Seakeeping

 

In the past, the effect of a ship’s seakeeping characteris­tics on her combat system performance has been diffi­cult to assess during the design process without resort­ing to time-consuming and expensive scale model testing and generic error modeling.¹ Therefore, until recently, little research was conducted on the comparative seakeep­ing characteristics of different ships and their effects on combat system performance.² Unlike conditions during World War II, today’s aircraft, submarines, and many weapons and sensors can be operated in severe weather conditions. Thus, the seakeeping characteristics of surface ships are more important than ever before. Fortunately, new analytical tools, including digital computers, are available, which enable naval engineers to simulate and assess the seakeeping characteristics and combat system performance of various hull forms and weapons suites in the design stage in a shorter time and at less cost than in the past. Thus, based on research using these tools, the new Arleigh Burke (DDG-51)-class destroyer will have a hull form that should offer significantly improved sea­keeping and combat system performance as compared to previous U. S. Navy ships.³

However, the bulk of the U. S. Navy ships that will be operational in the year 2000 already exists. Therefore, we must improve our assessment of these two qualities in existing ships in order to better determine:

► The operational availability of existing weapons and sensors in relation to the seakeeping performance of each class of ships

► The availability of each class of ships to conduct vari­ous operational missions in rough water ► The effects of the seakeeping limitations of different classes of ships on task force and convoy operations ► Improvements to ship design features that would enhance a ship’s seakeeping characteristics ► Realistic seakeeping design criteria that can be used in computer-aided simulations of new ship designs

With these objectives in mind, a questionnaire was dis­tributed to the commanding officers (hereafter referred to as operators) of 185 U. S. Navy frigates, destroyers, and cruisers (15 classes). The results of the survey should help fleet operators and ship designers establish warship requirements and select weapons and sensors.

The 100 responses to the questionnaire provided an excellent data base for statistical analysis. Various factors, however, affected the opinion of operators, which could influence the validity of the results. These include experi­ence in the ship reported on, experience in operating in rough waters, “preencounter expectations” of seakeeping performance, difficulty in assessing the seakeeping per­formance of other ships when separated by several thou­sand yards of rough water and, finally, the fact that the survey was conducted during peacetime. Nevertheless, the responses to the questionnaire allowed us to assess the seakeeping performance and operational limitations of these U. S. ships from the operators’ viewpoint.

Speed: The operators were asked the maximum speed their ship could make in head seas in various sea states. They were also asked to report the sea state in which maxi­mum speed would have to be reduced to avoid damage to the ship or personnel. The results are seen in Figure •

As a ship travels through ocean waves, she responds to the effects of the waves on her hull with a constant) changing series of motions. These motions can either angular—pitch, roll, and yaw—or linear—heave, sway* and surge. It is generally agreed that, of these six motions- pitch, roll, and heave are the most significant. It is ^als agreed that good seakeeping performance is characterize by the degree to which a ship can operate in rough se with a minimum of pitch, roll, heave, keel slamming, ^an deck wetness. Propulsive power, ship size, and hull fo^rnl influence the maximum achievable speed of various types of ships at very low sea states. A ship’s speed is limited l the propulsive power available and her hull resistance characteristics up to about Sea State 4. At higher sea states, speed is limited by ship motions. Therefore, ship’s speed in rough waters depends primarily on her si and hull form, which influence her motion characteristic^

In general, the responses to the survey were consist between ships in a class and between classes in relation size. Only small differences in maximum ship speed exi at the very lowest states. In Sea State 5, 400-foot frig^ateS^ such as the Knox class and Oliver Hazard Perry class, ^ca generally make a maximum speed of about 20 knots- Under these same conditions, a destroyer like the Sp^ll‘ ance class is capable of 25 knots, and a cruiser like ¹ Virginia class, about 30 knots. In Sea State 7, these typ^e^ of ships can achieve between ten and 17 knots, depends on ship size.

Figure 2 shows the percentage of time ship operators indicated their ships can make full power in different se states. For simplification, 400-foot frigates and 550-fo destroyers are used for comparative purposes. These da*³ are presented in relation to latitude since sea state ¹ closely related to latitude regardless of longitude. Ho^w ever, the correlation between latitude and sea state is ^true only for open-ocean areas. Enclosed or partially encfo^se seaway areas, such as 60° north latitude in the Atlantic an 50° north latitude in the Pacific, are not considered open ocean areas. The gap between Greenland, Iceland, and tn

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q * ^e(^ Kingdom (GIUK Gap) lies between 55° and 65°; Car hk^^atteras *'^{es at} ^5°; ^ant^ Guantanamo Bay, in the pe_r(! °^Can’ *'^{es at} 20° north latitude. At any latitude, the _a|_SQ^Cntag^e of time a shipboard operation can be performed an ^{Varies w}iA the season. The probability for performing Wint^^erat*⁰ⁿ *^s       'ⁿ the summer and lowest in the

^er when the seas are roughest, n, ! north latitude, a 400-foot frigate will make her enf ^lrnum speed only about 40% of the time over the nj ^r.^e y^r; whereas, a 550-foot destroyer will make her j_n ,^lrnuni speed about 70% of the time. A frigate operat- _Sp ^l^^e North Atlantic during the winter will achieve a ^e of 24 knots or more only about 25% of the time and

will be limited to speeds of 15 knots or less about 20% of the time. Under comparable circumstances, destroyers can achieve speeds of about 24 knots or more about 70% of the time and will be limited to speeds of 20 knots or less about 20% of the time.

Ship Operations: The responses to the survey indicate that at higher latitudes during the winter months, antisub­marine warfare (ASW) ships will be limited in their ability to use their hull sonars (Figure 3). For example, in the GIUK Gap, a frigate may not be able to use her hull sonar two days out of three during the winter months, and under similar circumstances, a destroyer may not be able to employ her hull sonar one day out of two. Year round, the operators’ responses indicate that, in the GIUK Gap, frig­ates can use hull-mounted sonars 50% of the time, destroyers about 70% of the time. Responses to the survey also indicate, however, that a variable depth sonar or towed array, once streamed, can be operated in very high sea states, regardless of ship size.

In the GIUK Gap, frigates can operate helicopters about 40% of the time year round, and destroyers about 60% of the time (Figure 4). However, during the winter months at 50° latitude, a frigate can operate helicopters only about one out of four days, and a destroyer only about two days out of five. According to our fleet operators, helicopter operations, particularly from ships of frigate size, are severely constrained at higher latitudes. However, this does not necessarily reflect ship performance under war­time conditions.

Operators indicated that ship size and ship motions had less impact on replenishment at sea than they did on sonar and helicopter operations (Figure 5). A frigate can replen­ish at sea about 55% of the time year round at 50° latitude, a destroyer about 65%.

Thus, the operators’ responses indicated that at higher latitudes, where we would confront a Soviet threat, our frigates and destroyers have very low availability during the winter season for conducting hull-mounted sonar, heli­copter, and at-sea replenishment operations. Between 15° north and 15° south latitudes, destroyers and cruisers appear to have little advantage over frigates in terms of their availability. The following are the approximate lati­tude limits for frigates, destroyers, and cruisers, for per­forming an operation three days out of four:

Limiting Latitude (North or South) Frigates Destroyers/Cruisers

Hull-Mounted Sonar	30°	45°
Helicopter	15°	35°
Replenishment at Sea	35°	40°

Combat Systems Performance: Many observers of the Soviet shipbuilding program have wondered about the mix of ships and the weapons and sensors the Soviets have chosen for their navy. For example, why does the “Krivak”-class frigate have two heavy gun mounts aft and a heavy trainable SS-N-14 ASW missile launcher forward? The “Krivak” also has a variable depth sonar,

the use of which the Soviets have emphasized. U. S. frig­ates do not have these design features.⁴ The designers of the “Krivak” class could have substituted a large helicop­ter hangar and landing deck aft for the two gun mounts and eight fixed missile launchers for the four-tube trainable launcher forward (Figure 6). This configuration would be more consistent with U. S. Navy ship design practices and would have enhanced the ASW effectiveness of the ship from the U. S. viewpoint. However, considering the U. S. operators’ responses to the seakeeping survey, the Sovi­ets’ weapon and sensor selections for the “Krivak” appear logical, because on a ship of the “Krivak’s” size, hull-mounted sonars, helicopters, and unstabilized missile launchers often would be inoperable in the rough waters of the high northern latitudes.

The seakeeping performance of ships and, hence, their combat system performance, can be improved by certain hull-form design features, fin stabilizers, and a helicopter recovery assist securing and traversing (RAST) system. For example, model tests have shown that a Soviet-style large-waterplane-area-type hull form provides better sea­keeping than a typical U. S. Navy-style hull form. Fur­thermore, based on past Soviet practice, we think all Soviet warships, including the large ships of the Kiev and Kirov classes, are fin stabilized; whereas, fin stabilizers have been fitted only on frigate-sized U. S. ships.⁵

As indicated by the survey, availability of U. S. frigates to conduct ASW operations in the winter season, using hull-mounted sonars and helicopters, would be very lim­ited at higher latitudes. If U. S. frigates had been designed with a seakeeping-type hull form, similar to the one cur­rently being considered for the new Arleigli Burke-class destroyer, and if they had fin stabilizers, their seakeeping and combat system performance would have been substan­tially improved. These two features would also provide considerable improvement in U. S. frigates’ ability to conduct ASW operations, particularly at higher latitudes.⁶ This ability would be further improved with installment of a towed sensor, a long-range ASW missile, and a helicop­ter RAST system (Figure 7). Even with all these improve­ments, however, a frigate would still be restricted in her ability to escort larger ships because of her limited speed in high sea states, as compared with the greater speed achievable by larger ships in rough waters.

Conclusion: The speed of frigates and destroyers will be affected more in moderate and high sea states than will the speed of much larger merchant ships and naval auxiliaries. A 20,000-ton, 21-knot merchant ship should be capable of doing about 16-18 knots in Sea State 6. A 400-foot frigate could maintain this speed in North Atlantic head seas only about 40% of the time in the winter season. Large destroy­ers and cruisers should maintain this speed about 80% of the time. In addition, the speed of frigates and destroyers could be further limited when towing passive arrays or operating helicopters. All this suggests that, since ASW escort ships must have a speed advantage over the mer­chant ships they are accompanying to conduct ASW oper­ations effectively, merchant convoys en route to or from Northern Europe could frequently be restricted to speeds

of 15 knots or less. This would be true also for amphih'¹’^. or replenishment forces. Carrier task forces, with ^la destroyers or cruisers operating as escorts, in high no ^ ern latitudes could, for similar reasons, be limited ^t0 speed of advance of about 20 knots or less.       ^

Current U. S. surface ships that are not fitted with^ stabilizers are often restricted by roll motions to co headings into the sea when conducting many operah This often limits tactical options. Effective fin stabih would provide these ships with the ability to operate greater course selectivity.  ,^₀

The U. S. Navy is introducing towed passive arrays i surface ships, which will provide long-range, converge zone detection capability. A towed array, once stream^ can be operated at much higher sea states, albeit , degraded performance, than conventional hull-moun sonars. Sonar contacts in the convergence zone can

^r°teed:

gaini ^SUccess^^u"y localized and prosecuted if the ship t^g ^ln'tial contact with her towed array can maintain d_Uc, _tu^{taCt w}*¹'*^e helicopters, aircraft, or other ships con- °iaint ^{£ 3ttac}*^c‘ T° maintain sonar contact, the ship must may _r^a'^{n 3 course} parallel to that of the submarine, which ti₀ne^^eC*^U’^{re a course} into beam seas. However, as men- _Const_r’ ^SaiP^{s w}hhout fin stabilization systems often are ^sea Th^{ned rod mo}ti°^ns to course headings into the active f^US’      frigates and destroyers not fitted with

tems ^{ln sta}'³'i'^ziⁿg systems and helicopter RAST sys- aiay not be able to maintain or prosecute a contact, destro ^Sea^^eePiⁿg limitations of current U. S. frigates, the ahT^fS’ ^{3nd cru}i^sers could have a significant impact on v₀y ^lt^ °f the United States to resupply NATO by con- C_0nv ^Unn® winter season via high northern latitudes. vj_a m°^{y resu}PPly of NATO could, however, be conducted 'ntprov^ ^^avora^*^e southern latitudes. This routing should ⁿess of⁶  operational availability and effective-

a]_So ^0Ur convoy escort forces. Soviet submarines would d_ec ^neec^ ^to transit further from the Northern Fleet, aircraft^ln^ ^tbe'^{r on}"^stati°ⁿ time. The availability of Soviet Worn i ^^{nd su}riace ships to support submarine operations

In _th ^S° ^{be reduced}-

‘tnpro ^{£ des}^ⁿ °f future ships, we must emphasize Water'u/’ ^{tbe sea}keeping performance of ships in rough n°t ₍₎' ^e must improve the seakeeping characteristics of may ^ ,v, ^Conventional monohull ships, but also ships that fo_rrn ° ^er significant improvements in seakeeping per- (SWath’ ^{SUCb as sma}*i waterplane area, twin hull and n i ^ ^S^'P^S- As an alternative, ships with oversized c°_st th ^erP°^wered hulls could be developed at a modest teris’tjc ,^ereby P^roviding the necessary seakeeping charac- escort ^{S com}f^)at system performance required of AS W ried _W^S' , ^{e s}i^{ze an}d cost of the combat system they car-

Bec°^{U d baVe to} rigidly controlled.) betw_c^{aUSe t}*^{le s}P^eed differential in rough water ing (j ^{a Sma}ll and large ships, we should consider provid- AS\y , ' aircraft carriers with greater antiair warfare and ^strainedh ^^e^^ense capability, so they would not be con­vict h’ ^ ^idower escorts. We should also consider pro- sy_St_enis ^'^sP^ee(l merchant ships with modular combat by _S|_0vv ^{or se}lf~defense so they, too, are not constrained c_0st of ^{Cr esC(}?ris. Compared with the acquisition time and mis_si,_es^Warship_S, towed sensors, helicopters, and ASW sil_{es c} ’ ^air defense missiles, and surface-to-surface mis- ^Uration ^U ^ ^be P*^{aced on} merchant ships in modular config- Xh_e l^easi’y and at modest costs.

5,ooo__t °^V'^et Navy has not built as many 3,000- to they h- °ⁿ ligates and destroyers as expected. Instead, °Pen-₍^{aVe c}°ncentrated on ships larger than 7,000 tons for 1,200 f^Cean operations, and small ships of less than ^ertlPhas°^nS *°^{r coasta}* ^zone ASW operations. They have ^Variab] '^Z^^{d tbe} waterplane hull form, fin stabilizers, thei_r ^ ^dePth sonars, and long-range ASW missiles in ttorther¹^^¹⁶?’ ^^estr°y^ers, and cruisers. Considering the U. 5 ⁿ atitudes in which future operations involving Sovj_et ^an,^{d Sovi}et naval forces will undoubtedly occur, ^aPpea_r ^C °'^Ces 'ⁿ weapons, sensors, and hull design Hen_Ce j¹¹¹⁰¹^⁶ *°gical than those of the United States.

’ arge Soviet ASW cruisers and destroyers, such as

h,

the Kirov, “Kara,” “Kresta,” and Udaloy, have the ship size, seakeeping characteristics, and the weapons and sensors necessary to conduct ASW operations in all but the most severe weather conditions. This suggests that the seakeeping performance and choice of weapons and sen­sors deserve more careful consideration in the design of future U. S. Navy ships.

Acknowledgement: The authors gratefully acknowl­edge the late N. K. Bales, David W. Taylor Naval Ship Research and Development Center, Bethesda, Maryland, and H. A. Meier and K. Walker, Spectrum Associates Incorporated, Arlington, Virginia, for contributing to this article.

'E. N. Comstock and R. G. Keane, Jr., “Seakeeping by Design,” Naval Engineers Journal, February 1980.

²J. W. Kehoe, “Destroyer Seakeeping—U. S. and U. S. S. RNaval Engineers Journal, December 1973.

³J. W. Kehoe, K. S. Brower, H. A. Meier, and E. Runnerstrom, “U. S. and Foreign Hull Form, Machinery, and Structural Design Practices,” Transactions of the American Society of Naval Engineers Destroyer, Cruiser and Frigate Technol­ogy Symposium, Biloxi, Mississippi, September 1982.

⁴J. W. Kehoe, K. S. Brower, H. A. Meier, and C. Graham, “Comparative Naval Architecture Analysis of NATO and Soviet Frigates,” Naval Engineers Journal, October 1980, December 1980.

⁵J. W. Kehoe, K. S. Brower, and H. A. Meier, “U. S. and Soviet Ship Design Practices, 1950-1980,” Proceedings, May 1982.

⁶N. K. Bales, “Optimizing the Seakeeping Performance of Destroyer-Type Hulls,” Proceedings, Thirteenth ONR Symposium on Naval Hydrodynamics, Tokyo, Japan, October 1980.

Captain Kehoe is well known for his work in conducting comparative naval architecture studies of U. S. and foreign warship design practices, for which he received the American Society of Naval Engineers Gold Medal for 1981 and the Legion of Merit. He is currently a partner in Spectrum Associates Incorporated, Arlington, Virginia. Prior to his retirement from the Navy in 1982, he served in three destroyers and three aircraft carriers, including command of the USS John R. Pierce (DD- 753). He has published a number of articles on U. S., Soviet, and other foreign warship design practices and the effects of design practices on ship size and cost.

Mr. Brower is a partner in Spectrum Associates Incorporated, which he founded in June 1978. He graduated from the University of Michigan in 1965 with a bachelor’s degree in naval architecture and has contributed to the design and construction of numerous merchant ships and warships, including the Ticonderoga-class guided missile cruiser, project Arapaho, the FDL and DX projects, and the new NATO Frigate Replacement for the 1990s, DDGX, and FFX projects, as well as several frigates devel­oped for Foreign Military Sales. Recently, Mr. Brower has contributed as an analyst, editor, and author of an extensive assessment of the engi­neering design practices of tanks, missiles, aircraft, ships, and electron­ics, and he has been the author of several articles on international defense affairs and coauthor of several articles on international ship design.

Mr. Comstock is currently Director of the Hull Form Design and Per­formance Division, Naval Sea Systems Command. He received his BSE degree in naval architecture and marine engineering in 1970 and his MSE degree in ship hydrodynamics in 1974, both from the University of Michigan. Mr. Comstock began his career with the U. S. Navy in 1974 as a seakeeping specialist in the Hull Form and Fluid Dynamics Branch of the former Naval Ship Engineering Center. He was previously Head of the Surface Ship Hydrodynamics Section, being responsible for recent hull form designs, including Arleigh Burke-class guided missile destroy­ers and the Navy’s new mine countermeasures vessel and salvage ship.