Air Lubrication Drag Reduction for Smaller Vessels

This whole topic is a perennial matter of interest for those seeking what is effectively "something for nothing".Which it can't be because there is the weight and complication of plumbing numerous outlets to the surface or finding as dog Cavalry has mentioned,that simply forcing air beneath the boat may lead to other problems.
I don't believe it is of much use at displacement speeds because the compressibility of air would see the boat float a little lower,with more immersed surface area and an amount o energy lost as bubbles rise to the surface.At planing speeds,not only do you have weight to fight but there would be less lift generated by a locally less dense fluid and perhaps it is the lack of bubble generating machinery in a test model that gives false hope of the concept.I can't visualise how a steady stream of bubbles would be generated within a torpedo,other than by building in a suitable compressed air bottle in an already limited space.Not an impossible task,but has anybody done it in reality and outside a study exercise?
I think we have seen that the best way to use air is to get as much of the boat into it as possible,by getting the greatest volume out of the water.As demonstrated by Moths,IMOCAs, Ultims and power boats such as the Candela 8.

 

Maybe this will help you visualize it...

 

Thanks for that,it does help.I suppose a service life measured in seconds is also beneficial.

 

Daniel asked:

"Does anyone know what percentage of hull drag is caused by water viscosity?"

That varies with speed and hull design.

 

The epitome of "use once and forget".

 

OK, I'll take this on as it is one of my trigger items.

effect : a change which is a result or consequence of an action or other cause.

affect : make a difference to.

Daniel...for the resistance force on the moving body there is only the pressure on the surface and the shear at the surface. Nothing else.

Everything else in the fluid is a effect of the energy imparted to the fluid by the moving body. All the following velocities, all the waves, all the vortices are an effect of the body. They do not affect the drag of the body (assuming quiescent water and ignoring pre-existing velocities, waves, vortices, etc...).

For most engineers and hydrodynamicists in this thread these are not "feelings" but demonstrated facts. Just because you can see effects in the fluid, does not mean that those items have any affect on the moving body...only the data from the moving body is meaningful.

This is a big problem for people just entering the field. There was a significant amount of early work devoted to the effects on the fluid to determine drag rather than the affect of the fluid on the moving body. This is no longer the proper way to look at the issue.

As far as the last question...it all depends. What portion is cause by the shear and what portion is cause by the pressure is determined by the kinematic viscosity and the density of the fluid. This is the whole reason behind the Reynold's Number; which is a dimensionless quantity that helps predict fluid flow patterns in different situations by measuring the ratio between inertial and viscous forces.

***Edit: corrected typo in 7th para. No change to meaning.***

 

Well, that is not an exact quote of my question.

 

Sort of glad that I triggered you, because that really is sort of flipping the way you think! Thank you!

As it comes to the "feelings", what I meant was that many people shared their views/ideas/opinions/some experience (and to make it clear - nothing wrong with that, I did that too), but didn't see anything that would be hard conclusive proof (experiment, test, data etc, etc) that would close the thread.

 

Of the many various air-lubricated projects we were involved in over the years, the data ownership was always retained by the "owner" of the project and seldom published. The same is true for the performance/technical data from other AMV types. While we work very closely with designers and builders, we can't run around publishing or otherwise disseminating proprietary performance or "how we did it" data. We've been involved in the design and construction of over 150 SES, ACV, SWATH, hydrofoil, catamaran and monohull craft (and hybrids thereof) and have published open-forum papers on maybe a half dozen of those over the years.

SES' happen to be sort of a company "specialty", if we have one, and remain my favored approach to putting air between hull and water. ;-)

 

I couldn't "Quote and Reply" because your post was on a different page.
While transposing your question, I edited it for clarity.
Doesn't change the answer.

 

I was looking at some threads on semi planing / semi displacement boats. HJF (sasdesigns.org) has done some work combining a midship interceptor on a 5.6m boat. The interceptor had a ventillator behind it. Tests on similar hulls with and without the interceptor were tested demonstrating a significant reduction in power for the same speed.
Design Challenge 2009 Professional BoatBuilder
An optimized motor boat with aftship interceptor

 

This is not air lubrication. It's just that it's dry aft of the interceptor. The ventilation is just to make it easier before the air comes in from the side.
JS

EcoSmart 83
8,3 x 1,9 m 1,0 t 30 hp
First test 23 09 07

 

I wondered about that. Your design looks vaguely like a stepped hydroplane, and being so far aft, I wasn't sure how accurately you had described it or how big the step was. Thanks for correcting my misinterpretation. I think some stepped hydroplanes were vented with some degree of success in the 60s. The Amateur Yacht research Society had some information on vented hulls back in the 60s. I may be able to dig up a copy, but you probably have it more easily accessible than I do.

I wonder if you could locate a vented interceptor further forward. Looking at your diagram, I thought that a rotating cylinder could close off the slot. Something like this:

First and only sketch. It needs work. I can't design a boat, but I can draw one almost as well as my 4 yr old granddaughter. Building from plans is much easier.

I have been tempted to look into building one of your fast rowboats for years. We are roughly the same age, and for some reason that increases the weight of my boat every time I go to lift it, so that plan has timed out. It's only 50 years old and does not appear waterlogged. Bit of a puzzle.

 

I wonder, whether anybody tested this old solution:


Looks interesting, but does it actually reduce drag?

regards

krzys

 

"Stepped planing hulls" have been around forever but have more recently become almost a required design gimmi....er ...feature on many performance watercraft being marketed and sold today. On the right hull and under the right circumstances, steps do reduce drag (or optimize planing performance...depends on your viewpoint).