Air Injection

Yup, this idea keeps popping up in every corner, where there is a discussion on flow phenomena. And it does not work.

The deciding factor is the kinematic viscosity "ny", which for air is about 15 times the water value. This is the "material" factor controling turbulence, hence friction along the hull surface.

For a mixture between water and air, ny is actually increasing with increasing air content. As ny is found in the denominator for the Reynolds number equation, it causes a reduction in Re, normally leading to an increase in friction factor.

On top of that, the injected gas volume acts as an additional body volume; the oncoming flow will "meet" a bigger frontal area and thicker boundary layer, all increasing resistance.

The only way to use air is where a "full" air cushion can be created, as in the russian air cavity crafts, the surface effect vessels, hovercraft and at a correctly ventilated step. Friction is NOT reduced by a film of bubbles.

 

what I say.................

 

I'm not arguing but what is it that makes a stepped hydroplane work?, How embarrassing, I now see that baekmo included a well ventilated step in his posting. My bad.

 

There are different mechanisms. One is gross as in a hydroplane where the thing is as much a plane as a boat.

There have been successful large scale trials with air injection but only in the form of microbubbles. The air does not create a low drag sheeting layer as some might envisage but it dampens the water movement in the boundary layer and thereby lowers the energy lost in the boundary layer to reduce drag.

Japanese researchers have been most successful as far as I know. This link has a snippet of some of the work:
http://www.nmri.go.jp/turbulence/group/040615Energy_saving_by_microbubbles.pdf
If you google microbubble drag reduction you will find various papers on it.

I have seen large scale model test data showing a drag reduction of 15% but that was largely offset by the energy to pressurise the air. The air needs to be injected as tiny bubbles not in sheets.

There are various studies being carried out on viscous drag on vessels. Some are effective but are regarded as polluting. Surface texture can have a positive effect as proven in AC racing. There has been some work to get surfaces that damping the energy in boundary layers in an attempt to copy what occurs in nature.

 

When I have read about stepped boats it is common to run across the description of the boats running on a sheet of foam. Is this what you mean by microbubbles?

So I just reread baekmo's post and found to my dismay I had overlooked his remark about a well ventilated step. If I am understanding him correctly and if I am understanding you correctly the "full" cushion of air that he speaks of and the comparison to a plane you speak of may be refurring to a similar phenomon.

"Surface texture can have a positive effect as proven in AC racing. There has been some work to get surfaces that damping the energy in boundary layers in an attempt to copy occurs in nature."

Is this similar to the sharkskin idea where a rough surface may be "slicker" through the water than a polished one?

And lastly, what is ac racing? LOL, I have so many more questions than answers.

 

In high speed applications there can be extensive mixing so the foam might be similar to micrbubbles but the microbubble injection technology is being developed for displacement hulls. I believe some of the benefit in these tunnels boats is aerodynamic lift as well as some recovery of the energy put into the displaced water by ramming it into the tunnel. It is not something I have considered in detail.

Here is a reference for the riblets used in the Americas Cup:
http://www.nasa.gov/centers/langley/news/factsheets/Riblets.html

 

AC, I get it. I think of a tunnel boat and a hydroplane as different types. There is a very knowledgable guy that hangs out here that is a tunnel master. The older hydroplanes were not traveling at the excessive speeds of today's racers but still were reported to get a substantial benefit from the step under smoother conditions.

 

Tunnels are designed to lift off Hydro's are not
Hence most speed records in small boats are done on hydros ( 3 pointers) as they are safer.

Fastest on the water - 317mph
Fastest outboard 178mph
drag boats 200+?

 

There is one very knowledgeable guy here on propulsion: baeckmo!

 

The Japanese example is interesting, it shows the situation at very high Re-numbers and very smooth surface.

Re for the towed 50m plate is ~3.5 e-8. The surface roughness (ks) is not mentioned; lets say it is a carefully painted "lab surface" with the mean roughness of 0.05 e-3 m, meaning a relative roughness of 1e-6. compared to the "normal" boat size this is a veeery smooth surface, and a high Re!

However, in this specific region, a slight reduction in Re may give a small dip in the ks/L curve, in the range of <10 %. But this is ONLY applicable within a short range of speed and roughness. If the surface roughness is increased just marginally from the test condition, the ks/L-curve is flat, or even increasing. Then you have a loss instead.

Now, there might be a change of streamline shapes close to the hull, due to the compressibility of the bubbles. For instance, if the geometrical aft lines are not optimal, the expansion of the bubbles in the deceleration zone, could reduce the induced losses. But again, what we see in lab scale has to meet the real world.

And Wally, these phenomena are not what we have in a step or tunnel; they are completely different, with a much deeper, continuous layer of atmospheric air.

Note: in the diagram attached, log 3.5e8 ~8.54, and ks/x = ks/length

 

Hi all,
Tried two experiments
Small hull with a lot of air, Boat almost sank. It looks like it build a bubble around boat and it kept sinking in bubble.
Larger boat with bubbles more spread out like in a step, yes boat seemed light, got on plane quicker. I think proper hull design will capture air better than injecting air.

 

Hey Guys' , L@@k At The Upper Right Hand Conner Of Your Screen And What Is That Boat ? I Guess All I Can Say Is , Been There, And It Has Been Tried , And This Is A Triple Digit Boat.

 

I doubt there is any air injection!

 

As a general comment, it is common to say "doesn't work" when "it didn't work under certain conditions" or "wasn't practical" is what actually happened.

http://docs.hydrofoils.org/SAS03.pdf

 

Vibration as a source of friction reduction is well known to seismologists. That is a key element in mudslides during earthquakes.
http://bsgf.geoscienceworld.org/cgi/content/abstract/178/2/149