ABSTRACT
The paper presents a status report on one of a number of advanced ship concepts that the U.S. Navy has under develcpmcnt. the Small Waterplane Area Twin-Hull (SWATH) ship. This concept represents a relatively low cost
option for small and medium size ships to carry multiple
ürcr.zít (helos or VSTOL). with high displacement ship speed capatili ties in advce seas. SWATH ships also offer potential for improved sliipioard sonar performance. Technology progress is discussed in the context of verified theories
nd techniques for prediction
of power and
seaway induced loads, definition of structural design criteria, andsrructLlrul design synthesis. Progress in the areas of seaway motions and control technology are covered ..Ship design tradeoffs are covered through a discûssion of design studies of small, high speed. and medium sized, moderate speed
SWATH ships. The results of these trade-off studies indicate
that balanced SWATH ship designs with good performance
and payload cttaracteristic can be-achieved. IN'T R O DU CT ION
For many centuries marine vehicles operating at the air/sea
interface have been pdominantly of one basic
tyçemouohull displacement ships. More recently; in the past 30 years. intensive technology development has focused ou other surface ship concepts so that, today, one has quite awide muge of choice between types of ship platforms. Each of these has certain characteristics and limitations which, after careful consideration, may make it the 'best' for particular appficatiòns.
-The purpose of this paper is to present a progress teport
on the ongoing development of one promising displacement ship concept. the Srnall-Waterplane.Area Twin-Hull or SVATI-I ship. This name and its acronym were selected to reduce confusion between this concept and another type of twin-hulled shipthe conventional catamarañ (which is du-ternt in many important respects) as well as to eliminate the
plethora of names coined by individual investigators for this quite old ship concept (e.g., MODCAT, modified catamaran, semi-submerged ship. S3, low waterplane catamaran TRISEC. SEdCAT. etc.)
lu content the paper will focus principally on overall progres.c in ship technology and design -development..Tó place these matters in context; brief mention will be made beforehand of the historical backgrouñd, why the Navy is interested in the SWATH concept, and its relation to other
ship concepts.
-SWATH HISTORY
In adverse seas ship motions, accelerations and slamming, green water on deck, loose- gear hying about, and crew easickness frequently results in damage, injury, loss- of operational effectiveness, and the necessity to slow- down or
to cake a more devious course. lt has-long been known that if the greater part of the ship's buoyant volume was placed below the air/sea interface, this would enable planform area at the interface ro be drastically reduced with consequent
THE SMALL WATERPLANEAREA TWIN HULL (SWATH) PROGRAM -A ST-ATUS REPORT
By
Mr. Seth Hawkins - Program Technical Manager Naval Ship Research & Development Center
and
Mr. Theodore Sarchin - Advanced Ship Studies Secuon Naval Ship Engineering Cnter
reduction in motion response due to wave excitation forces.
This is tIse fundamental ideabehind the small waterplane area ship concept. -Starting back at tIse end of the nineteenth century, several patents have been issued on variations of this
fundamental idea (Lundborg, 1880; Blair, 1930; Creed, 1946; Leopold, I969, and Lang, h97h, to mention a few). Sorne of these covered ingle-huU configurations, others
twin-hull, and still others both single- and twin-hull versions. Single hull. smahl-waterplane-area ship applications continue
to be the subject of relatively low effort studies within the
Navy.' However, in the late 1960's twin 1juli, small waterplane area ship concepts began to receive increased attention
outside of the U.S. Navy. A 1200 ton small
waterplane craft was built by the Dutch. for offshore drillingsupply work. Litton Industries model tested and publicized technical papers on what they called TRISEC2. Also, in the late 1960's, increased interest within the U.S. Navy mani-fested itself at the Naval Undersea Center (NUC), the Naval
Ship R & D Center (NSRDC) and the Naval Ship Engineering
Center (NAVSEC). As studies at these organizations pro-gressed and technology- efforts began to bear fruit, greater understanding of the SWATH concept and its potential became possible. Today, all of the previously diverse Nay
efforts are under a single development program fùnded, primarily, by the Naval Material Command as shown in
Figure I. -uSDC nd DIPVS SWATN TEdI SSP TEdI EI1
TDCK
bl13s-TECH1ISCIE UVEflSIThIT
.aboratoi1um 'icor ScheepshydmrnechanlcaArchief
.-Mekeiweg 2,2628 CD Deift
Lz O15-758873 Fac 015.781836 m51Oç swAt, Pias 001cl'C
MmS5mgaS/J!sl0N EL 15Cl $ CIUSO%TATIOUlE Opti. & SWATI Nils. tIALI CllTlACT0US-s'all.. (PROCEDU Rn I COiCEPTI) __ RAISEC
-) sisc
V
iocFigur. 1, SWATH PROGRAM ORGANIZAT!OS
The program objectives are to:
s- Develop essential SWATH technology ici the areas of;
l,ydrodynamics. structures,, machinery, mission equipment
interfaces; and design procedures to permit rational con figu- -
-ratiòns-- to. be- developed,, costed- and compared with: other
pia tío mt alternatives. - --
-- Develop-- and carri out technology--related trials on the. 190-ton SWATH. workboct. the SSE' KAJMALINO, being
built for the Naval Undersea Center.
Upare successfully achieving these objeçtive it- is pro-'
posed that an Advanced Development program be-- under-taken to demonstrate the SWATH concept technically and operationally with a ship of a size to be of tactical value to
the U.S. Navy.
WATI
WHY SWATH SHIPS?
When Navy planners have available to them such surface platform options as hydrofoil craft, air cushion vehicles and
surface effect ships, in addition to coiiventional monohulls,
one might, quite logically, ask: why SWATH ships?
As mentioned earlier, the unique feature of small waterpiane area ships in comparison to rnonohull.s is the relative decoupling of the body from forces imposed by a
seaway. If accomplished by means of twin hulls, with a cross struwre or box tying them together, a subset of design opr:ons result. - These design options are shown in Figure 2. When properly combined for a given set of mission
specifica-tions certain benefits accrue, the most important of which
will now be briefly discussed.
- CLEAR OK SSTR
-DECK PLAN
SINGLE & MULTI (DECKS)
CRO STRUCTURE r ACTIVE FINS
IF
MANUAL AFT OR FWD&AFT AUTOFigure 2.. SWATH SHIP DES(GN OPTIONS
First, and not necessarily in order of importance, SWATH ships tend to be roughly two-thirds to three-quarters the length of an equivalent displacement monohull. For small and medium size, high speed ships, this feature results in a deck plan which, instead of being long and narrow, tends to be twice as wide and thus much more conducive to accommodating multiple VTOL/STOL aircraft. Figure 3 shows a plot of number of aircraft as a function of ship size. Existing monohulls, as can be seen, accommodate only one or two helos in the size range shown: design studies to date show, ort the other hand, a great increase in the number of
aircraft which can be accommodated on small SWATH ships due to more flexibility ¡ri deck arrangement.. On such ships, it appears that the most favorable arrangement is a combination
of he!os with a hangar and superstructure forward; on larger
ships helos and/or VSTOL can be accommodated with either a SUperstrUCtUre or with an effectively clear deck for take off and an island on either side of the main deck.
HELICOPTERS HELICOPTERS & V/STO L AC
s
lo
15DISPLACEMENT (ICTONS FigureS. NO. OF AIRCRAFT VS SIZE. FOR SWATH
SHIPS ANO MONOHULLS
In addition, the majority of the useable volume of SWATH ships is in the cross Structure. Since the crocs
Structure is little more thin a box, it is highly conducive to not only simple modular construction but also to accom-modating modularized and interchangeable payload elements.
Achieving the latter-in a one- or two-level box has agreater potential than in a long, narrow, multi-deck ruonohull which, inaddition,has hull curvature to contend with.
- The short and wide deck planform is also common to
both types of air-supported craft, air cushion vehicles and surface effect ships: with the former, however, on-decl air screw propulsion schemes presently in use impinge on the capability to support multiple aircraft Hydrofoil craft,as presently configured (or conceived) are small flying mono-hulls and thus the remarks mentioned above apply. Further-more, hydrofoils much in excess of a few thousand tons displacement are not envisioned as being practical due primarily to foil size constraints.
The second 'why' of SWATH ships derives from the previously mentioned fundamental aspect of the concept:
the-reduction of waterplane area and redistribution of buoyant volume into submarine-like lower hulls tending to decouple the ship from the excitation forces of a seaway. When this inherent factor is comii'ed with the vertical plane control provided by active (manual or automatic) control surfaces,
greatly enhanced performance can be achieved in adverse seas
n comparison to monohulls (and also conventional
catama-rans) of equivalent size. Figure 4 is indicative of the
maximum vertical seaway motions potential of SWATH ships, as a function of sea state, in comparison to
conven-tional catamarans and monohulls.
I.-
u-j15
O I-O 10 X 5 SEA STATE-
Q'.-o'
s-.
HEAD SEAS 20 KNOTS 6.Figur. 4. MOTIONSCOMPARISONS
In Ilse head se case the conventional catamaran is the worst and monohufis of either the same-- displacement or length are only slightly better in performance. The SWATH ship motions from Ref. 3 are seen to be much better than either of the other ship types shown. Superior SWATH ships
motions result from a combination of reduced waterplaqe
area and the added effects of horizontal con troI surfaces. In addition to head sea performance, SWATH ships have
excellent motion stability in adverse- seas when- running at
quartering and beam headings, as deduced from observations of manual arid radio-controlled model experiments. -
-The reduced seaway motion qualities of SWATH ships make them, amongst other things, excellent- and- safe
all-weather, multi-aircraft- pbtforms. ' - - -
-A third 'why' of SW-ATH - ships is'- a by-product. of the-improved. seaway motion qulities.-Conventionak-tnonohulls
and marans,.wbexroperating:in ah dseas,.are knowit to'-be sea state limited as- far. as. speed i concerned.. Figure--5
shows comparisons of the ratio of speed ¡n waves- to speed in calm water for various types of craft as a- function-of, sea -
-state.. As: can-. be seen- SWATH- sbips (a -walias fully-
-submerged- foil hydrofod craft) manifest. superior ability to
-sustain speed in adverse seas-a. distinct and- Important' tactical consideration for small and-medium- size'ships. -
-'4 STRUTSINULL SWATH
f)
C SHIPS LOWER HULL C00
INBOX INHULLS MACHINERY FIXED FINS1000 TON
HYDROFOIL
Another reason for interest in SWATH ships derives from
their underwater configuration and its relationship to ship-borne sonar adaptability and performance. By placing trans-ducers in the bow of both of the widely spaced lower hulls,
and possibly conformal-type arrays along the outboard sides of the lower hulls, it is expected that superior detection. bearing resolution and classification capabilities wilt result. Such expectations are, of course, dependent upon the noise
characteristics of the platform itself; in this regard there are a number of features which are worthy of mention.
Sonar transducers need not be placed in appended domes as they are on conventional monohull desigas.
The sonars will
be deeper in the water than
inconventional moriohulls as SWATH ship keel drafts, for a given displacement, tend to be greater than those for a monohull.
The combination of thin struts at the waterline and their position aft, well away from forward transducer locations, should eliminate or greatly ameliorate bubble sweepdown problems.
Reduced quenching, due to superior motion qualities in a seaway, should extend sonar performan.
The fact that SWATH ships can be operated up to
cruise speeds using only the power plant and propeller on one side will permit a high degree of silencing of the other side of the ship thus reducing self noise. -.
lt is expected that, qualitatively, improved flow
characteristics into the propellers will exist onSWATH ships
due to their general shape and the fact that walce producir.g elements (strut, control surface) will be further in front of
the propellers than on monohulls. Therefore, onset of
propeller cavitation noise should be delayed to somewhat higher speeds.
Studies to date have tended to venfy the excellent
shipbome sonar potential for SWATH ships.Another area of interest is costs. Although a controversial topic and not one easily generalized certain cost-related factors have become evident. SWATH ships, since they are
not veri high speed ships (with the concomitant higher technology) will tend to cost considerably less per ton than hydrofoil and air cushion supported ships. On the other hand, investigations to date reveal that SWATH ships will
cost more than monohulis of the same total displacement. With regard to SWATH vs monohull costs (without GFE), studies indicate the following trends:
(I) Comparing medium size ships (say over 4000-5000 tons) where botir. are- of. hybrid (steeL and. aluminum) construction, it appease that. SWATH ships will tend to cost about lO percent more than an. equivalent displament rnonohull.
(2) For smaller ships (say around 2000 tons) where aluminum construction and relatively higher technology subsystems must be considered, costs on a dollar/ton basis will increase in comparison to medium size ships. (Whether the rate of increase will be the sanie. for monohulls and SWATH ships is not evident at this time.)
Costs per ton, even omitting GFE costs, can, of course,
3
be misleading. Ultimately, it is of interest to compare costs to achieve certain mission objectives: efforts are presently underway to do just this.
In summary, SWATH ships represent a relatively low cost option for small and medium size ships (2000 to 15,000 tons) with high displacement ship speed capabilities in
adverse seas and considerable potential for improved ship-borne sonar performance. Their inherent characteristics and proportions also make them highly conducive co- accgrn-modating and operating, under relatively severe weather conditions, multiple aircraft, whether they be helos, VSTOL
or a combination thereof.
ATH TECHNOLOG'( - REQUIREMENTS AND STATUS
As a prelude to covering the status of SWATH technology
development it may be well to discuss some general charac-teristics of SWATH ships and the exploratory development
program which has been followed.
Experience to date has shown that a good SWATH ship
must be fairly well 'tuned' to obtain a good balanced design. The necessity for design tuning results from a high degree of technological coupling which, in turn, has had a significant. effect upon the approach to technology development. It has necessitated close working relationships between technol-ogists in different areas. Furthermore, a fairly substantial feed-back loop between the designer and the technologist is
required.
Increasing the level of technological and design confi-dence for SWATH ships has been an implicit goal, and the
effort to date has been directed along the following lines:
develop (and expand upon as necessary at any given time) theories anddesign criteria in key technological areas
and verify them under controlled (laboratory) conditions probe and verify, both qualitatively and quantitatively, the concept as a whole with radio controlled or small manned con figurations.
With the foregoing in mind and understanding that any given
bit of technology must, ultimately, be viewed in the context of its e t'feci on SWATH ship design, progress in hydro-dynamics and structures and materials technology efforts will
now be discussed; machinery considerations as well as how
various design options (Figure 2) tradeoff technologically and in a design sense will be treated subsequently.
HYDRODYNAMICS The principal areas of interest are:
t. Drag and propulsion.
Seaway motion and control. Seaway induced structural load. Calm water maneuvering.
Drug and PropuLsion. The drag and propulsion characteristics
of any ldnd of air/sea interface vehicle are of interest from two viewpoints; first in calm water and secondly in rough
water.
As appropriately pointed out in reference 4, for a given displacement, SWATH ship configurations tend to- have roughly twice the wetted area of a comparable uionohutL At first glance this factor alone might seem to be enough of a. detriment to avoid consideration of the concept; however,
rtain factors militate against this.. The.totaI calm ws.tes drag.-... -' of a ship can be expressed as:.
RR+R
where R constitutes the- frictional componentof drag and is a function of wetted area and- Reynolds: Nimzber
As-.-"ReyDolds mbr Re - VI/., wbc V Is the thip . '
length, end p 1g thekIneInAØCYISCIY (1.2817 X 16 (L'/secCetviUu.
(emture oI59F, --
-0123 4
5 6SEA STATE
mentioned, the relative wetted areas of SWATH ships and monohulls of tise same displacement strongly fovors ¡nono-hulls.
Residuary resistance (RT) comprises hull wave-snaking
drag, hull form drag, wave interference effects between the
hulk arid eddy making resistance of tise bulls and appendages. lt is low wavemaking drag in particular where the greatest
effort has been placed, to permit offsetting, at high speeds, much of the inherent frictional drag penalty of SWATH ships.
Three separate efforts have borne fruit in the context of
SWATH hull form hydrodynamic design and drag prediction. Chapman5 has developed a simplified and rapid method of
predicting wavemakjng drag for SWATH forms. This
tech-nique has been used in the early stages of SWATH ship design studies. Lin, et al.6 have developed a more detailed technique for drag prediction which can be used during the early stages
of design refinement. And Pien7 has developed a highly complex hull form optimization procedure for the latter stages of design. Figu'e 6 shows example comparisons of theory and experiment for the former two procedures. Results are shown in the form of CR coefficient values, defined as drag divided by the dynamic pressure.
Experi-mental verification of these techniques has been achieved for
a wide range of configuration optioñs, i.e., I and 2 strut configurations, lower hull length to diameter ratios varying from appro,cimately 10 to 20, different lower hull cross
sections (circular and elliptical) and different strut shapes. In addition, the effects of cambering the struts (and lower hulls) has been investigated analytically and exnerimentally.
5.0 4.0 = 3.0 C., 2.0 1.0
Freude emb (nidL._..i..4 '/.JL. wfie,vV ithe ielodly, L the length, md g
DlIbae4 nset.
SHIP TYPE
PROPCJLSIVE
COEFFICIENT
Tabe 1. PROPULSIVE EFFICIENCY COMPARISON
Another powering-related consideration which proved to be a problem for a while had to do with theoff-desi-power
fuel consumption characteristica of aircraft derivative gas
turbines in combination with the desire (for acoustic reasons)
to be able to run with only the power plant on one side. This
necessitates having the propeller on tise other side of the ship Operate at a minimum drag condition. The results of an analysis of this problem shows that reduced drag propeller pitch settings can be achieved such that drag and fuel consumption characteristios are acceptable.
Rough Water Powering. A trend which isapparent in much of the advanced ship development workunderway in the Navy
(and for monohulls as well) is to design ships which have extended weather capability, i.e.,
they are
capable of sustaining higher speeds and remaining operational in adverseseas. The problem with monohulls has been thattheyare not usually power limited (particularly in head seas) but rather motions/acceleration limited, lésding to voluntary acts of slowing down or changing course radically
in order to
ameliorate the situation.lt is in adverse seas where the SWATH ship/monohull powering question tends to reverse itselfeven short of the point where a monohull will voluntarily slow down As was
poin ted out in Ref. 4, loss of morsohull speed in a seaway (or
increased power required to maintain a given speed) can be traced to a combination of factors increased-resistancedue to motion and wave action and excessive rudder control surface action; and. a decrease in propulsive-efficiency as a
result of air drawing accompanying propeller emergence due
to large motions, The reduction of motions dut to deaeasing
waterplane area and the fact that SWATH ships tend.to have -a high degree of direction-al st-ability (-and- thus. less. rudder
action) all tend to offset those. tactor which increase monohull powering requirements in a. seaway. Thus, in wear/re, where Ir cow, SWATH ship. drag and powering requirements tend to be-. superior;- or- at least equivalent, to those of a conventional monohulnd-in zea-conditions where a monohull would- be forced
to.voluntarlly-slow down but a SWATH shicouI.continue.to-opee-.
because of its superior motion characteristics theSWATKvs
monohuli powering argument-ia blunted- evem
fùrther-It is worthy of menüon that- thepbnnecLtrfaj, by ML'. and- NSR.DC, of the first. large mamted--SWATW
workboat,-SSP KAIMALINO, include, not only-- side-by-sidc way
motion experzmemts with-a monohull butalso.4etermjnjo!,-of the. relative. increase. in: por. required way foe
-both ships..
0.62 - 0.67 0.75
-i
Propulsion. The second factor tending to offset Che relatively
high wetted surface penalty ofSWATH ships has to do with the demonstratedly superior propulsive coefficients in
com-parison with monohuhls, particularly twin screw installations
which are common to high speed applications. The more deeply submerged hulls of SWATH conflgrations when combined with planetary reduction gears may permit larger
diameter, slower turning and therefore more efficient
propel-1ers ro be used. And, more importantly,the favorable wake contlicions bhjnd each of the lowerhulls of a SWATH ship will tend to produce higherpropulsive coefficients. Table I
which shows some comparativeresults between high speed, single and twin screw monohulls and SWATH configurations, reveals that improved SWATH ship propulsive efficiencies can be achieved.
ÌWIN OR SINGLE SCREW HIGH SPEED/LENGTH RATIO (VIVI >1.2)
PER DEMI HULL. SINGLE SCREW SWATH o 0.5 1.0 1.5 2.0 V/VI KTS/F14" -EXP ThEORY NSR DCSWATh Z z NUCSWATh I
s______
Fjjre 6. ORAG PREÒICTIOPIs- EXPERIMN-rAi VS ThEORY
Some of the more
lient general findings will bementioned here and others, which are more closely tied in with achieving a balanced design, will be covered
Subse-quently.
l_ In general, higher length-to-diameter ratios for the lower hulls are preferred, the penalty of slightly greater wetted surface (for a given displacement) being more than
offset by reduced residuary drag at high speeds.
Minimum strut thiclasem to lower hull diameter ratios (within minimum waterplane and structural. design and machinery arrangement constraints) are prefeaved.
Caxnberingof the hulls and/or struts doesnt seem to have a benefjcjaj effect at higher Froudenumbets5 but does.
appear to have- a minor benefit at. Froude numbers around
\torflS
IIIJ ControL The (kv.!oprnent of motions aitci.:(','Ç!)L t::IlnoL)y for SWATH ships has been approached trQ!
:t
lir(iis:
-.-. tr.fl;hIe-çh()oirt with ridio coutrokd acid small
SWATH contt&ritiots. and
4 vlrpruett of motions arid conrol theories
(inctdi;g veriliearior under coutrolld onditioiis).
lo u pr.Ce technical sense. the first approach tends to he
nor qua1iitive but it does provide a rapid means for
discoring troublesome questions in areS of operation and for getting a gross picture of overall seaway response of SWATH configurations as against the more abstract (but exact) responses in individual decrees of freedom. A 5-foot radio controlled model of the 190-ton KAIMALINO and a
7-1/2-foot model of a larger wo-strut-per-hu1I SWATH
configuration, boUi built by NUC, have been operated in San Diego Bay and test facilities with wavernaking capability. In addition a 20-foot, manned model, built by Litton Industries, has been operated in a wide range of wave conditions both in the Pacific Ocean and in the Seakeeping facility at NSRDC.. The development ofSWATHmotion and control theories
(and their verification under controlled conditions) tends to
be a longer term but more quantitative (and necessary)
approadi. The strategy being followed is first to obtain a
fundamental understanding of the freebody (no appendages)
inotions characteristics of SWATH configurations actd, with this as a foundation to develop the control requirements
necessary for any particular ser of mission specifications. Realizing fully that seakeeping prediction is an inherently
conplex subiect, an attempt will be -made nevertheless to
discuss freebody SWATH motions in simplified terms. For purposes of discussion it is convenient to separate ship motions into their six components: surge, sway, heave, roll,
yaw and pitch. The first three describe translational motions while the latter three are rotational motions.
Because the lateral directional stability ofSWATHships
has been observed in model tests to be excellent in head and following seas, only the vertical-plane motions are of interest at these two headings. Also, for ships with slender
hulls it can be assumed further that heave/pitch ¡notion will be independent of surge motion. lt follows that, to a first approximationS the mathematical description of the coupled heave and pitch motion of a SWATH shrpin ahead seas can be reduced to a pair of linear differential equations.
While heave motion generally predominates in sinusoidal ahead seas, there is also some coupling of heavemotion into pitch. Coupling of pitch to heave is most eviden.t in following
seas.
An important assumption on which. the equations of
motion are based is that the ship response amplitudes will be linear to the amplitude of the exciting wave. This means-that,
when the amplitude of the incoming wave is doubled, the
ship motion amplitude will also be doubted. A more
complete discussion of the SWATH motioa prediction theory developed at NSRDC is contained in Ref. 8. At the heartof it is the two-dimensional strip theory orinated by
Korvin-Kroukovsky, but with different forward-speed correction
terms. This was computerized to approximate hydrodynanaic
added mass and damping coefficients for parallel twin,
heaving cylinders of arbitrary section shape.Correlations of SWATH heave motion amplitudes predicted by this theory with test results for one NSRDC model arepresented in Ref.
-3, from which Figute 7 has been extracted. -A fundamental limitation of strip theorywhen applied to
SWATH ships is- that viscous effects are ignored. (With conventional ships damping of vertical motion results prin-cipally from wavemaking, and the contribution of viscous
damping is relatively minor.) But because SWATH forms have
low wave-making it follows that themagnitude of the viscous damping of heave or pitch motions is no Longer neglible.
Consequently, ship motion predictions based onwave-making damping alone will be inaccurate. Another source of error in
two-dimensional theory is that inceasing ship forwardspeed
reduces hydrodynarnic interactions between the two
demi-CIRCULAR FREQUENCY - ZITt
Table 2. COMPARISON OF REPRESENTATIVE ZERO-SPEED
NATURAL PERIODS OF HEAVE AND PtTCH MOTION
FOR 4,000.TOl'I SHIPS -
-The potential of SWATH ships for greatly improved
seakeeping ability is implicit in the- relath'ely infrequent
occurrences of ocean storm waves with long periods, making
it possible for ships with similarly long naturaL periods of heaving and pitching to- have less motion excitation in the--majority of seaway- conditions, and at mostship headings. la -a m-anner -an-alogous, to .mech-anie-a.l vibr-ations, l-arge syrl-. chronous motions occur when. the ship/wave- encounter
-frequency is close to one of the ship's natural frequencies of
motion. - - - .
-Moreover, because the- natural pitch period of a SWATH -
-ship will generally be widely separated-from theheave-natural -period, in a. particular seaway one. or the- othet- type of' -
-motion will predominate. On the other hand, the-ship's-
--natural roll period cart, if care is not taken, be close tothe. - pitch period so that roll-pitch coupling, in quartering se
-MONOPIUU. - 1.15 S ra 6 c - S.61o6.5..a
SWATh SHIP - 060i.c'1 IDta 11 16 ta 17
-HEAVE PITCH
NATURAL NATURAl. NATURAL
FREO.. PERI0O PEaIoo
1 2 3 4 e
WAVE LENGTH/SHIP LENGTH (AiL)
Figure 7. NONDIMENSIONAL HEAVE AMPLITUDES VERSUS WAVE LENGTH TO SHIP LENGTH RATIO
FOR-SWATH
hulls, shifting the resonance point to slightly higher
fre-quencies than predicted. During the past year, additional model forced o5ciltation tests have been conducted in an
effort to deve!op simple correction relationships to take these factors into accOunt.
Equally important as peak ¡notions and responses to the seaworthiness of a s1iip are the accompanying accelerations. In the case of heave, ii the peak response is known for a given
sea condition, the maximum heave acceleration will be
approximately Z; where w is the circular wave encounter frequency at which the greatest heave motion occurs and Z is the single-amplitude, in feet. The reduced waterplane areaof
SWATH ships results in natural heave frequencies about
one-half those of a monohull of the same displacement (see TabLe 2). The difference in natural pitch periods is even more pronounced. This suggests that the peak heave acceleration experienced by a SWATH ship, for a given heave amplitude,
will be about one-fourth of that for a monohull. Moreover, the peak pitch acceleration may be as little as one-ninth as
-cou'd result. In moderaeIy severe seas (up ro State 6) a S'VATH ship ofescort size vi11 have its minimum motions whcr Operating at or near full speed, rc3rd1ss of heading.
Th objective in head seas is to avoid heave resonance while in following Seas pitch resonance is Ehe principal concern. In Unusually severe head seas (State 7 or greater) SWATH ships
will be no different 1mmcOnventional monohulls in having
to reduce speed to, perhaps, 15 knots and possibly change
heading to avoid takinggreen water onboard. Siniilarhy, the
best operating strategy in severe. beam seas appears to be a change in heading to take the waves on the bow or stern qu2rter, combined with a decrease in speed. This will usually resuht in moderate coupled roll-pitch motion which can be controlheJ and dampened further by active fins at the stern.
Additional confidence in the supe'jorjty of SWATH
motion characterjstjc wen equipped with active stern anti-pitch fins haz been drawn fromoperating experience with Litton's 20-ft. manned model in choppy ocean wäves
oft the West Coast, at ail possib1eheadin Nevertheless, the high-speed seakeepirig ability ofa SWATH ship inmoderatety severe conditions will always be contingent on the existence
of sufficient ross-structure clearance above the mean water
t surface. Model tests indicate
that 17 to 18 feet is probably the minimum clearance foravoidance of unacceptable wave
impact pressures or frequency on SWATH ships from 1000 to
4000 tons displacement. This height is based on a design
criteria requiring sustained high speed capability in State 6 seas, which are exceeded only I to 10 percent of the time on a yearly basiswor!dwjde depending on the ocean region.
tSeaway Induced Loads. The designer is interested in
know-ing, at the earliest possible time, what loads will be imposed
upon a ship, and in sorting these loads out to détermine
which loads will tend to dominate the structurai design Since
j SWATH ships tend to be wideor spread out conuìguratiorts
wich much of their structurai weight high up in the box,
static stability consrderatjons dictatethat an effort be made to minimize the box size and weight.
In approaching the problem ofseaway induced loads a combination of experimental and theoretical means have
ç been used.
Experimentally, SWATH models have been fitted with
load and pressure sensing devices and tested under controlled
conditions at various speeds, headings and in various seaway conditions. The. resulting data have then been analyzed and used to not only provide qualitative insight into the
dominant types of loads but also to provide quantitative checks on predictions made from theoriesbeing developed.
(In both respects we were fortunate itt having a small but
developing base of conventional catamaran technology.)
Other than wave impact type loads, experiments and
theory both show the zero speedbeam sea transverse
bending moment ori the box structure (tending to push or pull the hulls apart or together)to be the predominant type of seaway load affecting primary structure design. it is also
worthy of mention that the maximum transverse bending
moment does not occur at roil resonance.
¡. Next in order of importance is vertical shearagnin at zero
. speedbeam sea conditiöns, at roil resonance..
Longitudinal bending and seaway-tnduced torsional loads
have not been found to be limiting loads in the context of
structurai design..
Lec,8 has developed a: theory forpredicting transverse
bending moments on SWATH ships; to date, correlation of
this theory with model experimentshas proven to be quite :good.. Less sues though, has: been achieved with
correla-tien of model and theory in predictingvertical shear, due to
axi over-estimate of roil amplitudes at.
rr.an..
¡t will be noted in referen 9 that,for two-level SWATH
ships of less than 4000-5000 tons, neithertransverse bending moment nor vertical shear toads- induced- by theseaway tend to drive. the primary suctcanUjn
Additional model experiments are being:conducted this fiscal year to determine the distribution of seaway induced
loads on SWATH ship Structures. Fór example, in addition to measuring the transverse berding moment at the centerline of
the cross structure, it will also be measured at the
intr-section of the struts and cross structure. Furthermore,
measurements arc going to be made of the forces at the iflter5ection of the struts and lower hulls. The first phase ofthese experiments will be confined to zero speed, beam sea conditions with others to be covered insubsequent phases.
Another type of seaway induced load is that-dueto wave
impacts on the bottom of the box structure. As mentned
earlier, for small, high speed SWATH
ships -it is highly
impractical to design the ship with a box height such that no
wave impacts will occur in any sea state Thus, in both the
theoretical and experimeritahwork to date close attention has been paid to factors relating to the frequency and magnitude of wave impacts, and how these change as a function of box heightas well as other design-related factors.
The results of SWATH model experiments, as well as
additional insight into this problem area obtained front
conventional catamaran model and full scale experiments,
have provided tentative impact loads criteria which ¡n turn
are being used in SWATH ship conceptual and feasibility
designs. itt addition, a series of model experiments with the
NI-IC built 5 1/2-foot model of the 190-ton SSP
¡(AIMA-LINO have been conducted to measure impact pressures;
these results will be correlated with full scale impact pressure
and load measurements during the upcoming P-hase i and
Phase I! trials of KAIMALINO.
Maneuvering Maneuvering or horizontal plane-control of
SWATH ship configurations has received some attention. The
approach taken to date has been to rely primarily on using
model experimental techniques, supplemented
with.analyti-cal studies as necessary.
The whole maneuvering question should be subdivided into two categoriesslow speed control,(i.e., maneuvering in
channels and around piers)-and high-speed maneuvering.
Slow-speed maneuvering charactetica-ofSWATH ships are, not surprisingly, excellent. The wide spacing of the propellers combined with the amount of thrust that each
applies, results in outstanding low-speedcontrol.. Indeed, itt
some experiments with radio controlled models, they have been able to rotate in a space equal to their àin length, rather than having to "back-and-fill"as octe is forced to do
with a monohufi. .
.The area of high-speed maneuvering control o. SWATH
ships is, however, different and a bitmore complicated: As
mentioned earlier, SWATH ships tend. to have a high degree
of directional stabilitythey tend towant to stay on a given
heading.. Crude measurements with radio controhled,modeis and a smaLl (20-foot) manned modelprovided rough m-urea of turning diameter as a functi of ship length ranging
from approximately 5 lengths fora two-strut-per-hall n
rion to 10-12 lengths for
a single-strutper.huuconfig-uration. In order to quantify these results a little more
accurately, rotating attn tests-have been conductedwith one demihuil of a single-strut--per.-haflconfiguration, fitted with a number of control surface .arivigemen The results of these tests, when analy-zed, generally confiîmed the less: rigorous,. free running model tests.. -. -. - -.
The conclusions that one' can draw fromall of the work to date on SWATH maneuvering and controlarer
-1. Slow speed control of SWATH ships is outstanding in
comparison to rnonohulls due to the wideseparatioeof the
thrusters; '
"
.. :
:. - ' :....2 High speed: turning diameters ofSWATIÇ ships are. equal. to- or in excess-of those for
equivalent'djpkcent
monohuils;
3. The fact that SWATH ships' tend to be. 2/3 to 3/4 the
length..of equivalent displacement monohullstends
toameli-elate any negative aspecta of the. second conclusionon-balance, SWATH'hjp maneuvering and high speed control
does not' appear to be- a decisive consideration iñ evaluating the concept for most applications..
STRUCTURES AND MATERIALS
Hull Structural Deiqa Weiqht
Twin-hull ships designed for a particuIa mission usually
have grcater encIoed vo'ume and deck area than an
equivalent rrionohull and, as a result, they tend to have a gjEer percentage of their displacement taken up by struc-turaI weight. This trend is accentuated in SWATH ships by
their deeper draft arid their need for comparatively wide hull spacing to ensure adequate transverse stability.
Because the struts of a SWATH ship provide relatively fitti reserve buoyancy, SWATH ships are weight sensitive.
The ultimate payload capabilities of the ship as eventually built, therefore, depends primriIy on the structural weight,
assuming that other ship weights will be similar to those for a
comparable mortohull. When the time comes to finalize
dimensions it is highly advantageous to be able to predict the
structural weight of a SWATH ship accurately. The penalty paid for uncertainty takes the form of larger weight growth
margins, resulting in greater dimensions with increased
wetted surface area, which degrades powering performance.
Complicating matters further, the structure of a SWATH ship is different in many respects from that of monohulls so
that previous structural design experience does nor provide much of a guide. There is uncertainty not only as regards the precise seainduced loading but as to how the ship transmits these loads to the various structural elements, the transitions and intersections being particularly important. lt was realized from the beginnings of the Navy SWATH investigations that
to perform a structural redesign of just one configuration every time better design information became available was
unrealistic inasmuch as the drawing board process required a
couple of months. If, as in exploratory design, many
configurations are to be examined, the traditional approach is
totally out of the question with limited manpower and financial resources. To provide timely structurai weight
information for numerous SWATH configurations, a
struc-turai design computer program is necessary. Fortunately, by 1972 such a computer program had already been developed
at NSRDC for analysis of the midship section of destroyers. Moreover, it was recognized that this program could be
adapted within several months to design the box structure of
SWATH ships. This was the long-term strategy decided on.
The necessary modifications are described in another paper to be presented at this conference.
While these alterations to the computer program were
being carried out, there was a need for immediate structural
weight information. The approach initially taken by
NAy-SEC was to size the structure of SWATH ships to withstand local load design criteria (e.g., deck loads, hydrostatic heads for watertiglitness, wave slap, etc.). Minimum scan clings thus
derived using, standard Navy factors of safety were then
checked for adequacy to sustain estimated wave-induced ship loads combined with appropriate local loads. A further rapid check on these results was made by estimating the structural
weight based on approximate minimum area and volume
structural densities from monohull experience.
Depending on the level of technology employed, as well
as the stress levels and configuration, from 2.0 to 8.0 pounds
of structure are required to enclose each: cubic foot of ship
volume.. The. fnst-ordes determinant of structural density is the construction material used. Whereas steel ships generally
have- structuraL densities of at least S lbs/cult, aluminum: :thip structures. achieve densities of at least 2' lbs/cult. or
more. Intermediate densities result when steel and aluminum
structural components. are combined in a single structure. Despite its greater weight, steel continues to be the- prime
structural material for Naval displacement ships because of its strength, lower cost, corrosion resistance and toughness.
7
If the same structural material is used throughout, about 50 percent of the structural weight will be contributed by the
box with the remainder divided fairly evenly between the
struts and hulls. lt follows that, relatively, the greatest payoff
in weight reduction will result from careful attention to the
design of the cross structure. As soon as the
SWATH-modified structural design synthesis computer program
be-came available, NSRDC embarked on a systematic
investiga-tion of the ensitivity of hull and, especially, cross-structure
weight to various factors so as to provide direction for
subsequent research efforts.
Even the most powerful and versatile computer program
is of little practical utility if the ship designer has no
confidence in the results. To instill confidence in the SWATH version of the MIDSHIP program it was essential to compare the predicted scantlings for a particular SWATH design with those obtained for the same ship using the laborious drawing
board approach. The first opportunity to validate the
MIDSHIP program in this manner caine in June 1973 with a structural design by NAVSEC. Cooperating in this endeavor
was the Boeing Company, which 'had an in-house effort co examine advanced structural concepts for SWATH ships.
Table 3, below, is a comparison of the resulting weight
predictions.
tAJI ,ho.m ,ncud. s 3% ailo..incs f u s4din md 15% fo,
Appumi,w. (ÑIie w,b'okm domt¡0Wesndo,d NAVSHIPS 3.dgit
,oUpn rath.r thu., by thip 5*omstry.>
Tab. 3. oeMPARISON OF WEIGHT ESTIMATEST FOR MED4UM ESCORT BUILT OF ALUMINUM
Agreement between the three independent estimates as to
the total weight of primary structure was excellent. Differ-ences in the weight breakdown probably result from (a)
inconsistencies in the.apportionment of some structure to the
box or struts and (b) the present inability of the MIDSHIP program to design, rather than merely simulate, the
trans-verse beam/stanchion/double-bottom type of' structure
selected by NAVSEC. Also, the Boeing weight estimate
should be the lightest because they used an unusual type of
plate stiffeners. . .
. .
These weight sensitivity studies fell generally into one of three categories, examining the effect on structural weight.of
changes in: (a) material, (b) box configuration; and (c)
design loads.. These studies were carried out during the latter part of FY72 and throughout FY73, but there is a continuing need to reassess previous corichisions as. new information becomes .available. Significant findings to date are presented in Reference 9. Nevertheless, several important points deserve
special mention. First, with current Navy design criteria for
the type of loads predicted for SWATh shipsi the usc of high.: yield steels will not result in any appreciable, main structure' weight savings.. This leaves aluminum: as' the oflly alternative.
to }ITS construction. Second, the structural scantlirigs. of SWATH ships of 3,000 tons or less are dominated by loeal.
design criteris. (deck load,. hydrosta tichead,. slamming
pres.-suie) rather than the n1mnimi pri'fl5y trfl5vbeflfl
moment Consequently, there- is lesc.need. to define this type-of wave loading precisely. Last, a cross structure havng.twe. levels (3 decks) is a more weight-effective means. of enclosing, a given volume than a. longer, beamier one level, configura -hort.
x.mTmuc.?u0I
1IGNT CTOf)
rrmjn
IG0IT rro.m L(0 ,ø.Il.I.23)Q4TrTOiu. TOTAl.
rrí
IIAVSSC 3OC lCJTtm) SOIINO. 113 das 444 t7V 715 . 217 1W 203 173ANATOMY OF A SWATH
The anatomy of a SWATH ship differs appreciably from
that of a monohull in several respects. Basically there are three major parts, the box or cross structure, the struts that tie the box co the hulls and the hulls which are always submerged. Figure 2 illustrate variations ¡n these as well às other elements in a SWATH configuration. The SWATH ship exhibits a greater degree of sensitivity to system and coritiguration relationships than che monohull. Cross coupling effects are more pronounced. By adopting a twin hull configuration vith a deliberate reduction in waterplane area for the pürpose of achieving superior seakeeping performance, a number of relationships are changèd. For example weight and moment sensitivity are increased sub-. stantially, the dominant effect of sea loads is changed from longitudinal hull girder bending to transverse bending, and the importance of frictional relative to wave making resis-tance is altered. Other relationships associated with
dimen-sions, arrangements, stability, propulsion and others are also
riifferent. Table 4 shows the Ñlationship between some of the ship requirements and configuration elements. A partic-War one peculiar to the SWATH can be seen iñ the case. of the strut and main propulsion. 1f the main propulsion is located in the hull then ducting rnustbe provided for intake and exhaust through the struts. Strut thickness and shape particularly in tise smaller ship sizes, can he governed by the
size of the ducts or by the minimum width necessary to pull
a gas turbine for maintenance. If the engines are topside, the strut width might be governed by the size of a zee drive necessary to transmit power to the propeller.
SHIP REQUIREMENTS CONFIGURATION ELEMENT
HANGER SPACE TRANSVERSE S rASI LITY HANGAR SPACE SUPERSTR'JCTIjRE WEAPONS ARRANGEMENTS AIRCRAF1' LANDING & TAKE OFF SUBOIV1S1ON . DAM. STABILITY FUGHT DECK FREEBOARD ENGINE UPTAKE & EXHAUST TOTAL AREA FOR SHIP FUNCTIONS MAIN PRÒPULS1ON TURBINE
GEAR ARRANGEMENT SHIP RESISTANCE (LID RATIOJ DISPLACEMENT REOUSREMEPdTS
STRUT TWCXNE/D1A. RATIO
SONAR
MACWY, FUEL, BALL. SONAR
ARRANGEMENT SHIP RESISTANCE L/D RATIO DISPLACEMENT REOWREMENTS
-STRUT LENGTH (NNECTIONP
TRANS.& LONG. GM-
-SHIP RESISTANCE
MAIN TURBINE REMOVAL
INTAKE & EXHAUST OUCTING (EEC DRIVEl
TRANS. & LONG. GM
HULLaBox !.ENGTHS CONNECTIONI PROXIMITY TO SONAR & PRELLER
TRANR.&
LONG.GM-SHIP REMSTANCE
MAtif TURBINS REMOVAL
INTAKE & EXHAUST DUCTING.:
ACCETO ENGR. & SONAR
- Thb 4. REQthREMENTSCØNF(GuRAT3OiRETropIp$:
Since the length-breadth ratio of a SWATU configuration ¡s rriucii lôwer than for a monohull, a requirement such as the capability to tiansit the Panama Canal limits the size to much
smaller ships tisan morlohuils.
Vitli these relationships in mind as vell as otherç, itwas imperative chat parametric studics be made to quantify them and ro determine the order of priority in terms of their
impact on the ship so that balanced ship designs coüld be ruade. A rough computer ship design synthesis model was developed and used to make prametric studies. With this information it was then feasible to initiate a spectrum of conceptual designs- to determine relacionthjps between
hypo-thetzcal ship msion and payloads and shipsizes S, ntlie-sis model conceptual designs were- ruade ofship sizes ranging
I'rom about 2000 tons for a small-escort to about 45000 tons
for a tactical aircraft carrier wich YSTOL aircraft. From a realistic -standpoint ships beyond 15,000 co 20,000 tonsare not believed to be competitive with monohull types.
Over a hundred studies were made, the majority being
computer design conceptual types. Feasibility designs, that is
designs in sufficient detail to quantify the major features of the ship, were carried out for twô ship sizes. One was a medium size helo ASW escort type in the 4000 to 6000 ton
range.- The other was a anall high speed helo escort between
2000 and 3000 tons. Tables 7 & 9 give the chacteristica o.
the designs which inciLde both single and multi strut configurations. Common items üsed in these studies are shown in Tables 5 & 6. A number of variations were made on
these two basic ship sizes iii order to determine the effect of changes in payload, structural material, endürance ecc. The results of these studies formed the basis for selection of the
two representative designi discussed belöw.: While the major
effort in the design studies was concentrated on the ASW
-helo mission with two ship sizes, the basic results can be applied to SWATH ships in general thus- forming a good
foundation for design purposes.
COMMON ITENS DESIGN CRITERIA
5 HELOS Puis 475 TONS MILITARY PAY LOAO MACHINERY, ARMAMENT, EdUIPMEÑT & OUTRT LOADS. FUEL, STORES. AMMUNiTION
COMPLEMENT (2671
-INSTALLED SHAFT HORSEPOWER -
-IWO LEVEL BOX
-LWPER BOX LEVEL ALUMINUM
MAIN STRUCTURE-STEEL - -PAN MAX
MIUTARY PAYLOAD - COMBAT SYSTEM. ARMAMENT;
AMMUN1ITON.HELØ FUEL.
TableS. SWATH MEDIUM ESCORT DESIGN DATA
COMMON ITENG..
-DESIGN CRITERIA.- - - -2 HELOS PLUS15O TONS MILITARY PAYLOAD MACHINERY, ARMAM ENTr EQUIPMENT & OUTFIT LOADS. FUEL STORES, AMMUNITION
COMPLEMENT (118t - - -
-INSTALLED SHAFT HORSEPOWER
-- ALL ALUMINUM -
-UGHT. WEIGHT SUBSYSTEMS
--- .MILITARY PAYLOAD COMBAT SYSTEM,ARMAMENT
--AMMUNION; HELO FUEL
Table & SWATh SMALI ESCORT (ALL ALWt4INUM
STRLJCTURE3-DESIGN DATA - -
-- MEDiUM ESCORT
SHIP--The medium size escort ship emerged as a fise belo ship--
-of hybrid construction utilizing the Si-t 3D helicopter and
'--:1
}
BOX BEAM}
Box LENGTh BOX DEPTH BOX AREA HULL DIAMETER HULL LENGTh STRUT ThIKNESS STRIJT LENGTh-STRUT:- - -WATER PLANEZcarrying sorne 500 toas of military payload exclusive of helos. TIic box is two levels and the structure is hybrid, that is hulls, struts and lower level of the box are steel with the upper level of the box of aluminum. A single strut perside proved to be the best selcction from a number of stand-points. Hull diameter was large enough to permit location of
main machinery below, however a topside machinery arrange-ment could be utilized with certain advantages.
-Structure
In the 5000 to 6000 ton size a hybrid structure was considered to be the most cost effective selection for the foreseeable future. The shipbuilding industry is well versed in steel construction. Furthermore steel is a more forgiving
material than aluminum and requires less quality control in design and fabrication. Two other factors mitigating against all aluminum construction, at present, are the increasedfire hazards and lack of experience in building aluminum struc-tures of this size. lt was considered important,nevertheless,
to make a study of an
all aluminum ship in order to determine what potential advantages could accrue by its utilization. Columns t and 3 of Table 7 show the results of trading off hull weight reduction. Everything else being equal, a trade off of hull weight for fuel provides almost-60%increase in endurance. If hull weight savings are utilized to
make the ship smaller, then displacement is decreased by over
900 tons and mnaxinium speed is increased. The potential advantages of aluminum are appreciable and further con-sideration must be given to its use, particularly in the smaller ship sizes as discussed later in the high speed escort case. Machinery
The SWATH configuration and its weight sensitivity suggests that gas turbines are the best selection of prime movers for propulsion. Comparative studies of gas turbine, steam, diesel and super conducting drive for a 60000 HP
9
plant were mude for the medium escort. Taking the as turbine design as a hase, the only steam boilers that would tit in the 23 foot high box were the 1200 psi type with the turbines, gears amid condensers located in the hulls. lt was necessary to lengthen the. box by 20 feet to accommodate the steam plant. Although the change to steam resulted in
approximately a 40% reductori in fuel requirements, the ship
displacement grew by approximately 300 tons due to the larger box and heavier equipment. If lower pressure systems were used, the space and weight wouldincrease further. The
increase in the box especially would be substantial as the
lower pressure boilers require a deeper box than that used for
the gas turbine designs. Another point is that steam plants
require a larger operating crew than gas turbines. This means increased weight, space and cost. Lifetime cost for personnel is a particularly significant factor.
14-P RANGE 3100-3670 * 3500.4400 * 4200-6000 120UG44000 * 17500-39000 * z7000oe * 30-36000
ALUMINUM STRUCTURE ADDED FUEL - ENGINES 1F-39140 FT 124 PROTEuS 0Th 990 LU 1500 LU 5100 FT 4C FT9 MANUFACTURER AvcO.LYDDMIN
PRATT & WHITNEY
ROLLS ROv
ROLLS ROYCE GARRETT
G.E. G.E.
PRATT & WIIIThEY
PRATT & WHFTNEY
75FC RATINGS AT 100 DAY
.NAv SEC PIPVGURATION APPROvAL
Table 8. SWATH MEDIUM ESCORT DESIGN DATA
STATuS MIO 14 IN PRODUCTION IN PRODUCTiON IN PRODUCTiON IN OEVELOPIIENT (16-fl PROOUCTIONI IN PRODUCTIGN (N PROOUCT7GU IN PRODUCTiON ODNIRACT IU DELIVERY 'EQ ALUW4UM STRUCTURE REDUCED 4IP ZE 2 5.HELO MULTI STRUT FEASIBILITY STUDY 5640 33.3' 3 5-HELO SiNGLE STRUT FEASIBILITY STUDY 5580 33.5' 4 5.HELO MULTI STRUT FEASIBILITY STUDY 5640 Q 33.2' 5 5-HELO SINGLE STRUT COMPUTER STUDY 40 Q 30.4'
SAME SAME SAME SAME
234x96x23 80x9.8, 90x9.3 (MULTI) 308x1 9.0 214x96x23 200x8.7 (SINGLE) -300x191 234x96x23 80x9,8. 90x9,8 (MULTI) 308x19,0 214x96x23 lBOx&7 -(SINGLE) -300x17.4 - -UPPER LEVEL OF BOX IS ALUM 2.LEVEL BOX ALL ALUM. 2-LEVEL BOX EXTRA FUEL ALL ALUM. 2-LEVEL BOX EXTRA FUEL -. ALL. ALUM. 2-LEVEL BOX REDUCED HULLS AND STRUTS CREW TOTAL 267 RELATIVE RANGE @ 1.0 ENDURANCE SPEED
TRIAL SPEED KTS. OVER
RELATIVE TRIAL SPEED 1_0
HYBRID STRUCTURE
COLUMN
i
DATA 5-H E LO
SINGLE STR UT
LEVEL OF EFFORT FEASIBIUTY
STUDY DISP. (TONS) @ DRAFT 5580 33.5'
(FT)
PAYLOAD (NO OF HELOS 5 HELOS +
+ TONS OF MILITARY 475 TONS PAYLOAD) GEOMETRY -BOX (FTxFTxFT) 214x96*23 STRUT (FT.x Fr) 200x8.7 (SINGLE) HULL (FTZFT) 300x19.1
NOTES UPPER LEVEL
OF BOX IS
ALUM 2.LEVEL BOX
1ht dies,t plant resulted in an increase in hull diameter .
frtn
L 9 f r 4f ft and art increase in the group 2 weg1it(nair proptz!..kc) by approxirnte1y I 100 tons. Diesel UnitS of tIi hu-:power are obviously out of the cuestion for smt or im.ucn sized SWATH ships with high horsepower
jnstl :itiOnS
S.zper-conthzring drive investigated only irr sufficient detail to determine that it
wiIi
would be somewhere bezween the gas rebinè plant and a conventional electricd-rF; systcrn. Furti':r development of superconducting drive m.i drr.,nstrate is compatibility for SWATH.The SWATH
coefiuration is well suited to the concept.
A problem ftiing the designer using gas turbines is the limitd number ard sizesof engines available. Table 8 is a list of available oç projected lightweight gas turbines for high performance type ships. Eliminating the ones with the poorest fuel rates reduces the list to five as shown by single
asterisk. This severely restricts the designer and forces him to
design around the engines. With such a limited selection, a problem exists in designing for good performance at both cruising and maximum speed with one type of engine. Fuel consumption is particularly poor at cruising where only a small percentage of the installed power is required. In the feasibility designs of the medium and high speed escorts, separate cruise turbines were included if the power required at 20 knots was 10,000 HP or less, since a new 5000 HP engine with favorable fuel rate is now in development. If the power was greater than this, it was necessary to use the main
engines at a low power rating. This produces a high specific fuel consumption hence a larger fuel load. Since the development time associated with a new turbine is a matter of several years, and no others are presently under develop-ment, the ones listed in Table are the only engines that will be available in the foreseeable future.
There are two locations for the gas turbines on SWATH ships, either in the hulls or in the box. Planetary gears were
used in the hulls for all the designs. With the main engines in
the hulls, the diameter in way of the engine is controlled by the size of the engine and reduction gear, and the thickness of the strut is controlled by the uptakes. Additionally, either
the intake or exhaust must be large enough to allow removal of the largest engine components for overhaul. This arrange-ment allows a smaller box size but does not provide for cross connecting one engine to both propellers.
Location of the engines in the box lessens the restrictions
on the strut and hull dimensions. By retaining the planetary
reduction gear, it is possible to use high rpm low torque right
angle drives and shafts. With this system the strut in way of the shafts can be reduced to approximately 5 ft and the hull diameter in way of the gear to aproaimately 12 ft.
Four other advantages derive from locating the machinery topside:
The main engines can be cross connected such that
either engine can drive either/or both props.
Arrangement problems are less with machinery top-side particularly where cruise turbines are used in
addition to main turbines.
3.. Hull borne noise is reduced thus benefitting sonar
performan.
4 The reduction in strut thickness and the increased hull L/D results in a substantial speed increase despite a 50 ton increase in ship size.
Speed-Power (ship system implications)
The long thin hulls and. struts. characteristic of the
SWATH configuration tend themselves to the use of thin ship potential theory for the prediction of wave drag Results from analytical methods developed by NIJC and NSRDC have been compared to model results and show excellent correlation. The NUC method is somewhat simpler than the
NSRDC methods and lends itself more appropriately to application during the conceptual and feasibility design stages. lt has been the principal method used in the ship design program at NAVSEC to date. The NSRDC methods are more complex and more refined. They are suitable for
lahr stages of design.
Both the medium and the high speed escort ship studies showed a significant difference in the power required for cruise speed between single and multi struts. Three separate
stt'dies were run to determine the cause of this ditfcrençe. In the first study displacement and hulls were common to
both contigurations. Commonality of the struts consisted of total length, thickness, waterplane area, wetted surface and
strut volume. Residual resistance per ton was calculated using the NTJC computer program. Generalized results are shown in
Fig. S and 9. Strut thickness is a significant factor in
wavema king resistance therefore separate calculations weremade for a range of realistic strut thickness to hull diameter ratios to determine if this would alter the relative shapes of the curves in Fig. 8 and 9. Results indicated that the relative
shapes were independent of thickness-diameter ratio. The real
significance of Fig. is that within the speed-length ratio range of 1.0 to 1.5, the multi strut pays a wavemaking penalty due to concept alone. This is of particular impor-tance for hull sizes between 175 and 400 feet cruising at 20
knots. lt is in this size range that the major potential of naval SWATH ships appears to lie.
COMMON TOTAL STRtJ1
LENGTH. STRUT THICKNEss. WATERPLANE AREA. STRUT
VOLUME MULTÇ.$TRUT I..
80.-z
na-
-.., 40. 4 u' 'w e 20. 1o.
O. e O.. 'O. KN0TS/PEEr SINGLE STRUT KULI. CONTRIBUTION COMMON HULL CENTERLINE SEPARATION 1.0 13 2.0 2.5 3.0SPEED LENGTH RATIO VtVt
KNOTS/FEET
Figure 8. STRUT CONTRIBUTION.
COMMON - HULL. CENTERLINE SEPARATION.
TOTAL STRUT LENGTH. STRUT ThICKNEss.
WATERPLANE AREA. 01SPLACEMENT
0.5 1.0 1.5 2.0 2.5
SPEED LENGTH RATIO VP%/L
Figure 9. COMBiNED HULLS ANDSTRUTS
03 1.0 1.5 2.0 2.5 3.0 3.5
SPEED LENGTH RATIO V?yt
C o S, 10 I-z w z -I a. UI 30 UI o z 20 U, UI e IO O
To evaluate the effect of strut gap the two ships shown in
Fig. 10 were developed. The box for the lotier ship vas
dç'rived by removing the upper level arid adding it to the
length of the lower level. sote that this required art
additiQiwl length of inner bottom. The strut dimensions and
box beams are. the sanie. for both ships. Displacement was efflarged to account for the increase in Group I (hull weight)
caused by the inner bottom change. Although the average cruise speed and range increased slightly, the top speed dropped one knot due to tile larger displacement and extra wetted surface of the longer thinner hulls. Thus, any gain realized by separating the struts was offset by these other
increases. Tandem strut interference effects (non wavemaking type) have not been included in the multi strut drag
calculations. Chapman. (5) found strut interference to be
minimal. Ort the other hand tests at NSR.DC of tandem struts showed ars appreciable effect due to interference. The issue of strut interference has not been resolved at this time.
Finally a study was made to determine the effect of varying box LIB. Strut thickness has a significant influence on powering and transverse stability, therefore by widening
the box and thinning the strut to maintain the same GM1., it c'ould be possible to reduce resistance. Controlling factoñ in
the study were the minimum box volume necessary to
provide the ship functions, constant box volume, overall extent of strut length per hull about equal to box length plus 20 feet (lO feet overhang at each end) and, with minimum
l'
lCLOSE LONG. ACING
GAP RATIO -2.0
WIDE LONG. RACING
Figure 10. MULTI STRUT 2000 TON SHIPS
goals of about 3.5 feet GM,. and 20 feet GML.. Using the
selected forms, mean speeds and endurances over the 1 5 to
25 knot range were calculated. lt is apparent from Fig. Il that the single strut is far superior in all three cases. While unrestricted limits on box dimensions would have permitted smaller multi struts, the box wöuld have been oversized resulting in greater weight, unnecessary box volume and
elimination of the Panama Canal transit capability.
GAP RATIO - 0,X1
H
Figi. 11. BOX.LI8 VARIATIONS
[n order to determine the cverall effects- of using-a multi strut configuration as compared tO the single strut version, a
II
feastoility design of the medium escort was made for a single
level box usinir hybrid structure. Common items were as
indicated in Table 5. The transverse stability requirements
plus uptake and engine removal sized the struts. Results of the study indicated a 140 ton increase in ship size with a 521
reductjön in endurance due to the substantial increase in
power required at endurance speed. in addition there was a
5% reduction in top speed for the multi strut. A two level
box version of the multi strut was about 80 tons less than the
orte level in part due to the trade off of aluminumstructure for steel in the upper level. The two level single strut box is
adequate for accommodating the functional requirements of the design. Furthermore the midportion of the strut is
suitable for either fuel or ballast rar.kage. Due to desis
constraints (stability, propulsion, strut separation) the two level multi strut box is 10% heavier than the single strut.
While the single Level multi strut box is smaller than the single strut by 28% the total box weight is 150 tons eater and the sùbdivision-damage stability features are worse. A single level
box requires doors in main subdivisión bulkheads thus
compromising the survival characteristics of the craft. On the other hand a two level box permits the lower level to be free
of horizontal penetrations, a substantial factor in terms of
damage stability.
HIGH SPEED ESCORT SHIP
Consideration of the smallest viable military or tactical ship led to the development of the high speed escort. The
initial hypothesis was that the ship should be an escort type
and should be ail aluminum structure with light weight
subsystems assumed to be available 15 to 20 years in the
future., The resulting ship was slightly lärger than 2000 tons
with two SH-2D helos, and a weapon system. Due to the
small hull size, topside machihery arrangement with zee drive was mandatory. Table 9 Column I gives the particulars oi the
original design. A more refined version of the ship with
lightweight subsystems is shown in Column 3. This was a
modified feasibility design, - that is, the structural scantlings and layout and the general arrangement were done by hand
in the detail normally associated with- feasibility work. The weights, power, stability, etc., were done by computer synthesis modeL
A more realistic version of the ship was made for near term application using existing subsystems.- This study .Ls shown in Column 4 of Table 9. lt is about 14% larger than the original version. A three helo version of the latter ship was made by enlarging the hangar resulting n a 30 ton
increase in ship size (Còlumn 5). For comparative purpos a hybrid structure study was conducted resulting in a
substan-tial increase in ship size (575 tons). Figure 12 is an artist's
concept of a hypothetical small high speed Navy tactical ship.
Figur, 12. ßX.l./B 180X110 180X110
,
GM9, CUr 12.8/208 4.1/19.5 STRUT. QT6P2L0
8915I RELML&NENOUR. 1.0 BOX-LIS 180X.146 145X146 4I1&7. 204118.4)
068.1GM, 5TRSIT. LITGAP 1/15/G 80/1 5/ R*L.MBAIÍ INDUS.. 0.37 8.46 SiNGLE MULTI Box-Ide 0X90 0X9O 3.5151.7 3.2/89.4STRUT- L(rlaAp 0/7.S#O 105111145
