# Golf-ball style "dimples"



## NightKnight (Dec 16, 2009)

I was just thinking about this the other day! I would think it would be doable, but not that easy to get the desired effect.

Technically speaking, you are trying to achieve near Laminar Flow around the object you are firing by reducing your Reynolds number to near 0.

Laminar flow can be attained with large dimples, or through smaller imperfections in the material that reduce the surface tension on the Boundary layer. Believe it or not, a slightly irregular (satin or matte) surface can create less drag than a polished surface because it is more porous. More porous = more boundary layer turbulence. More boundary layer turbulence (on a sphere) = less drag.

Here is a quote from the wikipedia article on Turbulence:


> Flow over a golf ball. (This can be best understood by considering the golf ball to be stationary, with air flowing over it.) If the golf ball were smooth, the boundary layer flow over the front of the sphere would be laminar at typical conditions. However, the boundary layer would separate early, as the pressure gradient switched from favorable (pressure decreasing in the flow direction) to unfavorable (pressure increasing in the flow direction), creating a large region of low pressure behind the ball that creates high form drag. To prevent this from happening, the surface is dimpled to perturb the boundary layer and promote transition to turbulence. This results in higher skin friction, but moves the point of boundary layer separation further along, resulting in lower form drag and lower overall drag.


BTW, Mythbusters did not discuss the other reason that a golf ball has dimples. Since a golf ball spins as it flies, a completely laminar boundary layer could cause it to curve. The dimples create just enough turbulence to make the flight more linear. To test this theory, throw a smooth hollow ball and then a similar size and weight dimpled ball and see what happens.

This simulator from NASA shows the "spinning ball" effect: http://www.grc.nasa.gov/WWW/K-12/airplane/lift2.html (select "ball")

At a high Reynolds number the boundary layer is already turbulent; dimples would be superfluous. At trans-sonic (250+ mph) or faster flight the dimples could cause some shock waves due to localized supersonic regions.

I hope that was helpful, and not too confusing!


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## Flatband (Dec 18, 2009)

I had often wondered about that very possibilty. Making steel balls with dimples to add speed to the projectile.Not very cost effective for the Steel Ball manufacturers, but it sure might be fun for us! A 20 to 40 percent speed increase if I'm reading my equations right? COOL!!!!!! Flatband


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## NightKnight (Dec 16, 2009)

I haven't done the calculations, but it could very well increase the *sustained* speed by 20%. The *initial* speed would not change by much, since it is controlled by the elastics. It would also increase the accuracy as stated above. By how much I don't know. I guess it would depend on the horizontal and vertical rotation of the ball after it exited the pouch. Someone like Joerg with a high-speed camera would need to paint some lines and then film a shot to find out how much rotation there really is.


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## NightKnight (Dec 16, 2009)

To find out just how much the airflow effects a sphere check out this applet:
http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html


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## NightKnight (Dec 16, 2009)

AaronC said:


> To find out just how much the airflow effects a sphere check out this applet:
> http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html


Ok, so I did some calculations. Here were my assumptions:
1. The ball rotated 1.66 times during its flight
2. The flight time was .5 second
3. The ball is traveling at 146 feet per second
4. The ball is .6" in diameter
5. The ball is 130 grain (0.018 lb)

With these assumptions, the weight of aerodynamic deflection would equal the weight of the projectile. If the speed, diameter, or rotations increased then the deflection pressure would actually be greater than the mass of the projectile. Crazy!


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## p4v_singh (Dec 21, 2009)

i dont really understand this but basicly if they put dimples in steel ablls it would make the ball spin more via air resitance making it go faster?


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## NightKnight (Dec 16, 2009)

p4v_singh said:


> i dont really understand this but basicly if they put dimples in steel ablls it would make the ball spin more via air resitance making it go faster?


No, the dimples reduce drag on the ball(by inducing boundary layer turbulence) thus maintaining the speed for longer distances. Also, since the ball is spinning, the lift and/or deflection is reduced by the turbulence caused by the dimples.


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## p4v_singh (Dec 21, 2009)

still dont understand but that cuz im jsut a kid thanks anyway tho


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## Flatband (Dec 18, 2009)

That's all right MR. Singh because myself and Aaron are trying to figure this stuff out also!







The distance overall would definetely increase at least 15-20%. The deflection pressure greater then the mass of the projectile? There wa s a reason I didn't take Physics in school!







This stuff is maddening! Flatband


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## NightKnight (Dec 16, 2009)

Flatband said:


> This stuff is maddening! Flatband


I know, but I love it!


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## p4v_singh (Dec 21, 2009)

AaronC said:


> This stuff is maddening! Flatband


I know, but I love it!








[/quote]
lol


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## Deimos (Dec 24, 2009)

I think the real question is: Is dimpling relevant to the relative short slingshot shooting distance?
In golf, you got seconds of airtime, and very large distances.
In Target shooting, impact is amost instant.

Tbh, you're better off working on your technique then spending hours dimpling ammo.


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## NightKnight (Dec 16, 2009)

Deimos said:


> I think the real question is: Is dimpling relevant to the relative short slingshot shooting distance?
> In golf, you got seconds of airtime, and very large distances.
> In Target shooting, impact is amost instant.
> 
> Tbh, you're better off working on your technique then spending hours dimpling ammo.


I think the effect is definitely reduced due to the shorter flight times, but also keep in mind that the relative pressures are greater since our slingshot ammo flys much faster than a golf ball does.

I agree that practice is more important than dimpling, unless you are already a great shot.







I am not that great, so I am hoping that a company will make what we are looking for. Something like this:
http://machinedesign.com/article/dimpled-bearings-run-smoother-longer-1108


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## ZDP-189 (Mar 14, 2010)

*Effect of Skin Friction on Accuracy:* I encountered this problem when shooting at long range. The balls would spin off to the side. Tex-Shooter revealed that it was down to my release (and possible differing strength of the bands). TBH, this effect is cumulative as the distance increases and the effect is small at short ranges.

*Significance of Skin Friction on Range:* I've been doing a lot of experimentation and analysis. I now know that form drag is much more significant than skin friction for a round ball. Unless you can throw a rod end on, then the way to reduce drag is to use a denser material, so that form drag is reduced as a factor of momentum. Even if you could cut skin friction to zero, a slight change in density would have a greater effect. Golf balls are tightly regulated. Dimple depth is regulated, but size and weight (together defining density) is more critical.

*Gaining lift through spin: *It's true that if you spin a ball it can rise due to lift. It happens in golf where backspin gives a higher trajectory, it happens in Airsoft, where some guns endeavour to extend range with backspin. Among Airsoft shooters, this effect is called Hop-Up, about which there have been many fruitless debates. I know of no slingshot that can consistently deliver rotation, so rotation effects are a source of inaccuracy, rather than a desirable attribute.


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## Pro-Shot (UK) Catapults (Apr 21, 2010)

Hi AAron
Only last week we received our master golf ball style slugs 2 sizes 11mm o/d & 15mm o/d
they are in brass at the moment . we are going to produce a aluminium mould with 9 impressions per size for self cast
our master pattern shop is producing the moulding patterns over the next 2 weeks
the open and closeing mechanism is designed and just needs the casting moulds to be complete.
had a look at the link very impressive.will keep al informed
all the best hogancastings uk


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## ZDP-189 (Mar 14, 2010)

Please PM me a quote for a set shipped to Hong Kong.


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## Pro-Shot (UK) Catapults (Apr 21, 2010)

ZDP-189 said:


> Please PM me a quote for a set shipped to Hong Kong.


Hi ZDP 189 will do once we have the weight of the mould total hopefully in the next two weeks
I know you will appreciate that these things take time and we all want it to be right?
will keep you posted
Hogancastings


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## Sam (Jul 5, 2010)

http://www.dankung.com/emart/300-kg-various-frosting-hunting-slingshot-ammosteel-ball-p-246.html

Came across that a while ago when I was looking for cheaper ammunition...

"*The steel ball has tiny dimples on the surface which helps the steel ball flys longer distance."*


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## ZDP-189 (Mar 14, 2010)

I have written a primer on external ballistics (link) and I conclude that drag is typically not material, particularly not with large lead balls such as the kind that one would cast dimples into.


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## Delaney (Nov 22, 2010)

This is some kind of weird psychic thing because i was just discussing this yesterday with my dad when we were cleaning out the basement and came across one of those dimpled batting cage balls. I agree that the effects would probably be minimal for the purposes of slingshot ammo but if nobody tried then we would never know. Plus i like to read these analytical breakdowns, we have some sharp guys on this forum!
Brian


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## Devoman (Oct 15, 2010)

What???? My brain hurts!
I am just going to shoot em! Let them fly as they want lol.


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## dgui (Jan 12, 2010)

Vinegretting your steel balls gives them a Matte like/type finish.


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## Darb (Sep 14, 2010)

I'm 100% in agreement with ZDP-189 and Deimos on the physics of this ... the effect does exist, but it's relatively insignificant in the realm of slingshots because of the far shorter distances involved (5-10x) vs driving golf ball, and the significantly greater inertia/mass-to-surface area ratio of metal ballbearings vs golf balls. Smaller size, denser material, and shorter flight time, all act together to reduce the significance of the effect. The effect should not be significant enough to be worth paying extra for specialty ammo that tries to capitalize on it, and would only become significant enough to warrant attention and adjustment if you were shooting less dense ammo over significantly longer ranges.

Although the math is complex, the conclusion is unambiguous.

It would be much more productive to focus instead on one's pouch grip position, cupping point, and release technique, than the extremely minor effects of ammo dimpling.


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