Ya'know, like a golf ball

[yipster]

thanks for the info, did read the links and punched "speedboat drag" in google but must be using the wrong terms
apart from waxing, grinding, aplying 303, smooth is best i understand but it keeps me puzzled
is not the turbulent airation benificial for higher speeds or should i be glad having an older unstepped hull for 50 knts
so for going fast best have a single smooth surface over laminair water does got me confused

[westlawn5554X]

I had the technologies ages ago... it is hulls growing barnicles..... drag... so you gonna use it in race now or clean it to smooth it? However, smooth surface with dimple is another... Its all about pressure.

[yipster]

stone skimming formula adds new spin

World record form
To match the world record of 38 bounces using a 10-centimetre-wide stone, Bocquet predicts it would have to be travelling at about 40 kilometres per hour and spinning at 14 revolutions a second.

He adds that drilling lots of small pits in the stone would probably help, by reducing water drag in the same way that dimples on a golf ball reduce air drag. "Although I suppose that would be cheating," says Bocquet, whose report will appear in the American Journal of Physics.

yeah, and you might say its all about pressure and cavitation is bad, but i'm still searching, planked viking ships benifited from cavitation discovery channel said, than there is the surface prop etc. the running pad(s) should be smooth i guess but other areas like nasa describes can gain up to a surprising 15% speed from riblets.

[Richard Hillsid]

No problem yipster.

I was in a bit of a hurry so just wanted to point out some controversy on the subject without having to actually think what I write so the links.

Freshly after finishing my Westlawn studies I got really exited about boundary layer friction in -88 after reading about it being used on boats, and actually filed a patent in -89, non boat related, using a thin grooved film as a mean of reducing drag by messing with the boundary layer flow.

[Mark 42]

Separated flow is bad... it's higher drag than turbulent flow.

Stall is one example of separated flow (also called boundary layer detachment & detached flow)

http://en.wikipedia.org/wiki/Flow_separation

Also, about blowing bubbles... that idea is not a bounday layer enhancement,
it is rather an attempt to move through a less viscous fluid (air is a less
viscous fluid than water)

Laminar flow enhancements rarely consist of blowing on the surface:

http://en.wikipedia.org/wiki/Boundary_layer_suction

[fewfish]

This might help a little.

The graph shows a Cd drop after critic value of Reynolds number of about 10e+5. This is why the golf balls have dimples... to force the air to flow past the ball faster increasing Reynolds number and decreasing Cd.

Similar effect could be achieved with boats but before any certain claims about required Reynolds number it is necessary to make an experiment to determine corresponding critical Reynolds number value. (transition from laminar to turbulent flow is gradual and irregular)

The graph (full line for sphere) can be used to evaluate Reynolds number of the hull traveling at certain speed just to see if it is near the interesting Cd drop.

There is also question of frictional drag coefficient. If the hull had dimples, water would have to travel additional distance and that means more frictional drag.

That is why smooth is better for certain types of objects in fluid stream and rough for other.

[Ari]

Originally Posted by resurrected

The theory is just starting to show up in the undertrays of racecars. Has it been done? Would it reduce the drag so much there would be no control?
Just been buggin me.
Any comments would be appreciated.
Thanks Jason

What you called the 'undertray' of a racecar (formula 1?) is for the purpose of creating vacuum. Not to reduce drag.It is for the purpose of holding the car on the track like a magnet holding a steel, with the tyre acting as the spacer in between, air are compressed (when the car move at high speed) to rush through the thin opening between the tray and the track(less than 30 milimetre), this high velocity flow created low pressure inbetween the air flow and the tray, pulling the tray thus the car down closer to the track.Refer to Bernoulli principal for a venturi.Without this tray the car will float away from the track at high speed.It is the rear spoiler that work to reduce the air drag by breaking the vacuum created when the car rush forward and split the air.

[westlawn5554X]

If everything is streamline... it would be long and sharp... like a needle. You need clean smooth surface with the right shape for speed and some drag to make it possible to steer and more control...

I know Russian Mig have bolt on the wings as to help them control at high speed.

Soo... which is more important? in race? Speed or control?

[Pericles]

The Exclusive ClackaCraft GulfStream® bottom

Why We Love Dimples. . .

http://www.clacka.com/gulfStream.htm

Pericles

[RHough]

Originally Posted by Pericles

The Exclusive ClackaCraft GulfStream® bottom

Why We Love Dimples. . .

http://www.clacka.com/gulfStream.htm

Pericles

Have these folk met P.T.Barnum?

[Mark 42]

Originally Posted by RHough

Have these folk met P.T.Barnum?

Here's what we found: the laminar flow past a smooth surface produces more
pressure and resistance than the turbulent flow past a textured or dimpled surface.

All you have to do is say "Physics" and you can sell any gimmick
no matter how ridiculous.

[innomare]

Originally Posted by tspeer

You don't find many healthy people going into chemotherapy in the hopes that it will make them more healthy, do you? The dimples on a golf ball are a cure for a very specific disease - laminar separation on a bluff body. The dimples add drag in themselves, but delay the separation so that the wake is much smaller, thus producing a net reduction in drag.

Most boat hulls are neither bluff bodies nor do they suffer from laminar separation in their boundary layers. Adding dimples will only increase the drag of a boat because there's no offsetting reduction in the width of the wake.

Agreed, but how about the propeller nut caps?
How about the propeller shafts when they have 8-10 degree inclination from the horizontal? (even more when planing with a trim of 2-4 degrees)

Would it make sense to have a "dimpled" propeller shaft?

Bruno

[Frosty]

[QUOTE
Would it make sense to have a "dimpled" propeller shaft?Bruno[/quote]

I always thought that it was best to have the shaft covered, ie inside a shaft tube. It takes HP to turn the shaft even without a prop.

I also always thought that 'egg shell' finish had least resistance to water drag.