How do we reduce skin friction ??

[yipster]

tunnels, probably youve seen it allready -if not- more testing at boattest.com

The Beneteau Flyer Gran Turismo 38's Air Step hull has a measurable difference in performance and handling. Air Step is no gimmick.

This was one of the most interesting tests we've ever done. We've come to have expectations on what a boat will offer in terms of performance and handling based on basic specs, but when we hit the water with this Flyer Gran Turismo 38, all those expectations went out the window. This boat was designed with Beneteau's Air Step hull, which brought a completely different set of handling characteristics to the boat -- all for the better. We immediately noticed a difference in this hull vs a conventional hull. There was no pounding through waves, turns were extra sharp, speeds were impressive, and acceleration was greatly enhanced. All of it spells a great handing boat, and it's all shown in our new test video before her premier at the Ft. Lauderdale boat show at the end of October. More... http://www.boattest.com/boats/boat_video.aspx?ID=2565

[tunnels]

yipster

Hi
Yeah i saw that last week ! I am right into the boat test site and gleened a lot from that site .
It sure is a interesting story and now matter what people say theres got to be something good about it . I am weary of the fact that air introduction is surposed to slow and cause more drag ! sorry i dont buy into that at all . If it was true the water speed record would never be as high as it is .

[m3mm0s rib]

Originally Posted by brucehallman

As to the question: "How to reduce skin friction?"

There is an implied presumption in this question that skin friction is the most important type of drag. There are different types of drag, and skin friction is not the most important type of drag for many boats.

I think that skin friction tends to be more dominant/important for slow, low powered boats (like some sailboats and some rowboats). Meaning those where the limiting drag is not the drag induced by the wave making of the boat that is seen with the faster craft.

So, with slower sailboats, and slower rowboats where skin friction is the dominant form of drag, I believe that the best way to reduce skin friction is to reduce the amount of skin. Hence, for equal displacement, you get the most payoff by having the hull be more compact (meaning less long/narrow). With the ideal shape being short round fat belly, versus long, angular and sleek.

It's the more correct to read so far

[Fanie]

Perhaps one must look at something completely different.

If you take an ultrasonic knife and cut through a piece of cardboard whilt the power is off, it's fairly hard to do the cutting.

When the knife is on and vibrating at 40kHz, the knife slits the cardboard like soft butter.

The trick would be to impliment something like that that would create a 'skin' effect around the boat so the boat 'runs on air' or the boat touching the molecules repell's it instead of the water clinging to it. The problem remains you have to displace the water somehow as you sail along. Now where's Moses when you need the man

[cyclops2]

DIMPLES on golf balls. To increase speed / distance by reducing drag.

Flip your boats over. Hit them 60,000 times with a tiny ball peen hammer.




Something for everyone.

[daiquiri]

Golf balls and boat hulls are aero-hydrodynamically two very different kind of animals. A ball is a so-called "blunt body", while a boat hull is usually a "streamlined body". Hydrodynamic characteristics of two types of bodies is explained here, for example: http://www.princeton.edu/~asmits/Bicycle_web/blunt.html .
At the end of that page an example of two balls is given, one smooth and one with a trip wire applied. Golf ball dimples have the same function as the trip wire - to force the transition from laminar to turbulent flow over the ball surface.
A turbulent flow acting on blunt bodies (like balls) gives a higher friction drag but a lover pressure drag. Since pressure drag is the predominant one in blunt bodies, the overall result of applying dimples is a lower total drag.
On streamlined bodies (like boat hulls generally are) pressure drag is much smaller than friction drag, hence it is more convenient to forget about dimples, keep the surface smooth and clean and reduce the total drag by reducing the friction drag.
Cheers

[cyclops2]

O K

Lets drill a hundred holes where we want to reduce water friction. We will release a friction reducing liquid very similar to that of the Salmon as it jumps water falls.

[tunnels]

.


Supercavitating Torpedo
A rocket torpedo that swims in an air bubble
By Eric AdamsPosted 06.01.2004 at 3:40 pm0 Comments
by John Macneill Several challenges remain for the supercavitating torpedo, including how it will be steered underwater. Water-tunnel tests have already proven that speed can be achieved: In 1997, the Navy tested a supercavitating projectile that reached 5,082 feet per second, becoming the first underwater projectile to exceed Mach 1.John Macneill

Submarines peaked in power and relevance during the Cold War; there has since been a shift in focus to aircraft-based combat, and subs have become budget-cut victims. But subs are still prized for their ability to sneak about global waters undetected and to defend surface ships from attack. Many U.S. subs are being converted from missile launchers into delivery vehicles for special operations troops.


But the supercavitating torpedo—a rocket-propelled weapon that speeds through the water enveloped in a nearly frictionless air bubble—may render obsolete the old submarine strategy of sly maneuvering and silent running to evade the enemy. The superfast torpedo could be outfitted with conventional explosive warheads, nuclear tips or nothing at all—a 5,000-pound, 230-mph missile could do enough damage on its own. The Russians invented the concept during the Cold War, and their version of this underwater killer—dubbed the Shkval (“Squall”)—has recently been made available on the international weapons market; the United States, of course, wants a new, improved version of the original.


The hard part about building a rocket-propelled torpedo isn’t so much the propulsion as clearing a path through the ocean. Water creates speed-sapping drag; the best way to overcome that drag is to create a bubble that envelops the torpedo—a supercavity. A gas ejected uniformly and with enough force through a cavitator in the nose of the torpedo will provide such a bubble, permitting speeds of more than 200 mph and a range of up to 5 miles (traditional torpedoes have slightly longer ranges, but lumber at only 30 to 40 mph).
Though submerged, the torpedo remains essentially dry, with a frictionless surface. “That sounds easy, but doing it is extremely difficult, especially if you’re trying to steer,” says Kam Ng, program manager for the torpedo at the Office of Naval Research, which has been developing the weapon since 1997. “If your torpedo moves in a straight line, you just aim and shoot,” says Ng. “That capability already exists with
Shkval. But the U.S. vehicle will be more capable—it will turn, identify objects, and home in on the target.” (Improvements to the torpedo to make it steerable likely froze when the Soviet Union collapsed, says GlobalSecurity.org’s Pike.)


Among the greatest challenges for U.S. torpedo researchers is developing detection and homing technology that will enable the torpedo to distinguish an enemy sub from, say, a rock formation, says Ng. Also tricky is finding a way to control the gas bubble to permit those course changes. “When you turn, the bubble distorts because it is no longer symmetrical,” he says. “So you have to compensate for that by putting more bubble to one side.” This is done, Ng explains, by ejecting more gas toward the outside of the turn.


Naval officials say the high-speed torpedo will enable submarines to attack enemy subs and surface ships without giving them time to respond. The U.S. military has tested a prototype, but combat-ready versions are not expected for at least 15 years.

[sdowney717]

create a 'skin' effect around the boat so the boat 'runs on air'

paint the hull with a superhydrophobic coating. A coating that traps a layer of air on its surface.
http://www.azom.com/materials-video-...aspx?VidID=281
http://www.youtube.com/watch?v=berp-odsKFo

perhaps would work keeping off barnacles
http://www.unm.edu/~solgel/publicati...0Truesdell.pdf