Tiny bubbles under a ship may help reduce fuel consumption | Hacker News Reader

Tiny bubbles under a ship may help reduce fuel consumption

smithsonianmag.com

107 pointspseudolus81 comments

Open articleView on HN

I remember reading about this 40 years ago, and have occasionally wondered why it hasn't been implemented.

Then I'm reminded of the history of the "Bulbous bow"[0] on ships, and how long that took to be adopted.

I've submitted the Wikipedia link[1] in case anyone wants to read about it.

[0] https://en.wikipedia.org/wiki/Bulbous_bow

[1] https://news.ycombinator.com/item?id=25091997

Bulbous bows sounds hard or impossible to retrofit on an existing ship but this does sound fairly easy to add.

It might be harder for larger ships but for anything under a couple hundred tons those are straightforward to retrofit in anything that doesn't have an exotic hull construction.

Are you referring to bows or bubbles?

bows

It isn't, but it also is not a fix-all. They work for a very specific, narrow range of speeds and wave conditions. If that's not in your shipping route it's not helpful.

Are you referring to bows or bubbles?

The bulbous bow.

I always wondered what that was for, cool!

For anyone that didn't read the article: Without the bulb, the prow slices through the water and causes the water to ride up the sides of the ship.

The bulb on the bow is under the waterline. This forces water up in front of the ship before the prow. The bulb is designed so that the bulb-rise of water cancels the prow-rise of water in destructive interference.

The bulb design only works at certain speeds, water conditions, etc.

> have occasionally wondered why it hasn't been implemented.

Because underwater hull part may be caused via cavitation erosion.[0,1,2]

[0] https://www.iboats.com/propellers/what-prop-cavitation

[1] https://www.thoughtco.com/cavitation-causes-and-remedies-229...

[2] https://www.southampton.ac.uk/engineering/research/projects/...

Those are two very different things. The OP article talks about introducing air bubbles into the stream along the hull, the articles you linked talk about cavitation bubbles, i.e. water that vaporizes when the pressure locally drops. The latter collapse spectacularly and do indeed damage to surfaces. The former don't.

[edit: clarified which articles I mean]

> introducing air bubbles into the stream along the hull

Placing air bubbles into the stream along the iron/steel hull in seawater... This is the worst idea as it would drastically increase oxidation.[0]

[0] https://pubs.acs.org/doi/10.1021/es0109242

Let’s forget about the thick layer of paint between the steel and the water...

> Let’s forget about the thick layer of paint between the steel and the water...

If seawater oxidation around the boat would be increased then thick layer of paint should be thinned to prevent steel corrosion, which also means increasing cost of the boat.

Most boats/ships already get wet with (sea)water, alternating with air at the water level. Hardly a new problem.

There is research into solving this with sacrificial material? Last i remember, there was the idea to have a coat of carbon fibre enhanced ice to ablate and soak up the damage, while regenerating from below?

> Last i remember, there was the idea to have a coat of carbon fibre enhanced ice

It may be used (probably with low profit) for small vehicles/boats, but not for big ships.

Related: Propellor Boss Cap Fins https://m.youtube.com/watch?v=a8PjWSeLUzY

Ships are already risky & expensive projects, and to modify them experimentally can have large follow up costs.

https://en.wikipedia.org/wiki/Galvanic_anode

>Large ships with bulbous bows generally have twelve to fifteen percent better fuel efficiency than similar vessels without them.

Wow, that's a much larger effect than I expected.

I know some ships have them, including Diamond Princess, aka. The COVID-19 ship. Look for Mitsubishi Air Lubrication System, if you’re looking for more information

Russian high-speed torpedos have used super cavitation to envelop the torpedo in a steam of bubbles in order to go much faster:

https://en.m.wikipedia.org/wiki/Supercavitating_torpedo

It has trade offs for torpedos which is why the Russians and other nations continue to develop and deploy conventional designs.

All the noise from the bubbles and rocket engine prevents the torpedo from listening to and tracking its target. Point, shoot, and hope the torpedo gets there before the target moves too far.

Don't the Russians have a guide wire back to the sub?

That doesn't sound like a terrible idea if you can make the torpedo go fast enough.

Yes, but torpedoes are meant to blow up after firing. Ships aren't.

The torpedoes don't have to withstand 60 years of supercavitation damage (unlike ship propellers do, which already can't, and need frequent maintenance and replacement)

(also this has nothing to do with supercavitation, it's just a regular bubbler at the bottom of the ship that lets air rush to the surface around the hull)

Seems like a clear cut and solid idea. Any ideas on why it is not in widespread use?

Less stuff grows under the ship, too.

A similar technology was examined in a class I took back in 2016. As I recall, the ultimate reason why this and similar technologies are not in use has nothing to do with whether the technology works or not. Apparently there is fairly little interest at present from the market for these technologies. That doesn't make a lot of sense to me (these technologies can clearly save a lot of money in the long run) but this is what I recall from the class. I'd speculate that the people with the power to implement the retrofits needed are skeptical of the benefits.

I've heard a story from someone working in naval logistics that they often ship loads of empty containers around the world (and back) just to avoid some contract clause that switch up the storage cost of idle containers after a few months. The incentive structures seem quite perverse.

Looks like there's barely any tax on fuel for ships and planes. I guess it's tricky to raise such a tax unilaterally...

"The extreme example of this price sensitivity was California’s 1991 decision to lift the fuel tax exemption and to tax interstate bunker fuel sales. Within a year, Californian bunker sales had collapsed as ships bunkered elsewhere especially Panama. The decision to impose a tax was reversed but California’s bunker business never recovered. "

https://ec.europa.eu/clima/sites/clima/files/docs/0036/taxat...

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’m a former naval architect. Not many of us around, let alone on HN. Got a website or more background about your work?

There must be a few! I guess I’m a hybrid though. (Software guy) Anyway, cheers!

Thank you for the detailed reply.

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).

1. https://www.marineinsight.com/types-of-ships/what-are-surfac...

Air compressors can explode if not maintained correctly, also.

I imagine many things would explode on a large ship.

What's great about ships compared to for example buildings is that some serious design thinking goes into ships.

Especially modern buildings seem to have very little thought hours per cubic meter spent.

> modern buildings seem to have very little thought hours per cubic meter spent.

"Thought hours?" "Cubic meter spent?"

Are you using your own units of measurement to communicate with people that aren't you? (And thus don't know how your units of measurement work.)

I couldn't find anything solid to back it up (although admittedly I didn't search very hard), but apparently this also reduces fouling of the hull.

>air lubrication can reduce fuel consumption by five to 10 percent

I wonder if this is just the air lubrications system, or if it also includes gains from the reduced fouling? Either way, it's an interesting technology and assuming it works anywhere near as well as the marketing suggest I'd expect it will become a standard feature on larger vessels.

I've analyzed performance data from air lubrication vessels with some retrofits included. Frankly, never witnessed over 5% gain. At specific velocity range the gain could be 5% on average if the installation has been really successful. They might be better nowadays.

I'd imagine fuel costs are a big part of the cost of running a ship. Wouldn't a 5% gain be something ship owners would jump at? I realize it would be lower once you account for the added cost of the system and you may not always get 5% but even a savings of 1-2% of a big number is still a big number.

> I'd imagine fuel costs are a big part of the cost of running a ship.

"A 12% reduction in at-sea average speed, known as “slow steaming,” led to an average reduction of 27% in daily fuel consumption and thus fewer greenhouse-gas emissions."

Slowing down sounds a lot easier and cheaper than retrofitting bulbous bows or fancy new air lubrication systems.

https://qz.com/1608527/the-shipping-industrys-emissions-coul...

I agree with the conclusion, but daily fuel consumption isn’t the right thing to track.

I think this comes out to a 17% reduction in fuel use per unit distance, which still seems quite significant, but maybe changes the trade off point a bit.

That does sound better but I think it would be harder to implement. Convincing people to accept later delivery times is tough. People don't want delayed gratification. US interstate speed limits used to be set to improve fuel economy due to the gas crisis in the 70s. Try to convince drivers that we should go back to a 55 mph speed limit.

Would this work on a kayak? Anyone want to team up and try it?

Seems like there would be a surface interface area versus propulsion force calc in there somewhere. Guessing the result wouldn’t be very significant for a tiny vessel.

It would likely have some effect but you'd have to pack and power the air supply. Might be something you could just do with compressed air and have a little foot pedal to turn it on when you want it.

Some whitewater playboats have a concave chine that I was told makes bubbles that have this effect, but intended to make turning the boat easier. No idea if it was the bubbles that actually helped, but I've been in a boat that had them and it was noticeably easier than normal to spin it around.

You can see the inverted chine here (the line running along the underside of the boat, at the edge of the flat bottom):

https://825355.smushcdn.com/1645402/wp-content/uploads/2019/...

You want to haul an air compressor with you in a kayak? O_o

It could be a foot pumped bellows. Release the bubbles when you really want to go fast...

Tiny bubbles on the propellor? (Rather than the whole hull.)

Propeller Cavitation. Friction reduction saves money on the glide not the drive

At 50km/h electric foiling boat uses comparable power per mile to an electric car (well something like 2-3x). See Candella’s boats videos.

Is the electric car driving at 50km/h too?

Don’t think so

Slight tangent but this BBC4 documentary about bubbles was great https://vimeo.com/213299679

Anyone know how this compares to the turbulent boundary layer “sharkskin” swimsuit concept that made a big splash at the Olympics a while back?

One is about turbulence (i.e. structure of the flow of the liquid) while the other actually relies on there being something other than liquid water in the flow (air or in the case of super cavitation vapor). Not sure if that answers your question - maybe you wanted to know about differences in performance (don’t know about that).

I'm not an expert but I can make an educated guess. They put dimples on golf balls to create turbulence, which gives less drag than laminar flow over the ball. I assume that is the same thing happening in the sharkskin swimsuit. For the ship hull I have another guess (sorry). In fluids, you can't reduce "friction" by having a "slipperier" surface. The rule is (for laminar flow) the speed of water at the surface, relative to the surface, is 0. That determines the velocity field around the object, which in turn determines the drag. My guess is that when the air is between the ship and the water, the air has 0 velocity at the surface of the ship. The air also has a 0 velocity relative to the water at that boundary. But the air has a velocity gradient so the net effect is that the surface of the water if now in motion relative to the ship, meaning the ship can cause a smaller velocity profile of the water.

Not very good if you are operating echosounders.

Hydrofoils is another way.

https://vimeo.com/358872215

Hydrofoiling most cargo vessels tends to be impractical.

Bubble systems can (in theory) be applied to any existing design.

Speaking of foils, there is active development in commercial and DIY personal electric foil boards. Check YouTube or a build forum like https://foil.zone/

Hydrofoils cannot deal with rough weather/strong winds, what shipping routes is this practical for (where it is far enough to compete with air, but short enough to not have weather conditions)?

Plus that video seemed very disingenuous, given the lack of hard data/facts and instead just vagueness on top of vagueness and a shiny demo. I wouldn't touch that company.

Jeremy here from that Boundary Layer Technologies. That company building hydrofoil cargo ships. Hydrofoils are not new and have been working for half a century in harsh weather conditions. Please look up the Pegasus class ships from the US navy. Technology has changed in the last 50 years which means we can greatly improve efficiency and practicality of foiling ships especially in the world of airfreight.

Foiling vessels can be built to handle any weather condition of a buoyancy based ship. And because we're at sea for no more than 6 days we have a much greater weather prediction to avoid storm swells compared to a tradition ship who might be at see for more than 30 days and cannot go around the storm.

For 99% of the time, the ocean is pretty flat so we can still have 99% on time delivery.

LOL, I appreciate your words about vagueness. We have not been specific about how we will achieve our goals, but watch out of the coming months for a lot more details. I do love a good hater, it's a promising sign.

Hey, thanks for coming on HN and commenting! Especially on a Sat. afternoon. That's really cool of you. Good luck on the business and thanks for putting all the hard work and effort in to all of it. Keep being awesome.

> I do love a good hater, it's a promising sign.

I'm not sure regurgitating well known limitations of Hydrofoils (vis-a-vis strong winds), and asking questions about the only presentation a startup asking for money has felt the need to release qualifies as hating.

It didn't go unnoticed that you didn't actually answer any of the questions, and instead doubled down on vagueness. Also pulling out the Pegasus-class really shows how little research has been done (a petrol boat class of ships specifically not designed for deep ocean that was retired early due to the limitations naturally inherent in Hydrofoils!).

Really interesting article! I wasn’t aware of this technology to be honest, but seems very promising...

Sounds like these bubbles are like little balls the ship is rolling on? Super interesting!

I wonder whether a somewhat scrubbed exhaust could be piped into the bubble system.

The think the new Sir David Attenborough polar research ship has this feature (though I may be getting confused with a different vessel, an icebreaker I might have seen described on YouTube) https://www.bbc.co.uk/news/science-environment-54597767

Aside: The Sir David Attenborough carries the Boaty McBoatface AUV.

https://en.wikipedia.org/wiki/Boaty_McBoatface

I think bubblers are just common in icebreakers in general. It's because you want to reduce the friction between the ship and ice.

A airplane's wings allows flight not because of a negative pressure over the foil but because of repulsive electromagnetic charges in the air below foil (same force that allows capillary upward movement of water in tree trunks). Can an electrocharged hull repel electrocharged bubbles?

Reply on news.ycombinator.com