I'm a marine engineer and on my last ship (a tanker) we certainly investigated the use of hull air lubrication to improve fuel efficiency of the vessel. There are a wide number of energy saving technologies (e.g. from propeller devices to hull appendages to intelligent engine control systems), and for each one the design team needs to perform a cost benefit analysis (CBA) in terms of both acquisition and operating costs.
That sounds easier than it is when you consider the many variables that need to be juggled during concept design phase (e.g. ambient conditions you expect the ship to operate in, exhaust restgriction, operating speed/power profile, etc.), but it's doable to a fair degree of accuracy.
There are a few primary reasons we did not proceed with air lubrication:
1.) Cost of compressed air. To generate the bubbles you need compressed air. Compressed air can actually be quite expensive to produce, so a fair amount of your fuel savings from reducing friction resistance is consumed by the need to generate the air. That being said, there is generally a decent amount of waste heat on ships, and that thermal energy (or potentially harvested energy from the evironment) could be capstured and used to reduce the cost of compressed air (but that's more equipment to buy & maintain).
2.) Hull shape constraints are very stringent because you need to be able to shape the hull go ensure the bubbles remain adhered to the skin of the vessel over the entire vessel length in order to gain the maximum benefit. Generally that means you want a flatter bottom. However, hull shape is very critical for the final powering resistance, so it's quite possible that if you design your hull for better bubble adhession, you prevented yourself from just using a more hydrodynamic hull shape that would perform just as well as well as the air lubricated hull in terms of total hydrodynamic resistance (friction & form drag).
3.) Real world conditions. You also need to consider that the ocean is not perfectly still, so the pitching/rolling of the vessel due to wind/waves/currents may further hinder bubble adhesion (to what extent is unclear to me).
4.) Concerns about inducing propeller cavitation if bubbles cannot be steered away from the prop in-flow. Cavitation can rapidly deteriorate expensive propellers, so the hull shape should steer bubbles away from the prop. Cavitation is a real, but it was not clear to me how much of a concern these air lube bubbles are; cavitation is really a function of propeller blade shape, size and RPM.
All of the above hull shaping and special curvature can be very costly in production.
I'm not saying air lubrication shouldn't be considered, it should, but a careful CBA must be completed to truly ensure you'll see savings in terms of ship lifecycle cost. Due to hull shape constraints, it's probably better suited to new ship designs vice retrofit.
This is great info! I'm envisioning a field of bubble emitters somewhere towards the front of the boat and possibly along the keel. It seems like they might be kind of fiddly from a maintenance perspective as well?
I wonder if magnetohydrodynamic systems could also be used to drive the boundary layer rearward and flatten the velocity gradient/reduce turbulence.
I do have one question regarding the effectiveness of the system when the ship is bobbing and weaving. Wouldn't the adverse affect of the ship movement be mitigated if the bubbles are generated at multiple points on the ship bottom?
That would be a mitigation for losing portions of the bubble field due to ship motion, but you'd be adding more air bubble ejectors, requiring a larger amount of compressed air which consumes more energy cumulatively. It's probably only an issue on higher sea states (e.g. > Sea State 5 or so). Another mitigation is fin-stabilizers (e.g. often found on cruise ships) to reduce ship motion. The goal seems to be to eject air from as far forward and then keep those same bubbles under the hull as long as possible to gain the maximum benefit from the energy you expended to compress the air.
If folks are interested in air lubrication, they'd probably be interested in surface effect ships (SES)[1]. Different principle, but still about rreducing friction drag. Air cushion vessels (hovercrafts also).
That sounds easier than it is when you consider the many variables that need to be juggled during concept design phase (e.g. ambient conditions you expect the ship to operate in, exhaust restgriction, operating speed/power profile, etc.), but it's doable to a fair degree of accuracy.
There are a few primary reasons we did not proceed with air lubrication:
1.) Cost of compressed air. To generate the bubbles you need compressed air. Compressed air can actually be quite expensive to produce, so a fair amount of your fuel savings from reducing friction resistance is consumed by the need to generate the air. That being said, there is generally a decent amount of waste heat on ships, and that thermal energy (or potentially harvested energy from the evironment) could be capstured and used to reduce the cost of compressed air (but that's more equipment to buy & maintain).
2.) Hull shape constraints are very stringent because you need to be able to shape the hull go ensure the bubbles remain adhered to the skin of the vessel over the entire vessel length in order to gain the maximum benefit. Generally that means you want a flatter bottom. However, hull shape is very critical for the final powering resistance, so it's quite possible that if you design your hull for better bubble adhession, you prevented yourself from just using a more hydrodynamic hull shape that would perform just as well as well as the air lubricated hull in terms of total hydrodynamic resistance (friction & form drag).
3.) Real world conditions. You also need to consider that the ocean is not perfectly still, so the pitching/rolling of the vessel due to wind/waves/currents may further hinder bubble adhesion (to what extent is unclear to me).
4.) Concerns about inducing propeller cavitation if bubbles cannot be steered away from the prop in-flow. Cavitation can rapidly deteriorate expensive propellers, so the hull shape should steer bubbles away from the prop. Cavitation is a real, but it was not clear to me how much of a concern these air lube bubbles are; cavitation is really a function of propeller blade shape, size and RPM.
All of the above hull shaping and special curvature can be very costly in production.
I'm not saying air lubrication shouldn't be considered, it should, but a careful CBA must be completed to truly ensure you'll see savings in terms of ship lifecycle cost. Due to hull shape constraints, it's probably better suited to new ship designs vice retrofit.