The problem is that its not just the dimples making the golfball go further – it is the spinning and dimples together.
It works because as the ball rotates with backspin because of the slope of the club’s face it pulls air over the top in the backwards direction and underneath in the forwards direction.
This means the effective air velocity over the top is increased and that underneath is reduced.
Sound familiar?
Yes, it is the same way a wing gets lift. The faster airflow over the top reduces pressure because of Bernoulli and the opposite happens underneath – the ball is really flying something like a wing .. it is gliding.
The old story about a “wing being longer over the top so the air has to go faster is wrong. Otherwise sails, bees, dragonflies, paper planes and balsa gliders which have the same distance both sides of the wing wouldn’t be able to fly.
So if you plan to spin your boat and it is ONLY in Air or only in water without touching the other … you might just be on to something!
There have been numerous attempts to get some “super surface” to work by reducing vortices and keeping the smooth (laminar) flow at the beginning of the hull going for more of the hull length, but none of them have proved practical.
The best solution is a surface that is FAIR – smoothness at the building level and then has a smooth surface as well.
There is not a lot of difference between a nice paint job or a nice paint job plus sanding with about 400 grit wet and dry sandpaper and water using a sanding block – NOT hand held.
This is particularly true for the first part of the hull and the first part of the foils because the water gradually moves from smooth (called laminar) flow to turbulent flow.
You can see the difference if you leave a tap running just slightly faster than dripping so there is a stream. The first part is beautiful and smooth, almost mathematical, but a little further down towards the sink the stream of water suddenly becomes rough and turbulent. This is inevitable. There is less drag on the hull from the smooth flow so it is better to have as much as possible. So it is a good idea to get rid of roughness.
Practically this means that with foils in particular and maybe the front part of the hull it could be sanded. They say spend about 70% of your time getting the first 30% of any boat surface smooth.
Personally I would almost ALWAYS sand foils but usually just try to get a nice paint finish on the hull.
I did see that “Mythbusters” found otherwise with a full sized car. Mythbusters is entertainment and only moves into the area of science occasionally where it won’t spoil the story. Testing flow and drag with something as complicated as a car is very tricky. A dimpled car just possible might have some local effect that might smooth some local drag.
For example if the air going over a particular car normally suddenly changes from smooth flow to turbulent going over the top of the windscreen causing a lot of drag it is possible that just through luck the placement of dimples might reduce that drag enough if the dimples are the right size and just the right position. That’s why such testing is normally done with much simpler shapes.
If dimples did reduce drag in this way, then Boeing and every cargo carrying ship would have been using it for years. But it don’t, so they don’t.
At least until I see a properly referenced and scrutinised scientific paper that says I am not correct (along with Boeing).
If you want to get into wing theory a bit more and want to know why it really works, you need to read Arvel Gentry, who I read when I was about 14 and digested over a couple of years.
He caused so many arguments at the time because people were reluctant to give up the old ideas of “flows faster because of longer distance” and the “air accelerates in the slot between the mainsail and jib” which had been held for a considerable time by aerodynamists and others that should have known better.
Start with Arvel’s “the origins of lift” and work your way through it on Arvelgentry.com
Hope this helps.
Thanks for that. But isn’t there something about micro-scales on sharkskin reducing drag? I seem to remember something about some Olympic swimmers being disqualified for wearing suits with that effect.
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Howdy Osbert,
You are correct, there was report of it. I never saw any data for how much the drag reduction was, it might have been tiny, but enough to upset people who are worried about 1/100 sec in a two minute race.
Sensibly the swimming officials decided it was all getting a bit crazy and stamped on it.
Michael
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The description of the dimple effect on golf balls is not entirely accurate. The spin of a golf ball is created mostly by the striking force, i.e. where the ball is struck in it’s travel vector…i.e. hitting it close to horizontal, but low. The angle of the club face has a lot to do with _where_ the ball gets hit and how much spin is transfered because of creater surface contact on the striking surface. Consider a basketball. When you bounce it nearly vertical, no spin. Add some angle to the bounce, and the greater the angle the more spin tranfered. The ball rolls a little across the surface when bounced and so some energy is transfered into the spin. Same with the golf ball. The higher angle the club, the longer the ball is in contact with the club and the more spin can be transfered. In the case of drivers, the spin is created when the top edge of the sweet spot is hit, biting a bit into the skin of the golf ball, therefore more effeciently transfering spin energy to the bottom of the ball while still giving it full velocity force to the face of the ball. The amount of air needed to create the amount of spin required to affect steerage of a dense object like a golf ball is magnitiudes higher than the airflow created by a golf club.
The dimples do not have anything to do with lift like in an airfoil. All they do is create local turbulence which reduces friction, which also in turn helps keep the balls direction of travel consistent. That’s all and nothing more.
Putting spin on _ANY_ ball, dimpled or near perfect smooth is going to provide the same amount of steering (lift in the case of backspin). Take a beach ball for example. Because it is so light, it really is effected by aerodynamic effects and exagerates quite nicely things like _lift_. Even a touch of back spin can cause a beach ball to appear to defy gravity. It’s really neat to play with.
Lift in a foil is produced by the air travelling faster across the top of the camber compared to beneath. This creates a suction on the top of the foil. A ball on the other hand behaves oppositely in this regard. Backspin is causing more friction beneath, with less air travelling across the top surface which in effect just pushes the ball upward into the path of least resistance. More resistance where airflow is higher, less resistance where there is no airflow. That’s all it is, no foil effect involved.
A fish scale or sharkskin surface on the bottom of a boat could indeed decrease local friction through biased micro turbulence in exactly the same way is does for the golf ball and also the mythbusters car. Sorry but in fact they duplicated the experiement enough times that although not strictly scientific method with 30 experiments and peer review, is still pretty much good enough. It’s still emperical evidence.
However…is it worth the cost? You’d have to experiement a LOT to find the right surface structure to use. I can’t believe it’s worth it, not in anything except the most expensive classes of racing.
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Howdy Scot,
Thanks for the detailed reply. Nice description of the mechanics of the club blow.
I do have to disagree with you about the effect of circulation. It will be in the sense (direction) that I put it in the original article. There are too many practical offshoots that work in the same way. Such as Flettner rotors. You are aware of circulation in the boundary area as the main means for producing lift of wings or anything else with a rotational flow centred on the body itself? This is why wings work and why topspin makes a tennis or table tennis ball dive for the court/table and backspin prevents it from dropping so quickly.
You can’t say that this effect of the circulation is not there in this case – it is. But I don’t know about the drag reduction. From millions/billions of hours of experience with boats and aircraft it seems that a smooth surface is the one that works the best. This statement neglects microgrooves and sharkskins at this point – which are very small scale and close to smooth.
I still think that Mythbusters using cars was a mistake. There really are far too many potential effects on such a complex body. There are far too many potential effects. As the lab “sharkskins” and “low drag film” tests show the reduction in drag is there but the special surfaces need to be very carefully designed and precisely placed. For the Mythbusters team to get the size right first time would be an incredible fluke.
I agree that surfaces can reduce drag – but I was more pointing out how difficult they are to use in a practical way – which is your point too. Even when the money and will is there such as in the America’s cup. It was tried, they thought it had an effect, but it got damaged and dirty too easily and would have been difficult to clean without damaging the microgrooves.
I really think it is much more likely that it had relatively little effect over the car surfaces themselves but altered the airflow by deflection at some critical point with a much more significant decrease in drag. I agree that Mythbusters found a real effect, but they are a long way from proving that it was the mechanism that they claim.
Thanks again for your reply
Michael
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Post Script … I misread Scot’s posting. I was on a long distance bus at the time.
Good point about the dimples not doing anything for lift and that lift depends on the amount of spin.
I think he knows more than I do (quite possibly) and I didn’t give him enough credit at the time of reading.
I’ll leave everything above unchanged so everyone can see what was meant. There is good information in both posts.
Michael
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I always found that mythbusters episode to be flawed, they only ran the car one way, in record setting where wind can change the results you have to run the course both ways, check out the rules for the bonneville salt flats racers, two passes both ways with a strict limit on the time between passes or it doesn’t count, the other example being for the worlds fastest human powered boat, MIT in MA, USA set the record but they had to do a run both ways and then average the times to get the speed. Mythebusters did scale the dimples in the golf ball to the scale of the car, that may be a wild shot that they got right, but they didn’t do passes in two directions.
Wings fly because of lift, the curved surface of the wing does make a lower pressure on the top of the wing and the pressure under the wing pushes it up, this works for slow moving foils like those on a glider, however most lift is done by slamming the big fat side of the wing into the air stream really building the pressure underneath it an forcing it up, this is easily seen when watching any jet take off. the ground run to build up speed then when there is enough control to force the nose up into the air, they haul back and the wing is rammed into the air at a steep angle forcing it to climb, just like your hand will if you hold it out the window of the car on the highway.
NASA did try laser etched wing panels on aircraft wings, copied from shark skin, they worked great at keeping the airflow attached to the top of the wing, which is important to keep the wing level and controlled. it also allowed a slightly different shape to the wing which made it more efficient for higher speeds, however it also allowed ice to grab on so that it couldn’t be removed, which killed the whole idea.
check out the Honda jet, they went for super smooth skin to give them more speed, they went to radical extremes for smoothness including a one sheet wing skin that goes from the trailing edge to the leading edge and around to the leading edge again for the whole wing span, there is only one mill in the world that can make a skin that big. From what they have published the odd shape to the aircraft nose, engine pylons and extreme wing smoothness have given them a more efficient aircraft than any boundary layer exciting can do.
my own personal idea has always been some sort of air pump blowing a sheen of bubbles out just aft of the bow, as air is much less denser than water, I am just not sure if at rowing speeds it would help or not. maybe a bicycle pump hooked up to a sliding seat or rigged up to the oars? over a long distance it might help….
Josh
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Howdy Josh,
Nice rundown of info.
I wasn’t aware of the Honda Jet construction method. I used the same idea for smoothness using plywood for a box keel on this boat.
The fat part of the foil “ramming through” is not really a good way to look at it as more bulk would divert air around the less bulbous side of the foil, however angle of attack and viscous effects (most old style modelling ignores viscosity) result in an a vortex eddy being dropped off the back of the foil, setting up the opposite circulation around the foil. This adds to the flow velocity on the upper side and detracts from it on the lower side.
“more bulk” doesn’t explain why golf balls, flettner rotors or flat wings like a balsa glider work as they all are the same shape on both sides.
See the text aligned with the second illustration on Arvel Gentry’s page on sail aerodynamics. He’s the one who wrote articles showing the old way that we used to think about sails was completely wrong … all the unsupported “slot effect” thinking.
I do know that at very high speeds bubbles can work – there were hulls developed that bled bubbles through their surface. Also there is a rocket driven torpedo technology where the nose of the rocket exuded high temperature vapour to create an “airspace” for the torpedo to fly in underwater. I think it has been tested, but my memory is foggy. At lower (sailboats etc) speeds I would suspect the bubbles would disrupt the flow of water around the boat too much … a bit like having a rough surface … maybe?
Really liked your other points a lot! Hope Maintence crews at NASA never takes a scotchbrite or buffer to their laser etched wings!
Best wishes
Michael
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