Advantages and limits for pleasure craft design.
The SWATH (Small Waterplane Area Twin Hull) is an unconventional hull form in which the waterplane area is very small in respect to the immersed hull volume. A quantitative definition has been given by German War Weapon Control Law as WaterPlaneAarea WPA< V2/3 where V is displaced volume. The small and slender waterplane results in reduced wave resistance, wake wash and small amplitudes of wave induced motions. SWATH vessels are generally catamarans in which the hull volume is mainly concentrated into two immersed bodies connected to the cross deck by very slender “struts”. Between the struts and the cross deck two slender volumes that are generally out of the water provide buoyancy reserve in case of transversal inclination or longitudinal instability. Swath configuration can present two or four struts . The latter option is preferred at present. The main benefit of SWATH concept is the small motion induced by sea wave and consequently the small vertical accelerations. SWATH seakeeping performances at zero or cruising speed are recognised as superior to any conventional mono or multi-hull craft as the increasing use of SWATH vessels for wind farm maintenance confirms. The small wave pattern and reduced wake wash are important when low environmental impact is requested.
Beside these very interesting characteristics, SWATH ships pose some problems. They are very sensitive to weight distribution and dynamically instable when relative speed increases. Main limits of SWATH configuration are small static stability and reduced load capability. They are specially felt in small size craft. Liquid ballast is managed to get constant depth and trim. Dynamic longitudinal instability is often present at high speed, so that active stabilizing fins are widely used. Motion resistance is similar to conventional hull forms, except at very low relative speeds when larger wet surface worsen SWATH performances.
Since the beginning
The idea of creating a SWATH hull dates back to 1880. The first project, conceived by US Charles Lundborg in 1880, presented not two but a single hull in which the basic principle was the same. In 1938 another project by the Canadian Frederick G. Creed was realized. Duplus 3″, the first representative prototype, designed by J.J. Stenger, director of the Dutch Boele Shipyard and built in 1968. This unit was 40 m long with a displacement of 1200 tons, designed to support oil exploration.
Later, this type of hull was applied mainly for military units (patrol boats and minesweepers) until 1988 when the first ferry SWATH, for civil transport, the “Navatek I” was commissioned by “Pacific Marine”. It is a 140’ fast ferry for 400 passengers with cruising speed of 16 knots. In 1992, the “Radisson Diamond,” a 131 m Finnish cruise ship with a displacement of 18400 tons was launched.
Some successful military applications followed. U.S. Navy Victorious class ships (1991) are acoustic discovery ships sized to long stays in the open ocean, in any sea condition, with an operating speed of 9 knots. The ‘A-Frame “stealth” Sea Shadow (1993) is the first example of unconventional stealth ship.
In 2008, “Silver Cloud” the first SWATH yacht, was built by German shipyard “Abeking & Rasmussen”. She is 41 m long and 17.7 m wide with a cruising speed of 12.5 kn. In March 2010, the pilot support ship “Elbe” was launched by the same shipyard. It is the largest ship of its kind built by “Abeking & Rasmussen”. She has a length of 60.40 m and a width of 24.60 m, is the first of two sister ships built for the German pilots.
SWATH today
At present several SWATH support and supply vessels are operating. The growing activities for offshore wind farms maintenance and survey, as well as the interest about small suppliers for coastal oil platforms, focused designer attention on the lowest possible wave responses at zero or very low speed. Reduced vertical motions become a design criteria and represent the main limit for the ship operability. The four struts configuration appears preferable versus the twin one and it is used for small size craft also. The reason is in better longitudinal stability due to the optimal longitudinal distribution of strut waterplane areas.
The main reason of the small number of SWATH unit realized in a considerably large time period of lies in the difficulty of applying the concept in a radical manner to relatively small size craft with adequate static stability and without incurring in dynamic instability at speed. Compromise solutions with larger waterplanes do not carry adequate advantages in terms of seakeeping. Recently, since comfort, interior usability and sustainable navigation have become significant design criteria for pleasure craft, yacht designers have start to considered SWATH concept as very promising. Silver Cloud, the SWATH yacht built in 2008 by Abeking & Rasmussen has cruised successfully for several years. The same Yard has launched in April 2016 a 25m SWATH yacht, Al Makher.
A few builders have developed SWATH proposals for maxi and mega yachts. Danish Yachts has presented a yacht version of their successful 24m SWATH workboat for wind farm service. Abeking & Rasmussen and Reymond Langton Design, have developed a 62 m SWATH yacht preliminary design with internal volume equivalent to a 80 m monohull for customer who expects to cruise in extreme comfort and to have a stable platform in almost all weather and sea conditions. In 2015 Fincantieri unveiled FC Swath75 a futuristic proposal for a 75 m LOA hydrogen powered ecological superyacht.
SWATH hullform design
After several developments of the SWATH concept have been tried since first applications in the late sixties, the present design trend features four strut and two identical immersed bodies torpedo shaped. Above the struts, twin longitudinal volumes – sometimes called “haunch” – with trapezoidal sections assures buoyancy and stability when longitudinal and/or transversal heeling angles reach dangerous values. This configuration, reported in Fig. 5, appears the most promising to exploit SWATH peculiar characteristics in terms of reduced wake and motions with adequate safety margins provided by the upper volumes. Strut height is a compromise between the highest encounter wave height and the maximum allowed draft. Strut camber must be very small to reduce wave pattern, but on the other side, waterplane area to allow positive metacentric height at rest is necessary. Cross deck bottom is V shaped to add useful damping in case of impacts with the sea surface.
When dealing with small size SWATH craft, some parameters have intrinsic limitations. The aft strut camber must allow access to the engine inside each one of the immersed bodies. Their diameter has to provide enough room for engine housing. To comply with such constrains the camber of the aft struts is generally larger than that of the forward ones. In the way of engine housing only, immersed bodies diameter is increased to accommodate main engines. The slenderness of the body provides negligible resistance increase.
The reduced displacement for inch of immersion and more in general the sensitivity to load modifications are the weak point of the SWATH concept, but they can be overcome by relatively simple ballast systems. Poor static and dynamic stabilities are further limiting factors for the SWATH application. While static stability can be carefully considered at design stage, dynamic instabilities are peculiar of this hull configuration and cannot be exactly predicted even when scale model tests are available. Stabilizing fins are the most common countermeasure to such problem, but need sophisticated control to be effective.
Static stability is not problematic for larger SWATH vessels, but it becomes a design criteria for smaller craft and represents the limit to small main dimensions for a practical application of the SWATH concept. When dealing with SWATH design procedure, it is preferable to identify the maximum allowable heeling angles (both longitudinal and transversal) due to heeling moments coherent with mission profile and ship characteristics. From this equilibrium condition, by Euler’s Theorem, the moment of inertia of waterplane area can be calculated for displacement D and vertical position of center of gravity VCG based on reliable data. The obtained moment of inertia is divided into two or four contributes according to the chosen number of struts and the single strut waterplane can be drawn. It generally has parabolic sides and slenderness (cord/camber) ratio not lower than 6, if wave resistance is significant. In case of very slow vessels larger and squatter waterplane can be accepted. With this procedure the strut waterplane area is set to the minimum possible value and SWATH concept used at its best. Reasonably it results very difficult to use SWATH hull for very small craft. In practice there is no manned SWATH vessel below 12 m LOA.When dynamic forces and moments are of the same order of stability ones the SWATH configuration became intrinsically unstable. Longitudinal position of centre of hydrodynamic forces can shift according to speed variation resulting in large trim changes. Although SWATH immersed bodies are generally axial symmetrical, the pressure field around them is not, as they are not deeply immersed. The pressure field is influenced by the free surface and the vertical position of the centre of hydrodynamic forces changes with speed and the pitch equilibrium with thrust will be modified. In case of SWATH the hull response to these phenomena is extremely weak implying not acceptable trim angles that can be observed at relative speed over Froude number Fr>0.4.
A full exploitation of the SWATH concept is possible only through an effective stabilizing device which action must be more powerful as speed increases. Transversal dynamic stability is generally easier to manage. Due to transversal symmetry of the hull configuration, the centre of hydrodynamic forces remains in the ship mid plane.
Scientific background
SWATH concept has been considered for more than fifty years but only recently significant progresses to spread SWATH concept also to small size vessels have been done. In the last three decades, significant contributes to SWATH development and understanding have been proposed. Lee and Curphey presented the mathematical model for motions, stability and loads identifying some critical aspects of SWATH dynamics as:
– the necessity of fins to maintain heave and pitch stability at high speed;
– peak motions in heave, roll and pitch are excited in longer waves;
– wave exciting heave force and pitch moment acting on SWATH are smaller than those acting on a mono hull of comparable displacement.
Salvesen et al focused on the wave resistance optimization of immersed bodies, keeping all other geometrical properties constant. Authors reported reduction of 38% in effective horse power between original ad optimised hulls at the considered speed. Papanikolau et al applied optimization procedure based on Salvesen [2] method for SWATH in the range of Froude numbers from 0.3 to 0.7. For this optimized SWATH, Schellin et al [4] reported seakeeping prediction compared against experimental data indicating good overall performances but also necessity of stabilizing fins. Beena et al presented seakeeping analysis of different immersed bodies, discussing on the appropriate criteria for pitch, roll and vertical accelerations. Yoshida introduced “resonance free SWATH” obtained by diminution of strut length to approximately one half of immersed bodies length. Brizzolara et al presented an example of the so called “2nd generation SWATH”. Authors reported that for Fr>0.5, the position of contracted section remains almost invariably at midship, independently from the prismatic coefficient CP, the slenderness coefficient L/V1/3 or ratio L/D. As maximum weight and dimensions were the assigned design constraints, the strut length decreased drastically, and to obtain the sufficient initial transversal and longitudinal metacentric height, four struts were chosen and positioned at the extreme bow and stern of the immersed bodies. Today, the major part of built and operating SWATH ships have two immersed bodies and four struts. According to Grannemann next generations, still in the experimental phase, are one strut and one immersed body. This concept is known as SWASH (Small Waterplane Area Single Hull).
Scale model tests at Naples towing tank
MAHY Marine Hydrodynamics Research Group at the Department of Industrial Engineering of University of Naples Federico II started in 2013 a scientific project aimed at the design and development of various SWATH hull forms for the identification of the optimal values of the geometric characteristics and hydrodynamic optimization.
Several SWATH configurations have been tested to identify the best SWATH configurations suitable for small size pleasure and working craft. Main hydrodynamic characteristics have been considered and peculiar aspects of model-ship correlation procedure have been identified. After a first phase of bare hull tests, the best performing model has been fitted with radio-controlled forward fins, so that the highest achievable speed both with and without active stabilization was identified. At present a self-control system for the active fins is on test.
Seakeeping Tests
Tests in regular head waves have been performed for wave lengths/length overall ratios (l/LOA) ranging from 1 to 8 with constant wave height of 0.045 m. Motions have been measured by two independent systems: Qualisys tracking system at sampling frequency of 60 Hz and for redundancy also by the system used for resistance tests, lasers, accelerometers and load cell at 500 Hz. Extensive report is given in [10]. Here, for space reasons, only results for speed of 10 knots relative to a 32m LOA ship are reported. Heave, pitch and vertical accelerations, are given in non dimensional form as Response Amplitude Operators. Experimental results confirm, as also noted by Lee and Curphey, that heave and pitch resonances occur at l/LOA >6 (wave period TW> 11.2 s for the considered ship length). This confirms the benefit of larger LOA for SWATH configuration. On the other side shorter waves are the most probable in any geographical area and specially in Mediterranean.
Ship Operability
From rough sea experimental tests the SWATH short term operability for the 32 m LOA m/yacht derived in geometrical similitude from the tested model has been evaluated. Calculations are relative to head sea only, considering JONSWAP spectra and varying significant wave height (H1/3 and zero crossing wave period (TZ) in the range of realistic service, i.e. H1/3ranging from 1 to 4 m and TZ from 5 to 9 s.
The criteria for SWATH seakeeping analysis of are based on limiting values of roll, pitch, vertical accelerations, slamming and deck wetness. With regard to human comfort vertical motions and acceleration values are considered. 3 deg is the considered limiting value for significant pitch and 0.1g for vertical accelerations. Heave motion will be directly related to cross deck clearance. The significant amplitudes for heave, pitch, acceleration for 10 kn full scale speed has been assessed. In sea state corresponding to Beaufort scale 6-7, heave amplitude is at max 0.85 m. Pitch limit of 3 deg is verified and vertical accelerations are still within 0.1g limit. For space reasons, only acceleration values are reported here. It can be noted that In these weather conditions SWATH configuration presents seakeeping performance in head sea quite better than any hull configuration of similar main dimensions. Further experiments and direct comparison with mono hulls have confirmed this sharp conclusion. Stabilizing active fins, beside the mentioned advantages in controlling longitudinal trim at high speed, are a valuable asset to further reduce vertical motions also.
An Italian SWATH Proposal, The Total Comfort Yacht
A significant part of the research carried on by MAHY has been supported by S&C Marinewith the aim to provide references for medium and large yachts characterized by superior comfort by SWATH concept exploitation. Total Comfort Yacht (TCY), this is the name of the brand are going to be pleasure craft focused on customer welfare and satisfaction trough a general sustainability. This trend, instead of striking performance aspects, privilege comfort, usability of the spaces and navigation environmental impact. The yacht’s Life Cycle Assessment has been considered also.
In TCY a clever use of SWATH configuration optimizes the perceived comfort by three main factors:
– The level of noise and vibration
– The motions and accelerations in the living areas, both at cruising and zero speed in rough sea conditions
– The breadth of the walking areas and the absence of steps.
The SWATH configuration allows to house any machinery inside the immersed bodies, in a distant area, and physically separate them from the living quarters allowing sound pressure reduction impossible for a mono hulls. Other cause of noise and vibration in navigation is the wave formation that is extremely reduced for SWATH hulls. This further contributes to lower perceived noise.
Motions and accelerations of a SWATH optimized configuration, are in the order of 40% of a corresponding mono-hull. Beside high navigation comfort, this feature is of particular advantage for craft intended to be anchored in open harbours, as is often the case for medium-large yachts
The inherent limitations of the SWATH concept that is the small value of the unit displacement and the reduced static stability are acceptable when, as in the case of recreational craft, the mission profile requires no significant weight changes or shifting and they are still managed for TCY main dimensions (15,80 m LOA 8.6m BMAX).
In the TCY proposal, the huge deck area is intended exclusively for living spaces that are set on one level only . All technical items related to the propulsion and operation of the craft are relegated into the immersed bodies or into the struts.
In the TCY proposal, the SWATH configuration allows different trim and heights of the living quarters over the sea surface, chosen according to navigation and weather conditions, manually and automatically controlled. The passengers can enjoy this unique feature passing from a stable island with low trim in still water, to a low motion platform set well over the waves in rough sea.
Finally TCY is fitted with low speed zero emission propulsion to cruise Marine Protected Areas (MPA).
Further Developments
At present SWATHs are considered superior to any conventional hull form when reduced motions in rough sea are requested. That’s because a SWATH presents the best seakeeping and comfort performances both at speed or when at anchor in open harbors.
SWATHs are sustainable for the high hydrodynamic efficiency and for the reduced wave pattern and wake wash. The huge deck area best fits pleasure craft layouts. The variable draft allows the most comfortable height of the living spaces above the sea level in any cruising condition. Such features are significant factors for the successful application of this concept to pleasure craft design.
This paper has considered hydrodynamic and geometrical aspects of SWATH design. In the next future structural, layout and propulsion aspects will be presented.
SWATH peculiar forms and proportions, as well as involved geometrical constrains, are a challenging task for yacht designers. The few SWATH proposals for medium size pleasure craft recall usual schemes or even workboat superstructures. After having established the benefits of this hull form, new original contributes by industrial designers, stylists and decorators are expected and necessary for SWATH wider diffusion in yachting.
