Air Lubrication Drag Reduction for Smaller Vessels

Hi All,

I am new to the forum. First of all I want to introduce myself! My name is Andrew and I am a mechanical engineer living in the UK. I grew up with Speed boats and Rhib, but unfortunately, my career has taken me a bit far from that recently!

I have read several studies both in experimental testing, CFD and full-scale results on Air Lubrication Systems, and I was wondering, if anyone has tested them on smaller vessels such as SY or pleasure crafts. I know that Veneteau have a model but I have never seen any data about it.

 

In lab conditions large reduction in drag has been demonstrated. However practical application isn't that simple. The bubbles need to be fairly large to make a difference, and the shearing forces in the boundary layer on a boat's hull tend to break them up. They also rise to the surface and escape, so that needs to be managed. And they need to be generated to begin with, and the associated equipment either makes enough drag to offset the advantages, or if integrated with the hull bottom increases the cost enough to make it pointless, and the ports are vulnerable to clogging.

 

Nailed it. We participated in several "air lubrication" projects back in the 90s, monohulls and catamarans both. All were essentially failures for the reasons you noted.

That said, back in the early 90s, I ran across an aluminum crew boat serving MSC ready ship crews anchored off Jacksonville. It caught my eye because it was obviously faster than the typical gulf-built crew boat. The owner/skipper came over to look at the vessel I was there with (a failed SES ferry design that we were trying to limp to New York) and told me about his air lubrication system. It was an ingeniously integrated step just aft of the original planing waterline. Air was fed to the step via a plenum (that was built to be above the static waterline) and an entire row of Detroit Diesel roots blowers, powered by a dedicated 4V Detroit diesel engine. He said it was good for a 10 knot improvement and I believed it. He was not the least bit worried about the power it required...he said he was the preferred charter for that role because of the extra speed he could offer.

Only truly successful air-lubricated craft I've run across in my 37 years of messing around with AMVs.

 

Super interesting @BMcF !

 

I have a Hickman sea sled , which has given me the brain madness: Sea Sled madness. It’s in my brain. https://www.boatdesign.net/threads/sea-sled-madness-it%E2%80%99s-in-my-brain.63079/

The hull form traps and retains so much air it's actually a problem for me, because the prop can't bite sometimes. No doubt the air is having useful effects, but I can't quantify it, because I can't compare it to anything.

 

Just to clarify, you do not have a Hickman Sea Sled, you have a modified, recreation inspired by Hickman, no?

 

That's correct. I didn't buy it from Hickman.

 

Evidently there has been quite a bit of new research and with large players like Mitsubishi entering the sector, it will be interesting to see how it works out.
Recently I had received a link to a ship being actually built with an Air Lube system but could not find it.
A quick google search for "Air Lubrication for Ships" reveals quite a bit of "new" research on the process

https://www.marineinsight.com/green-shipping/how-air-lubrication-system-for-ships-work/

 

The low Reynolds number "bubble lubrication" I've seen for large ships appears to offer a few percentage points of drag reduction. All of the projects I was involved with were for smaller, much higher speed, craft.

From that link, a key to the necessity of ALWAYS reading between and behind the lines in any of these claims:

"Though according to the experiments conducted by Mitsubishi there were negligible effects of air bubbles on the propeller, rough seas and changes in fluid density can produce unfavorable results."

"With the right ship hull design, the air lubrication system is expected to achieve up to 10-15% reduction of CO2 emissions, along with significant savings of fuel."

Frankly speaking as a very long time ship designer...if that last claim was actually true, every commercial vessel built these days would have that "air lubrication" system as part of the hull design.

 

Hi Andrew, welcome to the forum.

I concur with DogCavalry and BMcF on this; all data does not point to any significant gain for the increased complexity in real world applications (SES's being their own niche).

There is one somewhat successful(?) implementation of what could be considered "air lubrication" for small vessels. Multi-step Gold Cuppers ( 1920's) and some modern hulls (like Destriero or your Beneteau Air Step) use a passive form of "lubrication" to get the hull un-stuck and up on the lift pad. As said, this is not new and has been tried several other ways (sea sleds, cathedral hulls, M hulls). Unfortunately, these follow the common theme of small vessels where significant horsepower is tossed at the problem of wringing a few more knots out for the ad campaign or race course. It is done only for speed, not economy or efficiency.

See this thread.
What is world's biggest planning hull boat and how fast? https://www.boatdesign.net/threads/what-is-worlds-biggest-planning-hull-boat-and-how-fast.64476/

 

Likewise.
We were involved with a high-speed pax ferry, where the client wanted an air-lub system installed.
I told them it wont work, but he was 'sold', literally, on their Kool-aid....suffice to say, after 6 months of trying - and trying - and trying,... because they were told it would work...they eventually removed the whole system. A serious waste of time and money for all involved...

 

https://ww2.eagle.org/content/dam/eagle/advisories-and-debriefs/Air Lubrication Technology.pdf

An interesting read with some cruise ships already using it. (according to the document)
But certainly not precise data to support claims
Again, heavy hitters in the game.

 

Hi All,

Thanks for everyone's reply, I didn't think I would get so many responses!

To the best of knowledge there are around 7 Air lubrication system supplies currently including some starts up with the NET drag reduction being around 3-15%. As some mentioned the use of compressors will negate some of the drag reduction effects. As it was also mentioned it works best on slower speeds maximum 24 Knots ( 12-20 knots bringing in the best results) not what some of these ferries and catamarans can do. Of course the draft would also effects these results.

My thought was that for some day boats say in the 7-14 m that cruise around 20-26 knots it could be a possible solution. As some mentioned its more of an ad campaign to increase a knot or two then in actual savings. My thought was on the other end of spectrum to save in fuel costs.

 

Just a quick ad....every time we were presented with drag reduction numbers like 15% or more, they were obtained with model testing. We eventually learned the hard way that the model test results did not scale well at all. I recall a 38-meter catamaran ferry that was built with an air lubrication system, as specified by the "expert" a naval architect/marine designer from one of the former Soviet marine design agencies. His model testing data was compelling. The air lubrication system, with hull mods was quite expensive and the air was supplied by high pressure-ratio radial-diffuser air fans.

The vessel did not even achieve the top speeds of the previous builds that were plain catamaran hulls.

A disgusted shipyard removed the air lubrication equipment and reversed all the hull mods to deliver it in same configuration as their previous catamarans.

 

That ... all sounds very spookily familiar!!