Frequently Asked Technical Questions

1. How Many SWATH Ships Are There?

2. How Does the Draft of a SWATH Ship Vary With Size?

3. How Does the Maximum Beam of a SWATH Ship Vary With Size?

4. How Does the Length of a SWATH Ship Vary With Size?

5. How Does the Length/Beam Ratio of a SWATH Ship Vary With Size?

6.0 What Are the Speed-Power Characteristics of SWATH Ships Like?

1. How Many SWATH Ships Are There?

Currently, there are about 50 SWATH craft and ships around the world. Of these, precisely one half were built in the U.S. The chart below shows the composition of the worlds SWATH ships according to their full load displacement. There are 35 existing SWATH vessels that displace less than 500 m. tons, with 20 displacing less than 100 tons. Most small SWATH vessels are constructed of aluminum. The largest SWATH ship by far is the 11,500+ ton Radisson Diamond luxury cruise ship, which was built in Finland. There are also 9 SWATH ships between 2500 and 5500 tons displacement, most of which were built for naval or oceanographic applications.

2. How Does the Draft of a SWATH Ship Vary With Size?

The accompanying chart shows a definite trend for the increase in SWATH ship draft with increased displacement, in tons, but there is also considerable scatter in the data. The scatter is due, principally, to varying design requirements and different operating areas. Small SWATH vessels have disproportionately deep drafts because they must operate in some of the same seas as larger vessels. 

3. How Does the Maximum Beam of a SWATH Ship Vary With Size?

The overall beam of a SWATH ship is strongly influenced by the total amount of strut waterplane area chosen for the design at the intended operating draft. During the design process, if the total amount of waterplane area is decreased, the transverse spacing between the struts must be increased to regain adequate transverse stability to resist heeling moments due to wind or movement of all passengers to one side of the ship. Because of their smaller waterplane area, SWATH ships typically have a larger hull spacing and wider overall beam than a conventional catamaran of the same displacement.

The chart below shows the data for overall beam as a function of full load displacement for existing SWATH ships. Up to 500 tons displacement there is a consistent trend. As with draft, the smaller SWATH vessels have disproportionately wide beam. Above 500 tons there is more scatter in the data points. Not shown in the chart is the 32.2 m overall beam of the Radisson Diamond, which is the worlds largest SWATH ship. Its beam was constrained by the need to pass through the Panama canal.

4. How Does the Length of a SWATH Ship Vary With Size?

SWATH ships are usually shorter than a conventional catamaran or monohull of the same displacement. For low speed applications short length is necessary to provide a very long pitch natural period, which decreases ship pitch motions. For higher speed applications reduced length is advantageous because it reduces structural weight and, therefore, minimizes displaced volume and wetted area.

As may be seen in the accompanying chart, the length data shows a pretty clear trend up to about 500 tons displacement, but for larger SWATH ships the data points are scattered.

5. How Does the Length/Beam Ratio of a SWATH Ship Vary With Size?

The chart shows the effect of SWATH vessel size on the ratio of overall length to overall beam for existing SWATH vessels. There is considerable scatter among the data points for displacements of 500 tons or less. About all that can be said is that the L/B ratios for SWATH ships appear to fall between values of 1.75 and 2.90. In contrast, the L/B ratios for conventional catamarans typically fall between 3.30 and 4.0.

6.0 What Are the Speed-Power Characteristics of SWATH Ships Like?

SWATH ships are similar to most other types of surface ships in that a sizable fraction of their propulsive power is wasted in generating waves. A key factor affecting the magnitude of wavemaking resistance that must be overcome is the vessels speed relative to its length or, more precisely, to the square root of its length. This parameter is called the Froude number. For any displacement-type surface ship, like a SWATH, a Froude number value of 0.50 indicates maximum wavemaking resistance. At increasingly high Froude numbers above 0.50 wavemaking resistance decreases slowly.

When the design speeds of existing SWATH ships are converted into the corresponding Froude numbers and then plotted vs. vessel displacement, the data shows that a majority of SWATH vessels below 400 tons operate at Froude numbers between 0.60 and 0.80, which is in the high-speed regime. For this reason, about 50% of the total resistance of these small SWATH vessels, with design speeds of 20-25 knots, is wavemaking resistance. As a result, the total calm-water resistance of small SWATH vessels is higher than for a comparable monohull or catamaran. Typically, a SWATH ship will be 3 or 4 knots slower for the same power in calm water, but the speed differential becomes smaller in real-world seaways because of the SWATH ship's reduced pitch motion.

The design speeds actually achieved by existing SWATH ships are shown in the next chart. Currently, the 4 fastest SWATH ships have displacements between 200 and 350 tons and provide speeds of 27 to 28.5 knots. Two of these are diesel powered and the other two utilize gas turbines. In this size range speeds as high as 35 knots can be provided if the new, higher power small gas turbines are installed. For example, SI offers a high speed option for the Super 4000 class ferry, with a design speed of 34 knots.

Thus far, all SWATH ships above 500 tons are relatively low speed, 15 knots or lower. However, speeds well above 20 knots can be provided with reasonable amounts of power.