Ya'know, like a golf ball

I have been wondering this for a while now. Has anybody tried to dimple a hull(offshore high perf) like a golf ball to reduce drag? 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

 

Back in 12 meter days, I believe 3M had a surface treatment that did exactly what you are describing. It worked to some extent so was ruled illegal.

 

interesting thought but is it a phenomenon only occuring at relatively high speeds through air, a compressable fluid?

"Shark skin" suits from swimmer have been developed that reduce drag, despite that fact that a shark's skin and these replications are quite rough to the touch... I know bottom paint for some of the small vessels I work with is quite rough. I believe it is due to a phenomenon we know of on very large ships where a region of water is imparted the same forward motion of the vessel, forming a sort of water lubrication so that the actual friction of the vessel is between two layers of water. By having rough surface, you could "grab" the water around you and drag it along to form a water layer like what the large ships experience, which is I believe what sharks and these suit developers have successfully attempted to accomplish, but its been a few years since I was at the demo for that tech so my memory may be off.

 

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.

The ribletts used on the Stars and Stripes 12 metre are different altogether. They are very fine streamwise groves that damp structures like hairpin vortices in the turbulent boundary layer and thus reduce turbulent skin friction. Just any old fine grooves won't work, either. They have to have the right cross section (V grooves work best), size (h+ = s+ = 15), and be aligned within 15 degrees of the local freestream direction.

 

Can we get that in English?


All I know - fact - roughening the bottom surface of your boat with 800 grit sandpaper adds 1 mile vs a wax polished shiny hull bottom. Fact. Conniston 2005, time between runs 1 hour ( the length of time it took to get the wax shine off )

 

Golfball-style dimples won't help reduce the drag of a hull.

Nor will any other texture you're likely to be able to apply. Smooth is best. Smoother is better.

 

Wow, those groves with the 800 grit must be pretty deep to add a mile to the waterline length ...

There is a thread on fastest surface smoothness here: How smooth is smooth enough?

No one has suggested dimples.

Polishing foils and the forward section of the hull seems to be a good thing.

 

I actually remenber reading of a boat,mabe a kiwi america cup boat that used the dimples on the hull type surface.

 

Sheeeesh - a mile an hour extra in speed!!



I ran some practice runs with my boat at a fresh water lake very near to Conniston UK, in preperation to get the V24 National Speed record. My runs were very consistent, within .5 mph over three runs. I went home, and spent the whole weekend polishing the hull till it shone so much you could use it as a mirror - this included hanging it from the lift strops to get complete access. We then layed bed sheets onto the trailer so no dirt could affect the hull. When we launched and attempted the record we were ( over two runs ) just under a mile slower.

In talking to some other boaters, they suggested that the polish was the problem, and I should roughen the hull with 800 grit. This I did, as I knew something was wrong, and that was all I had changed since testing. I then did my third run and gained a full mile.

Thus I add to my my theory of life of a fast hull - a roughend hull with 800 grit!

(And now having the UK and World record!! for V24 offshore I think I am doing some things right to get the last little bit of speed.)

 

There's another, quite detailed discussion about golf ball surfaces here

http://www.boatdesign.net/forums/showthread.php?t=9566&highlight=golf ball

 

some basics given, for offshore high performance ( planing ) boat hulls -a stepped hull for example- i dont think these rules fully apply

 

Basically, the golgball dimples are to cure problems that a boat hull doesn't suffer from.

Essentially, it's an apples & oranges type comparison.
Different principles.

There's a reason airplanes are shiny and polished instead of rough and dimpled.
The flow over airplanes tends to all be in one direction (laminar). A golfball has flow
going all sorts of ways (partly because it is spinning) and is turbulent flow.

Laminar flow is really a bit more complex than that, but basically it is uniform
and creates a thin boundary layer. Turbulent flow is thicker and more drag.
And worst is separated flow, which is what happens to a golf ball w/o the dimples.

Here's more:
http://www.golfjoy.com/golf_physics/dynamics.asp
http://en.wikipedia.org/wiki/Golf_ball

 

horses for courses

as for fast boats thats whats its about, run/fly as separated from the water as you can,

 

Also the dimples are there to thicken the boundary layer, so the circulation from the spinning ball (backspin if all is well) will produce lift and the ball will stay up longer.

Dimples might work on a sailboat if you throw it in the air and spin it with a high value of rotational velocity. Backspin also

If the dimples are the right size the sailboat will return to the ground after a greater distance is covered.

To optimise the effect the sailboat could be made so it resembles the geometry of a golf ball more accurately.

Cheers big ears

MIK

 

From the simple perspective of drag reduction, a form which promotes laminar flow within the boundary layer is the best choice. The Carmichael hull, developed and tested by Dr. Bruce Carmichael of Rockwell International in the 1970’s is a good example of such a hull form. In laminar flow, fluid particles move in laminae or layers. Skin friction drag is much lower than in a turbulent boundary layer where the fluid particles move more erratically resulting in higher shear stresses between layers and at the surface http://www.ise.bc.ca/WADEhullform.html

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

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The type of boundary is a little harder to comprehend.
In general there are two types of boundary layers: the laminar and the turbulent.
The laminar boundary layer is a nice smooth boundary layer.
Very close to the hull its speed is 0 and the farther away from the hull the higher the speed becomes, until it reaches the ship speed.
This laminar flow has a low frictional loss, but is not easy to maintain when it gets thicker.
Even the smallest surface irregularities might disturb is, resulting in it becoming Turbulent.
A Turbulent boundary layers has a higher friction, and is in general not smooth layered, but is more turbulent, as its name already makes clear.
A turbulent layer has compared to a laminar boundary layer a smaller part in which the speed is very low. http://www.sailtheory.com/resistance.html

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And finaly http://www.boatdesign.net/forums/archive/index.php/t-689.html