Wave drag reduced with SWEEP hullform
Don Burg, the inventor of a new patent- pending ship hull concept, the 'Ship with
Wave Energy Engulfing
Propulsors' or SWEEP, has combined the advantages of a bulbous bow with those of an air-lubricated ship (ALS) to reduce significantly the wave drag of displacement hullforms; recently, he has proposed a ferry version of the concept. Here, David Foxwell explains the new concept further.
done on the air-lubricated ship (ALS), perhaps the most high profile of which is the SeaCoaster, a concept based on the use of twin air cushions and intended to offer fast speeds at high efficiency, good load-bearing capacity, a smooth ride and high level of stability. Mr Burg has more than 30 years of experience as a naval architect working on advanced marine vehicles, and 12 years experience in aerospace engineering prior to that. His naval architecture experience includes design and development of the SeaCoaster SECAT (Surface Effect CATamaran) and advanced technology waterjet and ventilated enclosed-rotor propulsors.
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Several versions of the SeaCoaster have been proposed, including high-speed ferries, crew boats, and patrol vessels, but the concept has been furthest developed in the AMH SeaCoaster technology demonstrator built for the Office of Naval Research (ONR) in the USA. Built by Austal USA, this 31.2m surface- effect catamaran is being used to demonstrate advanced hullform technology on behalf of the ONR, and is operated by American Marine Holdings.
The patented SeaCoaster catamaran design has air cavities in the bottom of each hull into which air is blown, providing lift to the vessel, reducing resistance, and thus allowing higher speeds to be obtained. The AMH SeaCoaster was designed to achieve speeds of up to 56knots and, following its evaluation by the ONR, American Marine Holdings expects various departments of the US military to consider using it for a wide range of applications.
The patent for the SeaCoaster hullform is held by Air Ride Inc, Miami, and was previously applied to a 19.8m SeaCoaster catamaran ferry that remains in operation with Island Express Boat Lines, which has been operating the 149-passenger SeaCoaster ferry on Lake Erie since 1999. Originally developed with assistance from US Department of Energy's Inventions and Innovation Programme, the air-assisted catamaran has 'wave-slicing' fine-entry sidehulls. The air cushions formed between the catamaran's side hulls and the water surface support approximately 80% of the vessel's displacement. Another advantage of the hullform concept is its reduced draught. The reduced-wetted-area resistance of the SeaCoaster is claimed to shrink powering
THE NAVALARCHITECT FEBRUARY 2006
ON Burg is known in the USA and internationally for the work that he has
An impression of a typical SWEEP high-speed ferry, showing the unique bow contours.
requirements by approximately 50% compared with conventional catamarans and monohulls at cruise speeds.
Large power reductions claimed Taking into account the power required by the blowers, the SeaCoaster only requires around 60% of the power of a standard catamaran, and the design has no flexible seals and no air cushions between the sidehulls, as is the case with surface-effect ships (SES). Air Ride says the SeaCoaster also has excellent platform stability without any of the 'cobblestone effect' experienced with some other hullforms. The vessel is also able to navigate in very shallow water, and in commercial applications, believes Mr Burg, the concept would enable more cargo to be carried at faster speeds with less horsepower than other high-speed catamarans or monohulls. Now, the inventor has taken the concept a step further, and a new company formed by him and his colleagues, SeaSpeed SWEEP, is proposing that the Ship with Wave Energy Engulfing Propulsors concept, which draws on the SeaCoaster concept but with a number of important modifications, could be applied to larger, displacement hullforms.
It is well known, of course, that wave drag increases dramatically as ships operate beyond what is sometimes termed 'hull speed.' As such, the operating speeds of displacement hulls - except for some very high-powered military combatants - remain, as Mr Burg puts it 'embarrassingly low'. Wave drag makes up 70% of total drag at 45knots on a 122m length waterline ship and, furthermore, wave drag has increased 25 times at 45knots over its value at 15knots for that 122m length waterline ship.
The bulbous bow is often employed to reduce wave drag, and is used on many ships where it reduces total drag by 10%-15% at high speeds. Friction-reducing air layers have also been employed on a few vessels; however friction drag makes up less than 20% of total drag for a 122m length waterline ship at 45knots, and air layers, while effective at low speeds where friction drag predominates, only help marginally at high speeds. What the patent-pending SWEEP technology does is utilise inlet water flow into large water
Comparison between sphere, bulbous bow and SWEEP bow hullform, showing how the potential efficiency gains of the latter are realised.
propulsors to engulf or 'suck in' wave energy, thereby reducing wave drag. It does this by taking in water aft of mid-point on a bulbous bow where the water is turning inward, thereby accelerating and increasing the water flow that would normally be parasitic wave-making drag.
The sketch reproduced here shows water flow around a sphere, standard bulbous bow, and a SWEEP bow. Note that the water flow detaches from the sphere in high-drag eddies after following the contour of the sphere inward and aft of the sphere's mid-section. The standard bulbous bow takes advantage of this inward turning of the water as a means of absorbing or sucking in part of the wave energy while avoiding the separating eddies of the sphere, but the SWEEP concept takes this a step further, accelerating the wave-making water flow into a high flow propulsor(s). The water flow into a SWEEP propulsor or propulsors is significant, with the water flow into a typical bow-oriented water propulsor(s)
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