Air Lubrication Drag Reduction for Smaller Vessels

And Andrew, if you want to know why this is true/common, go look at the Reynolds based skin friction/boundary layer turbulence differences between controlled tank experiments and real world chaos.

See this thread.
Skin-Friction Formulas https://www.boatdesign.net/threads/skin-friction-formulas.31280/

 

Are air lubrication systems like bow bulbs that are only beneficial at a narrow speed range?

 

Sorta kinda. The bubbles have to stay..."attached(?)"... "in contact(?)"...to the wetted surface in the TBL to change the skin friction properties. Very similar to riblets and sharkskin or teflon coatings, you are actually playing with the fluid/solid interface. Something do-able in a clean tank with a smooth well tended model. Not so easy to do on a rough fouled and slimed hull in a seaway full of seaweed and whale snot.

 

I'd almost forgotten (mercifully so) an ONR-sponsored experiment in the use of a "special polymer" that would be mixed with seawater and injected in to the boundary layer of a high-speed submerged body. The intent was to demonstrate a "sprint speed booster" capability.

"Whale snot" was one of the various derogatory terms my engineering group came up with as a proper name for the stuff. ;-)

 

This may help or not,

300knt torpedo https://www.boatdesign.net/threads/300knt-torpedo.10574/

 

I remember reading about torpedos and submarines. However, they are totally submerged which is a very different operating condition from a boat.

 

Yeah, I remember that study. We used the term (and whale s**t) for whatever viscous junk we'd have to clean out of small passages. I don't think most people really appreciate just how many, and the volume, of biologics there are in seawater...and they really love warm small spaces to live. Cleaning a ctenophore colony out of a filter that has been sitting for a week in drydock needs to be experienced.

 

Maybe concept could be helpful in speeding up a SWATH?

 

Not really different. Think about it this way, all "vessels" when moving through a fluid have two distinct components on their surface resisting their movement; the pressure perpendicular to surface, and the shear tangential to the surface. In broad terms you can think of these as the two different components of drag, the pressure/wave making drag and the skin friction; and it doesn't matter if we are looking at a torpedo, a boat, or a missle. The Shkval "torpedo" creates its own underwater high pressure but low density bubble to operate in, effectively removing most skin friction due to water, but substituting the skin friction of the gas in the bubble. Like I said in post #10, they are throwing a lot of power into generating the bubble (i.e. "air lubrication") just to go faster.

 

If all you are interested in is going faster, then just make the step to leave the high drag fluid behind....aeroplane or ekranoplan anyone?

 

Cargo capacity would be less , and cost per unit might be significantly higher going to an airplane?

 

Perhaps...for a very, very brief duration using techniques similar to those assumed for the Russian torpedo. Producing sufficient air to "feed and sustain" a bubble of sufficient size to be of any benefit for drag reduction is quite a challenge. Having personally been involved in a SWATH project that was supposed to achieve very high speeds from operating in an air "bubble".........

 

Warp speed Ferries at 5 miles per minute, ha? But, quite impractical for other reasons..

 

My sea sled collects all the air you could want in the tunnel, and rams it underneath so it comes out under the transom. So much air that I've spent months trying to get dense water for the prop to work in. Could barely get on plane because above about 15% power the prop would just lose grip and race wildly. So I bought and installed a 5" leg extension to get it down under the aerated water. Which led to other problems. The air is intrinsic to the hull form, so I don't really think meaningful comparison testing would be possible. And so I can't add anything there. The boat does ride easily over short steep waves that force everyone else to slow down. @baeckmo has advised us that that is due to the speed of sound in highly aerated water vs that in non-aerated water, but now I'm diverting the thread further.

 

Been recently thinking of air lubrication too and did read all your comments. Looks to me like a lot of "feelings" flying here around so I'm gonna add mine as well.

To my senses drag of hull is caused by 3 things: water viscosity, water displacing (?pushing?) and vortexes caused by non laminar water displacements (not sure if these are correct English names, but feel free to correct me). And air lubrication is addressing first one only.
So the formula would be like that: the bigger wet surface air lubricated the less drag. And then efficiency of using air as high as possible.

So I was recently thinking if there would be benefits for flat bottom hulls, if you applied on bottom something like skin of that airplane below to prevent air bubbles runaway.
https://external-content.duckduckgo...2f3a1d27395b78a4e04f20994d4286f115&ipo=images

Also as a man of numbers will ask, does anyone know what is some average percentage of hull drag caused just by water viscosity?