Viscous Drag-Reducing Cladding Airbag System
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Solution Overview
Problem
Current materials and technologies fail to effectively reduce viscous drag on marine vessel hulls due to limitations in withstanding hydrostatic pressures and maintaining non-wetting properties over extended periods, leading to increased fuel consumption and environmental impact.
Innovation Solution
A cladding system comprising airbags with a pressurized plenum and a water-expelling outer surface layer, featuring a reinforcing fabric sealed with vulcanized rubber or thermoplastic polymer, and restrictor holes to maintain air pressure equal to or greater than hydrostatic pressure, ensuring the airbag remains effective under harsh marine conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If hydrophobic materials with complex surface topography are used to trap air on the hull surface, then viscous drag is reduced, but the materials cannot withstand hydrostatic pressures at depth and the trapped air diffuses into water over time
Solution Approach 1:
The hull is divided into multiple segments, each covered by independent airbags that can be individually pressurized and maintained. This segmentation allows each airbag to independently withstand hydrostatic pressure while maintaining the air layer for drag reduction, solving the durability problem of continuous hydrophobic surfaces.
Solution Approach 2:
The invention uses pressurized airbags filled with air at pressure equal to or greater than the surrounding hydrostatic pressure to maintain a stable air layer between the hull and water. This pneumatic approach replaces the passive hydrophobic surface approach, enabling the system to actively counteract hydrostatic pressure and prevent air diffusion into water, ensuring long-term reliability.
2Object-affected harmful factors
If air is blown through the hull to create bubbly flow, then viscous drag is reduced, but the power requirement to produce large volume flow of air bubbles offsets the power reduction from reduced drag
Solution Approach 1:
Air is introduced into the hull structure in advance and stored in the airbags before the vessel encounters high-drag conditions. The air is maintained at the required pressure continuously, so no additional power is needed during operation to generate bubbles, eliminating the ongoing power consumption problem of bubbly flow systems.
Solution Approach 2:
The airbags are self-contained structures that maintain their own internal pressure equal to or greater than the surrounding hydrostatic pressure. Once pressurized, they automatically maintain the air layer without requiring continuous external energy input, making the system energy-independent during operation and eliminating the power offset problem.
3Reliability
If a gas-permeable ply is used to connect the air-retaining layer to the reservoir, then air can be replenished, but the ply lacks robustness to survive in the marine environment and presents a barrier to air flow
Solution Approach 1:
The vulnerable gas-permeable ply is completely removed from the system. Instead, the airbags are directly connected to the air reservoir through robust, impermeable connections. This extraction eliminates the weak link while maintaining the air replenishment function through alternative, more robust pathways that can withstand the marine environment.
Solution Approach 2:
The airbag structure uses composite materials including reinforcing fabrics and robust sealants to create a structure that is both strong and air-tight. The combination of these materials provides the necessary robustness to survive in the marine environment while maintaining effective air flow pathways from the reservoir, eliminating the need for weak gas-permeable plies.
4Object-affected harmful factors
If holes are made in the hull to blow air bubbles, then viscous drag is reduced, but the holes are large causing air to form large bubbles that wet the hull surface
Solution Approach 1:
The air release system is segmented into multiple small restrictor holes distributed across the airbag surface, rather than using large holes. This segmentation creates numerous small air pockets that maintain the non-wetting surface condition, preventing the hull surface from becoming wetted while still achieving drag reduction through the distributed air layer.
Solution Approach 2:
The airbag surface is equipped with locally distributed restrictor holes that create small air pockets at specific locations. This local quality approach ensures that air is released in a controlled manner through many small openings rather than large holes, maintaining the non-wetting property across the entire surface while achieving effective drag reduction.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution significantly reduces viscous drag by maintaining a non-wetted surface area, leading to decreased fuel consumption and environmental impact while withstanding the harsh marine environment.
Implementation Method 1
the airbag further comprising an outer surface layer adapted to be water-expelling
Implementation Method 2
Water has a surface tension of 72 mN/m. The hydrostatic force at 20 m depth is equivalent to the surface tension force of almost 3 meters length of a completely hydrophobic material in each square millimetre.
Implementation Method 3
the pressure in the plenum is adapted so as to pressurise the air in the water-expelling layer to a level substantially equal to, or greater than, the adjoining hydrostatic pressure
Implementation Method 4
The gas-permeable ply is between 0.5 and 5 microns thick, and lacks the robustness necessary to survive in the marine environment. The porous media of the replenishable reservoir presents a significant barrier to air flow, and significant air pressure gradients occur when air flows through it.
Data Source
AI summary
A low-cost viscous-drag-reducing cladding for a ship's hull comprising airbags whose outer surfaces are adapted to be water repelling. Each airbag comprises a plenum that comprises substantially open space, which does not obstruct the flow of air through it. The airbags are inflated with compressed air to a pressure higher than the adjoining hydrostatic pressure. The airbag material comprises a reinforcing fabric, which is adapted to withstand the forces encountered during operation, and is sealed with a sealant so as to be made substantially impermeable to air. The outer water-repelling surface of each airbag is connected to a plenum by means of restrictor holes. Air flows from the plenum through the restrictor holes into the water-repelling layer.