In order to understand the principles underpinning SLICE™ one must understand the physics of resistance. Resistance of a ship comprises two principal components: (1) viscous resistance stemming from the friction of the water against the hull, and (2) wavemaking resistance leading to the formation of the waves following the ship, known as the Kelvin wake. At high speeds, wave-generating resistance composes 50 to 60 percent of total resistance. This produces the distinct Kelvin-wake pattern behind the vessel.

The transverse wave component of the Kelvin-wake, observable as the large periodic waves within the V-pattern, is the larger part of the total wave drag. Until the 1970s, designers could do nothing to counteract the effect of wave drag on resistance and total performance. The hull parameter governing wave resistance is known as the Froude number. The Froude number relates the speed of a vessel to its length by the formula, where V is the vessel's speed, L the vessel's length and the acceleration due to gravity.

The illustration at right (click to enlarge) shows that at low Froude numbers (low speed-to-length ratios), the wave resistance is low and the viscous resistance dominates. As speed (Froude number, or F) increases, wave resistance becomes a higher percentage of total resistance--until at the critical or "hump speed," wave resistance exceeds viscous resistance. This large increase occurs when F = 0.4, and is maximum at F = 0.5. Conventional ships always operate at Froude numbers below this primary hump speed To achieve high speed, naval architects design their ships to operate below the F = 0.4 threshold by incorporating long lengths. Only Navy ships with high installed-propulsion power can operate at a Froude number above 0.4.

The first attempt by naval architects to reduce wave resistance was the bulbous bow, which is widely used on cruise ships, ocean tankers and cargo vessels. This design cancels a segment of the wave created, thereby reducing the energy in the Kelvin wake. The bulbous bow lowers the height and increases the period of the transverse wave created by the ship. While the bulbous bow reduces the energy of the Kelvin wake by about 10 percent, this reduction occurs only at the design speed of the ship. Therefore, this design improves the efficiency of transit at cruise speed but provides little improvement at other speeds.

The SWATH ship Sea Shadow is an extension of this cancellation idea. This ship's lower hulls have a bulbous, "Coke-bottle" shape, with thin struts that connect the lower hulls to the superstructure. This shape counteracts wave resistance, reducing the energy of the Kelvin wake by about 20 percent. The Sea Shadow's sculptured lower hull doubles the limited cancellation effect of the bulbous bow. But again, this effect is realized only at the optimum design speed.

Theory shows that at high Froude numbers, the transverse portion of the Kelvin wave is virtually eliminated, reducing the wave resistance to low Froude number values. This leads to the idea that to significantly reduce wave resistance, a ship should operate at large Froude numbers, thereby surpassing the limiting hump. The idea is analogous to a supersonic jet overcoming wind resistance by surpassing the sound barrier. SLICE™ is designed around this high-Froude-number principle.