Finding a Frigate for the Future

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The deep-Vee hull might be the answer to the dilemma of finding an afford able frigate with a good seakeeping ability. The current frigates’ prob­lems in the rough waters of the North Atlantic had raised ^^g****¹*^ doubts about whether there would be future frigates.

The Garcia (FF-1040)- and Brooke (FFG-l)-class frigates were the first to go during the first stage of military budget belt-tightening. The 46-ship Knox (FF-1052)-class frigates are now 20 years old. If the Navy should decide to design a replacement frigate, it should be a ship that:

► Is capable of countering the threats of the 21st century, with a combat system that can be rapidly upgraded with advanced technology

► Has significantly better seakeeping ability

► Is more survivable, particularly in the area of signature

reduction

► Is more reliable and operated by a smaller crew ► Is affordable in large numbers

Experience dictates that if a future frigate is to be af­fordable in large numbers, it must be relatively small. Many U. S. Navy planners think an affordable frigate should be no larger than 4,500 tons in displacement. How­ever, a ship of this size has only limited seakeeping ability when operating in the higher latitudes of the North Atlan­tic and Pacific, particularly during the winter. Improving the seakeeping capability of ships of this size, therefore, is a vital requirement for an affordable future frigate.

The commanding officers of U. S. Navy frigates report that because of poor seakeeping performance above 50° north latitude, today’s frigates can make their maximum speed only about 40% of the time annually and can achieve a speed of 24 or more knots only about 25% of the time during the winter.¹ They also report that during the winter they are limited to speeds of 15 knots or less, 20% of the time. As a result, battle groups and convoys of larger ships will often be restricted in high sea states to a speed of about 12-15 knots, to allow escorting frigates the speed advantage required to maneuver relative to the battle group or convoy.

In the Greenland-lceland-United Kingdom Gap, the commanding officers of today’s frigates report that they may be able to use their hull-mounted sonars only about one day out of three during the winter, and operate their helicopters only one day out of four. Thus, the seakeeping limitations of today’s frigates could limit the ability of the United States, particularly during the winter, to resupply NATO’s Northern Flank by convoy or to support strikes by carrier battle groups.

Current Design Concepts

Recognizing these limitations, a study was conducted to determine what size platforms and improvements in tech­nology could be used in designing a future frigate in order to achieve significant improvements in seakeeping perfor­mance.² This study, which included innovative design concepts, as well as the familiar displacement type monohull with a round-bilge hull form, has shown that it is possible to achieve significant improvements in seakeeping performance by using a small waterplane area twin-hull (SWATH) concept.

The study included a frigate designed with a SWATH- type hull, the combat system and endurance of an Oliver Hazard Perry (FFG-7)-class frigate, and about three knots less speed in calm water. The results indicate that a SWATH design would be about 7,000 tons in f^u‘ load displacement, almost twice the displacement of <h^e FFG-7, as originally designed. This greater displacem^£n| is partly the result of the inherent requirements of ft® SWATH concept and partly the result of the impact o U. S. Navy design practices and criteria on ship ^size. Also, experience in SWATH design, construction, an operations indicates that there are more technical risks an higher costs associated with a SWATH design than with^a conventional monohull. SWATH frigates, however- would have significantly better seakeeping ability and ^L fully operable in the North Atlantic more than twice often as today’s frigates.

The study also indicates that the seakeeping ability ° monohull frigate can be further improved by deliberate. designing an oversized ship. For example, a 9,000-n’¹’ monohull design, with the same combat system as '

7.0-            ton SWATH and the good seakeeping hull form ^ the Arleigh Burke (DDG-51)-class destroyer, will ha^v seakeeping performance comparable to the 7,000-t SWATH and much better than today’s 4,000-ton frig^ate^

But the Navy cannot afford to replace today’s fl^{eet 1}

4.0-            ton frigates with a riskier 7,000-ton SWATH ot much larger 9,000-ton monohull ship design, both ¹ which would be considerably more expensive. Howe^vc j this does not mean the inadequate seakeeping performs ^ of today’s frigates must be perpetuated if a future frig^atl" to be restricted to an affordable size.

Deep-Vee Hull Form

In recent years, a new hull form has been develope Europe which, based on model test results, will p^r°^v _s better seakeeping ability and higher speed in rough ^s than any existing round-bilge hull form. Model tests¹ ^ cate that the new deep-Vee hull exhibits less heave, P¹ and yaw, experiences lower vertical accelerations, comparable roll motions and lateral accelerations, ^antl _s a much lower probability of slamming in high sea st than a round-bilge hull. This new design concept i^s ^ HRS deep-Vee hull form, developed by Hydro Resc’^arL Systems S.A. of Geneva, Switzerland.        ^

Unlike the hard-chine, V-shaped hull form used m ^ small, high-speed planing craft with which we are U¹¹’^ iar, the deep-Vee hull form is designed for operatic¹¹ ₍ speed-to-length ratios associated with larger displace'^111, type hulls. A deep-Vee hull is characterized by:

► A bow that extends below the keel line                         jn

► After sections with constant deadrise that increase* the area immediately adjacent to the transom

► Reverse keel drag, with parallel buttock lines

► A stem skeg that acts like a bilge keel

► Short bilge keels located aft in the way of the ^c g Head Seas Performance: Figure 1 shows the seakee’P j

model test performance of a deep-Vee hull form m ^ seas. The model test data are representative of a 4,5o ^ frigate with a length of 455 feet, operating at speeds ^ to 25 knots in sea state 6. The values shown in Figure j_:t well within the U. S. Navy’s seakeeping limiting ^Cl1

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Proceedings

/June

for full power operations: 8° roll, 3° pitch, 0.4 Gs vertical acceleration (bridge), 0.2 Gs lateral acceleration (bridge), and 20 slams/hour.³

Slamming: The commanding officers of today’s frigates generally reduce speed and/or change course after they experience only a few successive slams in a short period of time in order to avoid structural damage or physical inju­ries. A ship with a deep-Vee hull has a much lower prob­ability of slamming than a comparable-size ship with a conventional round-bilge hull (see Figure 2). This appears to be the result of a combination of lower ship’s motions and a relatively deeper bow immersion. Therefore, the deep-Vee ship should be capable of maintaining a higher maximum speed in rough seas.

Speed in Rough Seas: Based on a limiting criteria often slams per hour, the speed of a 4,500-ton round-bilge frig­ate is limited by slamming when operating in waves higher than about 13 feet (sea state 5). See Figure 3. By compari­son, the speed of a deep-Vee frigate is limited only by resistance when operating in waves of up to about 30 feet (high sea state 7), not by slamming. Consequently, be­cause the commanding officer of a deep-Vee frigate will not have to reduce power in low and moderate sea states in order to reduce slamming, the deep-Vee should be able to maintain significantly higher speeds in higher sea states than a comparable round-bilge frigate.

Roll Motions and Lateral Accelerations: Deep-Vee ship designs generally have much more stability (GM) than comparable round-bilge designs (i.e., a GM/Beam ratio of 15-20%, compared to 8-10%). Yet, the roll motions and lateral accelerations of the deep-Vee and round-bilge frig­ates are comparable. Figure 4 indicates that a deep-Vee frigate will not exceed the U. S. Navy’s limiting design criteria of 8° of roll and 0.2 Gs of lateral acceleration when operating at speeds of about 22 knots or more on any head­ing. At speeds below 22 knots, the deep-Vee frigate will be limited when operating within about ±30° of beam seas. Similarly, a round-bilge frigate will not exceed the criteria when operating at speeds of about 20 knots or more on any heading. At speeds below 20 knots, the round-bilge frigate will be limited when operating within about ±60° of beam seas.

The use of a fin stabilization system will allow a deep- Vee frigate to conduct unlimited operations on all head­ings and speeds in waves of 15 feet or higher (low sea state 6). Because of its hull form and high GM, the fins used in a deep-Vee design would have to be larger than those used in a conventional round-bilge frigate of comparable size.

Deep-Vee Hull Features

Calm Water Resistance: For ships of equal displace­ment and length, the resistance of the latest deep-Vee hull form is somewhat higher than that of an FFG-7 round- bilge hull form at speed-to-length ratios below about 1.2 (i.e., about 24 knots for today’s sized frigates) but is less at higher speeds. Model tests indicate that with a slight increase in the angle of trim by the bow, the powering requirements of the deep-Vee design can be reduced by about 5%, with only a limited impact on seakeeping per-

formance. The calm water resistance of the deep-Vee de­sign could be further reduced if a reduction is made in the prismatic coefficient of the hull. Therefore, it may be fea­sible to further reduce the calm water powering penalty associated with a deep-Vee hull form.

Stability: Its Vee-shaped lower sections give a ship with a deep-Vee hull a relatively higher vertical center of grav­ity than that of a ship with a round-bilge hull and the same depth amidships. However, a deep-Vee hull has a wider beam and a much fuller waterplane area than a round-bilge hull. This provides much more transverse inertia. A ship with a deep-Vee hull, therefore, will have much more sta­bility than a ship with round-bilge hull.

Deck Area and Arrangements: The full waterplane area of a deep-Vee hull provides more useful deck area than a comparable round-bilge hull. However, the potential ben­efits of the full waterplane area on available deck area will be partially offset by the auxiliary machinery spaces of a typical 4,500-ton deep-Vee frigate which may result in having to extend one deck higher because of the Vee- shaped lower hull sections. Therefore, the superstructure of a deep-Vee frigate should be comparable in size to the superstructure of a similar size round-bilge frigate.

Comparison With a Round-Bilge Frigate

Roll Angles and Lateral Accelerations .

------------------ 11 Knots

£,k>

In order to compare the deep-Vee and a round-bilge hull form, two new ship designs were developed for a frigate of about 4,500 tons. See Figure 5. They were modified repeats of the Oliver Hazard Perry-class frigate. The de­signs were the same length, and had the same internal deck area, number of accommodations, propulsion sys­tems, and endurance cruising range. They had comparable lightship weight and speed.

Both ships were designed with the combat system of the FFG-7, except an Arleigh Burke-class destroyer 29-cell vertical launch system was substituted for the FFG-7’s Mk-13 missile launcher. The FFG-7’s combat system was selected only because it provided a typical budget of pay­load weights, vertical centers of gravity, deck areas, and electrical loads.

Both designs include a twin screw, gas turbine propul­sion plant consisting of two LM-2500 gas turbines rated at 22,500 brake horsepower each, with a mechanical cross connection between the shafts to permit cruising opera­tions on one shaft. The FFG-7’s machinery box arrange­ment was used for both designs. In recognition of the Vee- shaped lower hull sections, the depth of the forward auxil­iary machinery room was increased in the deep-Vee design. The endurance cruising range of both designs was set at 4,500 nautical miles at a speed of 20 knots.

Light Ship and Load Weights: The light ship displace­ment of the deep-Vee design, with margins, is about 5% more than that of the round-bilge design. Most of the in­crease is in ship structure. The greater light ship weight of the deep-Vee design is primarily a function of its higher hull girder bending moment.

An increase in light ship weight is usually associated with an increase in cost. However, experience in the con­struction of the Turkish Coast Guard’s SAR-33 class pa-

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a_nce^lorm on annual operating costs, model test resist­or, f,^ata were used to estimate an annual fuel bill for kills ^{66 anc}i round-bilge frigates. The estimated fuel ^f°_r f_rr^cre based on a typical speed and operating profile in ^ ^^ates> operating in the various sea states encountered thi_s h ^Nortb Atlantic over the period of a year. Based on ffjg_at^ata’ the estimated annual fuel bill for a deep-Vee ffig_atg ^Was virtually identical to that for a round-bilge

tfo) L

⁽he °^ats’ ^w*^t'^{1 a} deep-Vee hull, suggests that because of ^catj^SlrTlP'^{er s}h^aP^e °f the deep-Vee hull the platform fabri- t(i_an°^{n cost} per ton of a deep-Vee frigate should be less f_aL . ^aat of a comparable round-bilge frigate. The overall to ^^at'^or> cost per ton of a deep-Vee design is estimated /bout 7% less than that of a round-bilge design. t°t_aj ^ore> based on the cost per ton of the FFG-7, the b_e Pktform acquisition cost of a deep-Vee design should j °^ut 5% less than that of the round-bilge design. ^tifV \h^{d in <}~^a^^m Reiter and Rough Seas: In order to iden- ^ee him impact of the calm water resistance of the deep­en form

Discussion

Comparison with Other Concepts: The study discussed previously shows that a SWATH frigate, with a payload­carrying capability equivalent to that of a round-bilge frig­ate has significantly superior seakeeping performance. However, as illustrated in Figure 6, it is about 30% larger in displacement than the round-bilge frigate, requires about 30% more fuel, and has about 6% less speed. A round-bilge frigate, with equivalent seakeeping perfor­mance to a SWATH, is about 30% larger in displacement than the SWATH, requires about 10% less fuel, and is one knot faster in calm water performance.

From the standpoint of the seakeeping performance of frigates with the same payload-carrying capability, a deep- Vee frigate: will provide seakeeping performance consid­erably superior to a similar size frigate with a round-bilge hull form, and similar to a larger SWATH frigate.

If the two modified repeat frigate designs discussed ear­lier had been developed not only to carry the same pay­load, but also to have an equivalent seakeeping capability, the round-bilge frigate would have been much larger in displacement than the deep-Vee frigate, required more endurance fuel, and had less calm water speed. If a SWATH frigate design had also been developed to carry the same payload and to have an equivalent seakeeping capability, it would also have been larger in displacement, required more fuel, and had less calm water speed than the deep-Vee frigate. Therefore, when seakeeping capability is the basis for comparison, designing a future frigate with a deep-Vee hull should achieve a significant savings in size, acquisition, and life-cycle cost, compared to a round- bilge or SWATH frigate.

Operational Capability: With deep-Vee frigates acting as escorts, it should be possible to increase the speed of convoys and battle groups operating in high sea states by nearly 50%. See Figure 7. A typical 20,000-ton merchant ship or Navy auxiliary ship with a maximum calm water speed of 21 knots will be capable of achieving speeds of about 16 knots in sea state 6. This is about the same speed a 400-foot round-bilge frigate with a calm water speed of 28 knots can achieve. Therefore, since antisubmarine war­fare frigates must have a speed advantage over ships in company if they are to conduct ASW operations effec­tively, convoys and battle groups en route to or from Northern Europe could frequently be restricted to speeds of 12 to 15 knots in order to allow escorting round-bilge frigates a speed advantage. However, because of its supe­rior seakeeping performance, a deep-Vee frigate of the same size should be able to maintain a speed advantage up to sea state 7.

The availability of a future frigate could be significantly improved by the adoption of an improved round-bilge hull form, like that of the DDG-51 class, with roll stabilization fins. (See Figure 8.) Use of a towed sensor and a locally stabilized stand-off missile system that can launch missiles in high sea states would further improve availability. With a deep-Vee hull form, the availability of future frigates to conduct ASW operations in the higher latitudes of the North Atlantic and Pacific is estimated to be about 95%.

^Ings / June 1989

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