SWATH, The Art Of Compromise

M&O has worked on both the implementation and the failure analysis on quite a number of SWATH (Small Waterplane Area Twin Hull) projects in the last decades. SWATH vessels hold great promise and continue to intrigue naval architects and potential ship purchasers. When we first became involved in SWATHs, as far back as 1981 with the Monoform concept, SWATHs were described as: “The best solution for small ships in big waves”.

That truism has not changed, but it is clear that not all small ships in big waves are SWATHs today, and so how can that statement be true?

The truth of the statement is related to the disconnect between physics and design. Anything can be most efficient for any one condition, but that does not mean that it will be most efficient once it is turned into a real life object.

SWATHs are the poster child for this disconnect. While, for its size, a SWATH hull will have the best motion in waves, it does not necessary result in the most efficient overall vessel. The SWATH design is inherently expensive to build, has lots of wetted hull surface (which results in high drag at high speed), has limited payload capacity, tends to have deeper draft and is incredibly weight sensitive. Its inherent limited payload capacity limits SWATH applications to small vessels that do not carry high weight cargoes. That still leaves a large group of possible uses such as long duration slow speed patrol vessels, station vessels, crew delivery vessels, small offshore passenger excursion vessels and offshore helicopter support vessels. Still the number of successfully operating SWATHs is small. While the concept may be right, the execution often is left wanting. Almost invariably the problem starts with poor weight control during design and construction, and, once the design ends up on the overweight side, thing tend to fall apart. In addition, propulsion systems can be very complex, and the structural design and hydrodynamic design is often quite complex. Furthermore, once the vessel moves at speed it often requires additional ride control surfaces and technology.

Once in service, a SWATH tends to require more maintenance than less wave capable vessels of equal capacity and once the vessel makes any speed at all, fuel consumption becomes a problem.

Historically these design and operational problems could be a serious deterrent for SWATH vessel purchase, but there have been positive developments. Today there are propulsion solutions, such as electric propulsion, that can almost be described as off the shelf and that actually would be a joy to operate with generator engines placed in nice deck engine rooms, and the drive train (requiring very little maintenance) placed deep inside the submerged hulls. Ride control packages have become so much less expensive that these systems can almost become a trivial consideration. As a whole, despite the very limited successes in the past, today a clean sheet SWATH design should always be considered as an alternative when there is a need for a small low payload vessel in big waves.

The Monoform concept developed in the 70’s by Dr. Adorjan Szeless (Virginia Tech naval architecture professor) was a clever attempt to deal with some of the SWATH disadvantages at the concept level. Dr. Szeless reasoned that since the waterplane area of struts provides the vessel static stability, it might make more sense to put the struts in a V arrangement between a below water single hull and the above surface deck. Dr Szeless called this a Monoform hull (I never figured out why, because, SWASH, Small Waterplane Area Single Hull would have worked just great).

As a young engineer I tested this concept and at the time there were serious issues that would be difficult to solve with the technologies of the day. (Mostly it would require very complex structure and control surfaces) Today there are viable solutions to the these problems, (composite struts, and cheap control systems) although it is not clear if a Monoform would make a good ship, mostly because the narrow below water hull, the angled struts and the wide deck are a berthing nightmare.

However, it is an intriguing concept and I particularly like the variation that I was toying with that has only three struts.

Today, I would make the forward struts a complex (and complex is expensive, where the question is: How expensive?) curved or stepped shape, which would provide better wave interaction and would allow better control surface integration.

Regardless, while a Monoform may have advantages over a classic SWATH, at speed in modest waves, a catamaran will beat a SWATH anytime in overall efficiency. However, an all SWATH approach, or an all catamaran approach, may not be the answer.

At the last SNAME annual meeting, I attended a paper presentation by Dr. Stephano Brizzolara called “Comparative Performance of Optimum High Speed SWATH and semi-SWATH in Calm Water and Waves”. Here the SWATH concept is morphed into a very complex catamaran shape, which thereby trades wetted surface off against somewhat reduced performance in waves. Some pictures of this hull and a comparable SWATH shape as shown in the paper are provided below.

The paper indicates that this semi-SWATH approach may be hydrodynamically more attractive than most SWATHs in higher speed applications in waves. In making the next leap, to a real world design, it can be seen that the semi-SWATH would be a less radical vessel mechanically and thereby likely more attractive to the market. This too is a very complex approach, and requires lots of computation, testing and compromise, but it shows that the SWATH concept is not dead; it is simply waiting for designers and technologies to catch up.