of a 122m length waterline, 12,000tonne displacement vessel of the type described above being approximately 22,650m3
/min. This
tremendous flow of water into the bow-oriented propulsor(s) in a SWEEP vessel reduces the energy in the bow wave, thus reducing the power required by the vessel. In order to maximise its efficiency gains, an optimum SWEEP design discharges the flow from the bow-mounted waterjet into an air layer beneath the hull (this air layer is similar in concept to the ALS, as mentioned above, where the air layer rises, moving from forward to aft, and is disposed aft of a displacement hull's submerged bow).
Little blower power required
Compared with some other concepts, such as an SES, this air layer requires very little blower power to be maintained. In a SES design, the ship's bow is at the water surface so that its blowers must supply air at sufficient pressure and flow to depress the water going from bow to stern, whereas the rising air layer in the ALS concept requires only around 15% of the blower power needed by a similar size and displacement SES. As a point of interest, notes Mr Burg, a very large SES can reach the point where its blower power requirements equal its propulsive power requirements.
What all of this means is that a SWEEP can be
efficient over a large operating speed range because it takes advantage of ALS technology at low speeds, where friction drag predominates, and then takes advantage of SWEEP technology where wave drag predominates. The SWEEP concept actually utilises both ALS and SWEEP technology at all speeds. For example, the 122m SWEEP can obtain a total drag reduction of around 15%-20% at 20knots and a 40% reduction at 45knots, because it is reducing mostly friction drag at 20knots, while having a very large effect on the predominating wave drag at 45knots. Discharging the bow waterjet flow into the air layer beneath the hull has another significant advantage compared with simply discharging underwater, because doing so avoids turbulent mixing losses that would occur if the discharge jet or jets were underwater. Yet another advantage of discharging into an air layer is that a steering and reversing system can be positioned at the jet discharge that is well forward in the hull, thus providing a very high level of steering and reversing ability. Moreover, this can be achieved without any increase in high-speed drag since the steering and reversing systems do not make contact with the water during forward, high speed operation. The accompanying illustration shows an underwater view of the forward end of a SWEEP; note the waterjet discharges (without steering and reversing systems here to simplify the drawing) internal to the air layer. Another illustration shows an underwater view of the complete SWEEP that is presented only in part in the other figure. Note that the air layer rises, moving from bow to stern and, in this preferred configuration, extends aft to the stern.
Potential efficiencies What, then, are the potential efficiencies to be gained by a SWEEP? Making a comparison of
8 Typical SWEEP bow section.
137m 2500tonnes displacement littoral combat ship (LCS) 35knots
Conventional SWEEP
Conventional SWEEP
Conventional SWEEP
29,800kW 19,090kW
137m 3200tonne displacement ferry 35knots
38,030kW 24,300kW
137m 12,000tonnes displacement cargo ship 35knots
141,700kW 89,500kW
A typical SWEEP hull, showing the air-lubricated space underneath.
45knots
49,200kW 29,530kW
45knots
62,640kW 37,660kW
45knots
220,700kW 132,700kW
Efficiency comparisons of three ship types, expressed in propulsive power (kW).
several 137m high-speed ships having waterline lengths of 122m - conventional hulls versus SWEEP hulls - indicates significant reductions in powering requirements for the SWEEP hull. These comparisons of 137m ships were made based on potential applications of the SWEEP concept to fill several high-speed needs including a littoral combat ship (LCS) at 2500tonnes, a vehicle/passenger ferry at 3200tonnes, and a commercial freighter of 12,000tonnes. The results, in terms of propulsive power required in kW, are shown in the table. The blower power required to maintain the SWEEP's pressurised air layer is only a few per cent of its propulsive power, and if the blower
power is added to the SWEEP's propulsive power, the SWEEP still offers a considerable advantage. Studies show that the 122m length waterline SWEEP hull - including its air layer blower power - requires only approximately 63% of the power of a similar conventional hull at 35knots, and 60% at 45knots. Mr Burg says he believes that the studies he has carried out to-date show that the SWEEP concept is technically feasible, and further analysis and model testing is planned as the next step in development. Tests and demonstrations of high speed SWEEP freighters, vehicle ferries, and/or military combatants will then follow.
THE NAVALARCHITECT FEBRUARY 2006
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