Motion is one of those things you learn to live with when cruising, as the boat rolls and pitches in the waves. There are solutions out there, such as fitting the boat with gyrostabilizers or, if you are old-fashioned, flopper stoppers. You can also take seasickness pills, which I find a great benefit.
But there is possibly a better way to reduce uncomfortable motion. It uses technology that dates to the 1930s and that promises a relaxed ride in adverse conditions. It’s called SWATH, and it’s starting to make its way from commercial vessels into the cruising-boat marketplace.
SWATH stands for Small Waterplane Area Twin Hull, which is a complex name for what is simple, logical technology. A boat rolls and pitches because it floats on the surface of the sea, where passing waves affect the full buoyancy of the hull. However, if you put the buoyancy that you need to keep the boat afloat under the water instead of on the water, then the effect is reduced. SWATH does that, and connects the underwater buoyancy to the boat’s working deck above water by way of a vertical structure. The design keeps a boat quite stable in lively seas.
Those connecting struts are narrow, hence the use of “small waterplane area” in the SWATH name. “Twin hull” obviously describes the catamaran-style structure that makes the vessel stable. It tends to create a rather ungainly looking craft, with a relatively wide beam for stability and an almost square shape when viewed from above. In the workboat market, nobody really cares what the boat looks like, but only now are we seeing designers starting to develop SWATHs that look reasonably attractive to everyday boaters.
What spurred designers to take another look at SWATH? Before, they were handicapped by limited space for engines inside the lower torpedo-shaped hulls. Modern diesels that produce adequate power in a smaller package can be installed in the confined space, on SWATHs up to about 70 or 80 feet in length. Advanced construction methods may also help: A yacht-caliber SWATH would likely cost half as much again to build as a conventional monohull. The shape does not lend itself to fiberglass construction.
Houston Pilots, which guide ships through the Texas port, were one of the first to use a SWATH as a pilot boat. They just replaced their aging boat with a pair of new ones, both SWATHs built by Abeking & Rasmussen in Germany, which also builds luxury yachts. A&R has been building SWATHs for 25 years, and more than 20 of its boats are used as pilot boats in the North Sea off the German coastline, not only to transfer pilots onto ships but also as floating bases for pilots awaiting incoming ships.
For that market, A&R developed SWASH (Small Waterplane Area Single Hull), with the same principle as SWATH but at a length of 65 feet. The vessel rides on a single underwater torpedo-shape hull; stability is created by adding two slim side hulls (the hull is in fact a trimaran). This prototype has been developed for private and commercial applications, but so far, there have been no takers.
John Kecsmar at Ad Hoc Marine Designs in the United Kingdom also is playing with ideas. He designed a couple SWATHs for the British military to use on crew transfer duties. He was also involved in designing the Lockheed Martin Sea Slice, an advanced SWATH for servicing oil rigs and platforms in the Gulf of Mexico. Today, he has a couple designs being used for crew transfers in the North Sea wind farm market, which is a challenging place, particularly in the wild winter months.
“Our 24-meter design has proven to be the best-performing crew transfer vessel on the market and is the only vessel still running when all the others are in port owing to bad weather,” he says. “No other vessel comes close in terms of seakeeping ability.”
For its latest design, Ad Hoc specified four engines so that the two engines in each hull could be staggered, reducing space requirements. This arrangement also leads to better fuel economy, as just two engines can be used when the vessel is alongside the wind farm pylons; and it allows the lower hulls to be slimmer and thus more efficient.
Meanwhile, in the United States, Richard Guglielmi of Stability Yachts in Florida has had a 60-foot SWATH running for 10 years now. He uses it as his private yacht.
“With regard to leisure, you get three times the enjoyment and time, especially offshore, due to the vessel’s immunity to the weather,” he says. “My S60 has two 135-horsepower diesels, and in the power catamaran mode, it will achieve 5 miles to the gallon at 3 knots and 2 miles to the gallon at 15 knots.”
The S60 uses a ballasting system so draft can be varied. With the ballast tanks empty, the yacht rides like a catamaran with the twin hulls on the waterline. In livelier seas, the yacht is ballasted down so that the slim hulls ride underwater and the craft behaves like a SWATH. This concept could point the way to the future of SWATH in pleasure boats; Guglielmi has a 66-footer on the drawing board that combines the SWATH concept with yacht styling.
Another taste of things to come is the design from Swiss company Solar Impact. With a length of 78 feet, this concept not only is based on a SWATH hull, but it is powered by the energy from 350 square yards of solar panels, has a large battery pack, and has twin 500-hp electric motors as the drive system (with a pair of diesel generators for when the batteries get low). Top speed is reportedly 22 knots for short spells, with a cruising speed of 5 knots.
Designer Werner Vögeli says he came up with the Solar Impact concept when he realized that no environmentally friendly yachts were suitable for the high seas.
“The decoupling of the yacht from the wavy surface results in 90 percent less rolling and heeling,” he says.
Now, despite all this excitement, SWATHs do have a downside. In addition to the small machinery space and higher construction cost, there’s the issue of what happens when the waves get high enough to strike the underside of the cross deck. As with catamarans, this situation creates a sort of cutoff point where skippers have to start considering tactics. The larger the SWATH, the higher the under-hull clearance, but there is no escape as the waves get higher. You could turn downwind, or lie stopped in the waves; some SWATHs have the underside of the deck shaped to deflect waves. A ballast system could increase the under-deck clearance, perhaps.
No boat is going to be fully stable in rough seas, but SWATHs are going to be better that most. One day, the perfect SWATH will be designed. I can’t wait.
This article originally appeared in the November/December 2019 issue.