Introduction to SWATHs – Part 1

Brief History

Firstly, what is a SWATH? What does it mean? SWATH stands for Small Waterplane Area Twin Hull. In simple terms, it refers to a vessel with two hulls with a small waterplane area.

The first recorded Swath was around 1905 with a US patent by Nelson [1] with what is considered the modern DNA-looking Swath – that of a thin hull with a torpedo-looking tube under the waterline, but was ostensibly designed for storage. This later led to a proposal to the British Admiralty by Creed and Lewis in 1942, called the ‘Mobile Seadrome,’ for an aircraft carrier, ultimately leading to another patent [2] after their unsuccessful attempt with the British Admiralty.

There followed a period of little interest in the hull form until the 1960s and 70s by the Office of Naval Research in the US, where they realised the potential for a vessel with excellent seakeeping characteristics at its core. This renewed interest led to additional patents [3]. The type of vessel was named a Semi-Submerged Platform (SSP), which resulted in the design and build of SSP Kaimalino. Coincidentally, during the same period, Japan also showed interest in similar designs, introducing their own term: Semi-Submerged Catamaran (SSC). This also led to the creation of the world’s smallest SWATH, the Marine Ace [4]. During this period, the term SWATH eventually became more commonly adopted, rather than the SSC or SSP acronyms, to define this hull form.

The world’s fastest Swath is currently MV Patria, designed and built by FBM Marine in the UK in 1989 [5]. She achieved a top speed of 32.1 knots on trials, with a service speed of 30 knots.

Geometry of a SWATH

Now we have a basic background to a SWATH or Swath; how do we know what a Swath is? Is there any metric that can define a Swath – hydrodynamically – compared to any other vessel? The answer is yes. Everything in naval architecture has its roots in simple coefficients and ratios to help establish a trend or to create a group or class of hull types; a Swath is no exception.

This is the waterplane area (WPA) ratio – (WPA/Δ ^2/3) – with typical values for a Swath ranging from 0.50 to 1.50. This ratio is a measure of a vessel’s restoring force. To put this into context, most vessels, whether a small sailing dinghy or the Queen Elizabeth 2 (QE2), have a WPA ratio of 4.0 to 6.0.

Below is the basic configuration of a Swath hull:

It can be seen that it consists of a lower pod or tube, which carries most of the buoyancy, followed by the thin strut, which is then attached to the haunch region and, in turn, to the main raft section [6].

The differences can be seen in more detail when viewed from above, in plan view, of typical hull forms and their WPAs:

Seakeeping

Now that we have defined what a Swath is in terms of its hull form and how to “measure” this to be classified as a Swath, why does this hull form perform significantly better than any other hull form in a given sea state, compared to its conventional hull form equivalent?

To answer this question, we first need to define seakeeping. Simply defined, seakeeping is the ability to remain at sea in all conditions and carry out the duties of the vessel. However, the subject of seakeeping tends to be a complex and subjective performance measure in the absence of real hard data. What this means is that we need to understand the relationship between the sea and waves and the response of a vessel to the waves. Therefore, we need to look at the basic equation of motion of a floating body in a sea state.

The natural periods of a vessel are associated with the 6 degrees of freedom: 3 in translation – heave, surge, and sway, and 3 rotational – roll, pitch, and yaw. Only those with a natural restoring ability, heave, pitch, and roll, have natural periods associated with the vessel since this is the basics of simple oscillatory motion.

The period of motion of a floating object is given by the following expression [7]:

These coefficients can be summarised as:

a = Inertia moment → virtual mass moment of inertia, inc. added mass
b = Damping moment → a force in opposite direction, like a bilge keel
c = Restoring moment → relates to shape of hull

The forcing moment is the wave. When this term is set to zero, it provides the ability to find the natural period of motion of the vessel.

Rolling Period:

Pitching Period:

Heaving Period:

The GM_T is determined from the transverse metacentric height (BM = I/V), which is a function of the WPA. Similarly, GM_L is the longitudinal metacentric height, and the AW is simply the WPA. So, we can see that in each term, whether it be rolling, pitching, or heave, the waterplane area has the biggest influence on the value that is associated with the hull’s geometry.

To put some context to these equations, take a typical fast catamaran that is 30 m in length and displaces 90 tonnes, like that of Red Jet 3, which used to run between Cowes on the Isle of Wight to Southampton. She has a roll period of 2.7 seconds. Changing this roll period is extremely difficult with a conventional hull form, as we can simply demonstrate. If all parameters of the vessel remained the same, increasing the roll period requires a low GM_T or a high mass inertia (displacement) or a combination of the two.

Let us investigate changing the displacement, this seems an easy fix, however, the resulting displacement required to move the natural period sufficiently far from its current 2.7s, keeping all other hull attributes the same, requires a change from 90 tonne to one of at least 600 tonnes, because being a multihull, it has a very large GM_T to overcome. Not forgetting, this is still the same 30m vessel!

If we were to look at the hull form, changing this shape to lower the GM_T sufficiently enough to yield a similar change in roll period is equally difficult, keeping all other parameters the same, such as the overall beam of the vessel. Merely reducing the WPA (water plane area) pro-rata (B/T) in order to lower the GM_T results in a hull form with a draft of more than 5 times its existing draft to maintain the same volume. This does not take into account other factors affecting the hull from this change, such as the significant amount of added drag owing to the hull forms increase in WSA (wetted surface area), as it more than doubles, or its inability to enter shallow harbours, etc., and just an ‘odd’ shape in general. It’s not so easy down this route, either.

Because of its size, the QE2 was famous for its long, lazy roll period due to her large displacement relative to its low GM_T.

It is a similar situation with regard to heave. The mass, Δ, is the displacement of the boat, and the A_W is the area of the waterplane at the DWL. Δ may be effectively increased by the ‘added mass’ (the ‘a’ coefficient), which is the amount of water that is attached to the hull and moves with the hull (it is somewhat more complex than this, but this is a fairly accurate summary of its effects) [8]. So, a high mass (displacement) with a small WPA would yield a long heave period. One point to note is that the key finding of experimentation into the added mass is that it is independent of speed, i.e., whatever the speed, it remains the same. So, the shape of the underwater hull is important.

It is clear that whilst the basics of a vessel’s natural period of motion can be calculated from the hull form characteristics, manipulating it on any conventional hull form to create longer periods yields next-to-impossible results.

Now we know the basics of what a Swath is and how the hull shape can play a significant role in the natural period of motion of a vessel. In the next article, we shall investigate why this matters and what it really means.

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About the Author:

John Kecsmar studied at Solent University and later did his postgraduate studies at the University of Southampton, where his Masters and PhD focused upon the effects of fabrication on the fatigue design in aluminium structures. Starting his journey as a naval architect at FBM Marine on the Isle of Wight in the late 1980s, and then several years at Austal Ships and WaveMaster International in Australia in the early to mid-90s, he returned to FBM Marine, later to become FBM Babcock Marine, in the mid-90s until the design department was closed down in 2005. He formed his own design consultancy company with late Nigel Warren in 2005 and continues to this day. He was previously the chairman of RINA’s High Speed Vehicle Committee and is a fellow member of RINA and a member of both SNAME and JASNAOE. He sat on the technical editorial board of RINA’s IJSCT publication and is a member of Lloyd’s Register’s Technical Committee, RINA’s High Speed Vessels and Safety committees, MCA’s High Speed Advisory Group, and SNAME’s SD-5 Advanced Ships and Craft Panel, to name a few. He has been designing high-speed aluminum vessels such as patrol boats, fast ferries, SWATHs, and crew boats for over 30 years and has authored many papers in the field of high-speed hydrodynamics and the design, fabrication, and fatigue of aluminum structures.

References:

[1] Nelson, A. 1905, “Vessel” US Patent 795,002.

[2] Creed FG, 1946, Floating Structure, US Patent 2,405,115.

[3] Lang, TG. 1971 “High Speed Ship with Submerged Hulls”, US Patent 3,623,444.

[4] Oshima, M. et al., 1979, “Experiences with a 12 meter Long Semi-Submerged Catamaran (SSC) Marine Ace and Building of SSC Ferry for 446 Passengers”, Proc. AIAA/SNAME Advanced Marine Vehicles Conf. Baltimore, US, paper no.79.

[5] Milner, R., 1990 “The World’s first 30-knot Fast Displacement Catamaran (SWATH) Ferry”, Proc. 7th Int. High Speed Surface Craft Conf.

[6] Kecsmar, J. et al., 2007, “The evolution of advanced SLICE® Technology adapted to satisfy the HSC Code and commercial requirements”, RINA H.S.A.M.V. Conference, Shanghai, China.

[7] Bhattacharyya, R. 1978 “Dynamics of Marine Vehicles”, Wiley-Interscience.

[8] Rawson KJ & Tupper EC “Basic Ship Theory – Vol II”, Edn 3, 1984, Longman.