted landing and takeoff operations is to use devices which minimize the time that the helicopter is not secured on the deck, e.g., rapid securing devices during landings and/or pilot-activated singlepoint tiedown release during takeoff. Roll stabilization to 12.8 degree significant double amplitude- roll will, of course, also extend the helicopter deployment capability of destroyer or other naval ships.” Based on FF-1052/SH-2F trials, the selection of a pitch criterion of 4° appears appropriate. However, it is apparent that the effect of pitch at the helicopter landing area is a function of the distance from the ship’s center of gravity (CG) to the landing area, as well as the pitch amplitude itself. Naval Air Test Center personnel indicated that the vertical motion amplitude at the flight deck was the factor of concern in safe helicopter operations. Using straightforward geometry and the distance from the CG to the helicopter deck of the FF- 1052-class ship, it was determined that the 4° pitch criterion was equivalent to an 8.34-foot vertical displacement at the flight deck.
One other helicopter compatibility criterion, based on the vertical velocity at the helicopter landing platform, attempts to identify the maximum impact forces that the helicopter landing gear can absorb during a landing. A 1974 Naval Air Engineering Center study implies that the maximum allowable vertical velocity of the ship is approximately seven feet per second. This criterion is substantiated by E. Comstock’s and P. Covich’s study, "Assessment of Carrier Hull Form Performance” (ComNavAirPac-ASNE Aircraft Carrier Technical Symposium paper, October 1976), in which a limit of 6.5 feet per second velocity at the flight deck is suggested.
Table 2 Seakeeping Criteria and Categories _________________________________
General criteria
Monohulls and SWATH
(1) 12° single amplitude average roll
(2) 3° single amplitude average pitch
(3) Motion sickness indicator (20 percent of laboratory subjects experience emesis within 2 hours)
Monohulls only
(4) Bottom plate damage
(5) Three slams in 100 motion cycles
(6) One deck wetness every 2 minutes SWATH only
(11) 18-foot average of highest 1/10th relative bow motions
(12) 12.8-foot significant relative motion at the propeller
Helicopter operating criteria for monohulls and SWATH
(7) 12.8° double amplitude significant roll
(8) 8.34-foot double amplitude significant vertical displacement at the flight deck
(9) 7 foot-per-second significant vertical velocity at the flight deck
Hull-mounted sonar criterion for monohulls only
(10) Sonar dome emergence criterion (three-out-of-five detection opportunities)
► The size of the helicopter landing area is not considered. ► The impact of ship obstructions and wind turbulence is ignored. ► The interaction between ship mo- | Ship Speed (Knots) | Following Sea 0 15 | 30 | 45 | 60 | 75 | Beam Sea 90 | 105 | 120 | 135 | 150 | 165 | Head Sea 180 | |
5 | 8 | 7 | 7 | 7 | 7 | 7 | 3 | 8 | 8 | 8 | 8 | 8 | 8 | |
tion and relative wind envelopes is not | 10 | 8 | 8 | 8 | 7 | 7 | 7 | 3 | 8 | 8 | 8 | 8 | 8 | 10 |
considered. | 15 | 8 | 8 | 8 | 8 | 7 | 7 | 3 | 3 | 8 | 8 | 8 | 10 | 10 |
► The effects of ship motion on air- | 20 | 8 | 8 | 8 | 8 | 7 | 7 | 3 | 3 | 8 | 8 | 8 | 10 | 10 |
craft handling and maintenance tasks | 25 | 8 | 8 | 8 | 8 | 7 | 7 | 3 | 3 | 3 | 3 | 8 | 10 | 10 |
are ignored. |
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Thus far, we have identified a |
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| Acceptable Significant Wave | Height (Feet) |
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number of quantifiable seakeeping | Ship |
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considerations that are applicable for | Speed | 0 | 15 | 30 | 45 | 60 | 75 | 90 | 105 | 120 | 135 | 150 | 165 | 180 |
monohull designs, but there are two | (Knots) |
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criteria unique to SWATH. The first, | 5 | 21 | 15 | 9 | 6 | 5 | 6 | 8 | 8 | 8 | 10 | 13 | lS | ~1T |
based on the occurrence of wave im- | 10 | 24 | 22 | 19 | 7 | 4 | 5 | 8 | 8 | 8 | 9 | 12 | 14 | 15 |
pact on the SWATH cross structure, | 15 | 26 | 26 | 21 | 16 | 6 | 5 | 8 | 7 | 8 | 9 | 12 | 13 | 13 |
limits the extreme relative motion be- | 20 | 27 | 26 | 22 | 18 | 10 | 4 | 8 | 7 | 8 | 10 | 12 | 12 | 12 |
tween the SWATH and waves to a | 25 | 26 | 23 | 22 | 19 | 13 | 3 | 8 | 6 | 7 | 10 | 13 | 11 | 11 |
FFG 7
ALL SEAKEEPING CRITERIA (1-10) 7.0 SECOND MODAL WAVE PERIOD
These criteria for evaluating helicopter compatibility of naval ships ignore some important factors, such as:
► No distinction is made between day and night flight operations.
maximum of 18 feet. The 18-foot figure is simply the design clearance between the bottom of the cross structure and the waterline of the representative 3,400-ton SWATH which we evaluated. Due to the geometry of the 3,400-ton SWATH, this slam criterion subsumes any reasonable criteria based
Table 3 FFG-7 vs SWATH .
on either deck wetness or dome emergence. The second criterion, based on the phenomenon of propeller emergence, limits the significant relative motion at the SWATH stern to a maximum of 12.8 feet. This figure is quite conservative when one realizes that, if the 3,400-ton SWATH were to trim down (by the stern) 1° degree, the distance between the propeller tip and the ocean surface would increase by 2.6 feet and eliminate propeller emergence in all but near mountainous seas.
Although our ability to quantify the impact of ship motion on the overall effectiveness of a ship is limited, we can identify motion thresholds or seakeeping criteria. These criteria are summarized in Table 2 and reflect considerations of seamanship, personnel effectiveness, and the ability of a ship to operate embarked helicopter and sonar systems. With these criteria we can compare the seakeeping effectiveness of various warship designs.
Using detailed ship motion statistics, developed by the Naval Ship Research and Development Center and available as functions of the heading of seas, the ship's speed, and the modal wave period (i.e., the period of the waves associated with the waves of greatest energy), and a specified set of seakeeping criteria, we can answer two questions: (1) What criterion is first exceeded for a given ship speed, modal
SWATH
ALL SEAKEEPING CRITERIA (1-3,7-9,11,12) 7.0 SECOND MODAL WAVE PERIOD
Ship Speed (Knots) | Following Sea 0 15 | 30 | 45 | 60 | 75 | Beam Sea 90 105 | 120 | 135 | 150 | 165 | Head Sea 180 | ||
5 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 |
10 | 8 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 |
15 | 8 | 8 | 8 | 7 | 7 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 |
20 | 8 | 8 | 8 | 8 | 8 | 12 | 12 | 12 | 12 | 12 | 12 | 12 | 12 |
25 | 8 | 8 | 8 | 8 | 8 | 8 | 12 | 12 | 12 | 11 | 12 | 12 | 12 |
Ship
Speed
(Knots)
5
10
15
20
25
Acceptable Significant Wave Height (Feet)
Ship Heading Angle in Degrees
0 15 30 45 60 75 90 105 120 135 150 165 180
22 22 20 7
23 23
9 12
22 22 23 15
11
8
23
23 20 16 11
25 27
26 28
26 29
25 30
21 30
28 28
29 29
30 30
30 30
28 29
29 29
29 29 30 29
30 29
29
29
28
28
29 28
29 28
30 29 29 28
Period, and heading of the seas? (2) At what wave height is that criterion exreeded?
Answers to these questions have been developed for four classes of naval rnonohulls (CG-26, DD-963, FF-1052, and FpG-7) and a conceptual 3,400-ton S^Ath. Examples of the type of information available are shown in Table The data in the upper matrices of cbe table identify the limiting seakeeping criteria as a function of ship’s speed and heading. The data in the lower matrices identify the wave heights at which the “limiting' rriteria are exceeded. By using information of this type for a variety of modal wave periods, it is possible to determine a broad seakeeping assessment for ships operating in a unique ocean environment. For example, from historical data on observed wave heights and periods available for eight Points in the North Atlantic, we can develop an average for these points and estimate the likelihood of occurence of sea conditions having a specific Wave height and period. Then, if we assume that a ship is equally likely to encounter a sea at any heading angle, We can calculate the percent of time Chat a ship can operate without ex-
ceeding one or more of the seakeeping criteria. The results of these calculations are contained in Table 4 and illustrate both the unique seakeeping qualities of SWATH and the significant seakeeping degradations of the monohulls in the North Atlantic winter environment.
These results take on added significance when we consider the dimensions of the various ship designs. (See Table 5 for data on the subject classes of ships.) The representative 3400-ton SWATH, while less than half the displacement of the DD-963 class, is a much more flexible platform in the face of adverse weather conditions.
Table 4 Seakeeping Box Scores in the North Atlantic
Speed
Ship | (knots) | Summer | Winter |
SWATH3 | All | .96 | .87 |
(3,400 tons) | 5 | .99 | .92 |
| 10 | 1.00 | .95 |
| 15 | .97 | .88 |
| 20 | .94 | .83 |
| 25 | .92 | .79 |
FFG-7b | All | .78 | .48 |
| 5 | .73 | .40 |
| 10 | .76 | .45 |
| 15 | .80 | .50 |
| 20 | .81 | .53 |
| 25 | .81 | .53 |
FF-1052b | All | .84 | .58 |
| 5 | .81 | .51 |
| 10 | .82 | .55 |
| 15 | .86 | .61 |
| 20 | .86 | .62 |
| 25 | .84 | .60 |
DD-963b | All | .92 | .72 |
| 5 | .92 | .69 |
| 10 | .91 | .68 |
| 15 | .93 | .73 |
| 20 | .92 | .73 |
| 25 | .92 | .73 |
CG-261' | All | .89 | .67 |
| 5 | .87 | .60 |
| 10 | .87 | .63 |
| 15 | .90 | .68 |
| 20 | .92 | .72 |
| 25 | .90 | .71 |
aCriteria for the SWATH are: 1,2, | ^1 00 VO |
11, 12
bCriteria 1 through 10 were used to evaluate the FFG-7, FF-1052, and DD-963 classes ‘Criteria 1 through 3 and 7 through 9 were used to evaluate the CG-26 class. (Motion data for criteria 4, 5, 6, and 10 were not readily
The monohulls having a | displacement | available.) |
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Table 5 Ship Data | CG-26 | DD-963 | FF-1052 | FFG-7 | SWATH |
Full Load Displacement (metric tons) | 7838 | 7822 | 4246 | 3578 | 3408 |
Length Between Perpendiculars (meters) | 160 | 161 | 127 | 124 | 93a |
Beam (meters) | 17 | 17 | 14 | 14 | 31.7 |
Draft (meters) | 5.7 | 5.9 | 4.7 | 4.5 | 8.0b |
“The length shown is the length overall.
(The box length is approximately 60 meters.) hThe SWATH's draft is 4.8 meters when lightly loaded.
more comparable to the SWATH (i.e., the FF-1052 and the FFG-7 classes) run a very poor second in the competition with the SWATH design. The greatest difference between the monohull and SWATH concepts appears to be in the lower speed regime. Such speeds are frequently required when towing surveillance arrays or employing underwater sweeping systems. They are also the speeds for which mechanical motion damping devices (e.g., fin stabilizers) are usually ineffective. A detailed analysis of the data also revealed that the dominant factor in determining the seakeeping qualities of the ships was the consideration of helicopter compatibility, although the incidence of motion sickness was also important. We might reasonably expect that the motion envelopes for vertical/short takeoff and landing (V/STOL) aircraft will be at least as restrictive as those currently implied by helicopter operations.
Although ship motion data are not available to reflect the potential impact of stabilization devices on the seakeeping qualities of the monohulls, it is instructive to point out that only roll can be significantly reduced by motion damping devices, and that these systems have enjoyed only limited success. Thus, while both monohulls and SWATH can receive some benefit from mechanical motion reducing devices, the relative difference between the seakeeping qualities of the divergent design concepts would probably be similar to those shown in Table 4.
The significance of the seakeeping qualities of naval combatants will continue to increase in the years ahead in light of the growing role that helicopters are playing as sensor and weapon platforms. Seakeeping will become even more important if the Navy continues to place strong emphasis on the development of V/STOL aircraft. If sea-based V/STOLs are to be operated from small- and medium-size combatants, these ships must be able to provide for the maximum availability of these aircraft in all ocean environments. There are, of course, many other compelling arguments for a good seakeeping platform, such as the employment of towed sonar arrays for ASW. The effectiveness of these arrays is highly dependent on their orientation with respect to the submarine target. A poor seakeeping platform therefore may be forced to forgo an optimal course if the motion of the platform is excessive. Finally, we have entered the age of the cruise missile and supersonic missile warfare. Now, and for the foreseeable future, the ability to rapidly react and defend against attack will determine the survivability of the warship. It seems clear that both the ships’ crews and their systems will be more alert and ready if they are functioning in an environment where the debilitating effects of ship motion are reduced to their lowest possible level. Thus, the effectiveness of the warship in performing a variety of missions, particularly when the support of embarked air vehicles is required, can be significantly enhanced if the platform possesses superior seakeeping qualities.
It is unfortunate that the small waterplane area twin hull has appeared on the scene in the same time as the surface effect ship and the hydrofoil’ While these latter concepts offer major increases in potential speed for the small combatant, they also involve considerable technological risk and raise the nagging question of the util' ity of speed in the naval warfare environment. Conversely, SWATH does not offer a dramatic calm water speed improvement over the conventional monohull, nor does it require a sophisticated technology. SWATFl simply possesses better seakeeping qualities and potentially greater deck area to sustain air operations. Because of the chronological coincidence of SWATH, surface effect ships, and the hydrofoil, these differences are frequently ignored, and the concepts are throw'n into the same bag, labeled "new technology” platforms.
SWATH is a low-risk concept that offers a dramatic improvement in seakeeping qualities when compared r° conventional monohull designs.
Unfortunately, the Navy has not yet decided to do so, although there are at least two mission areas—mine countermeasures and surveillance towed arrays—where a SWATH platform could have offered a significant improvement over the conventional monohulls that are currently planned.
SWATH is ready for the Navy- When will the Navy be ready for SWATH?
Tactical ASW & Ocean Acoustic Forecasting
By Lieutenant Commander Alden B. Chace, Jr., U.S. Navy. Oceanography Department, Naval Postgraduate School, and Lieutenant George V. Galdorisi, U.S. Navy. Helicopter Anti-Submarine Squadron Light Thirty-Two
. . . even with our great modern strides in technology—lasers, homing and guided ordnance, sonar, electromagnetic devices of many kinds, and their countermeasures —further significant improvement in the performance of these devices and weapons
The ability of naval operating forces to detect, localize, and track submarines is mainly dependent on their proficiency in predicting the behavior of underwater sound and then employing their acoustic sensors based on this
may only come from an increased knowledge of the environment in which they operate. (Rear Admiral C.O. Holmquist, U.S. Navy [Retired], former Chief of Naval Research, November 1973 Proceedings)