Being that SWATH is an acronym of Small Waterplane Area Twin Hull, such vessels typically have two submarine-like lower hulls which run completely submerged.  When in the water, a SWATH resembles a catamaran. There are two other components to the hull: 1) The Struts, 2) The haunches.  The haunches, which blend into the decks and bridge, are connected to each submerged hull by one or two relatively thin vertical members, called struts.  The longitudinal cross-section of each strut roughly half the width of the submerged hull, and streamlined to decrease wave-making resistance. A typical SWATH cross-section is shown below.

Classic SWATH hull design

As any marine architect will know, ship motions are caused by the waves on the ocean surface which frequency and amplitude determines the effect on slowing and rocking the hull as it moves through the water.  If you can go below these waves, you will achieve a smooth ride as you might expect with a submarine.  Of course wave effect, which creates as much or more drag as frictional (pipe) resistance cannot be eliminated totally, since a degree of reserve buoyancy is needed.  However, wave forces can be lowered if the volume and water-plane area at the design waterline is kept to a minimum. Hence, by carefully calculating strut size it is possible for the net vertical wave force on the struts and hulls to be near zero over a chosen range of wave frequencies.

The design compromise, is to achieve similar payload carrying capacity with reduced motions, yet still maintain speed.  This has the effect of reducing seasickness, hence enhancing crew performance and comfort levels.  Thus, unlike mobile oil drilling platforms, the waterplane area of a SWATH vessel is small, to be about 50% as much area as a monohull of equal displacement.

Typical submerged hull and strut sections

The spacing of the submerged hulls is an important consideration during the design process.  There is a relationship between the area of strut waterplane and distance from the centre of roll, to maintain  adequate transverse stability, hence to resist heeling moments due to wind or movement, or positioning of cargo or passengers, from one side of the ship to the other.  A wider hull spacing results in greater structural mass for a given vessel length, hence, a waterplane area of insufficient volume to cope with roll could give an uncomfortable ride, whereas a greater area could result in a more expensive vessel.

It should also be remembered that the struts house much of the propulsion machinery, and that adequate access must be maintained and strength. Thinner struts decrease wavemaking resistance, but also increase the hull’s wetted area, which results in increased frictional resistance.  For all the above reasons  research and development effort on SWATH vessels has focused on the correlati on waterplane area and sea-keeping performance.

Sufficient wave clearance to the underside of the deck structure is also important to allow any  SWATH ship to ride above surface waves that are typical of the area the vessel will be operated.  Naturally, a coastal vessel will need less clearance and a blue water cruiser, more.

SWATH LINKS:

• CCGC Frederick G Creed, a Canadian Coast Guard survey ship.

• USS Sea Shadow, a US Navy experimental stealth ship.

• Radisson Diamond, a 350 person cruise ship.

• R/V Kilo Moana, a University of Hawaii reasearch vessel.

• Cloud X, a passenger ferry running between Florida and the Bahamas.

• Nekton Rorqual/Swacat 80, an 80 foot cruise ship operated by Nekton Diving Cruises.

• Cetus and Perseus, pilot vessels for the Netherlands Loodswezen.

• Döse, Dunen, pilot vessels for the German Brotherhood of Elbe - Pilots.

• Elbe, pilot station vessel for the German Brotherhood of Elbe - Pilots.

• The Swath Ship Concept

• Swath Ocean Systems