[MHml] Friction Reduction

[Alan Christoffersen]

Best Regards/Med venlig hilsen

Alan Christoffersen

Se de spændende tilbud på: www.insatech.com

Telefon: +45 5537 2095



-----Oprindelig meddelelse-----
Fra:	Tom Speer [SMTP:me at tspeer.com]
Sendt:	2. december 2006 06:58
Til:	Informed discussion of multihull issues
Emne:	Re: [MHml] Friction Reduction



Gary Pearce wrote:

>http://www.newscientisttech.com/article.ns?id=mg18925391.600&print=true
>
>I am not aware of it being tried on multihulls or indeed of any scientific 
>results other than the article above
>  
>Tom Speer wrote:
There's lots of literature on this.  For starters, take a look at:


>A key question, though is, "Is the power required to pump the air 
>reducing skin friction of the hull?" It's not enough to reduce drag, 
>there has to be a net reduction in power for the technique to be 
>worthwhile. Otherwise, you'd be better off sending the power to the prop 
>instead of the air pumps.

That is not a correct assumption. A well sorted merchant ship will be running at a
alot less than 40% efficiency when considering the diesel engine - that is not counting
the propeller efficiency! So any use of the energy that gets wasted, or where one can use
"low density" energy will be a welcome. Alot of energy is lost up the stack,  
as it takes about 15 kgs of air to burn one kg of fuel (in a diesel engine)

An example: 25000 bhp engine consumes about 0,2 kg/bhph = 5 tons/h fuel

5000 x 15 kgs air = 75000 kg/h exhaust gas (primarily Nitrogen plus some nasties)
 at a density of 1,3 kg/m3 this gives around 57700 m3/h 

Use of the kinetic energy (velocity) of the exhaust gasses, the overpressure available,
and judicious use of the vessels speed through the water can solve the issues.

Back in the 70's they started to use scoops for cooling water instead of pumps
saved lots of energy, a reverse scoop can be used as an ejector. Our old friends 
Mr. Bernoulli and Mr. Coanda working with us yet again!


And that brings up issues like what volume of air is required to get a 
given llocal drag reduction, how long do the bubbles or air cavity 
persist before the air is swept away from the surface, will the 
technique work on a vertical wall as opposed to a flat bottom, etc. All 
of which are aimed at getting the most out of the amount of air injected.

Bottoms can be made concave, sides can be designed to retain bubbles - just like the old viking ships!

I like Kodama's assessment, "Microbubbles can be called 'a big child -- 
immature, but with a bright future.'"

I agree - not only micro bubbles, but actual bubble entrainment in the surface structure

I am starting some tests with hollow hulls for a catamaran to attempt to exploit this issue - but on a sail boat!

Initial tests prove that I can get air under my hulls with out the use of external power.
Cheers

Alan