Boundary Layer Texture?

A long time Surf Board builder uses texture to break Boundary Layer surface tension.
Laxauskas Doctoral Paper implies maybe it's worth doing on ships.
Other than "go fast paints" are there reasonable, durable surface texture options for hulls ?
Surf board and sail boats are different kettles of fish, or just use fish oil?
Bill PKS.

 

Of course, this "Laxauskas" d00d is not to be confused with me. I don't recall mentioning anything about "Boundary Layer surface tension".

 

A boat ain't a golf ball. Textured surfaces on boat bottoms are a sure way to make the boat slower. This has been the subject of long term speculation and mostly wishful thinking. There is an Everest sized, mountain of emperical evidence to support the smooth is better argument. Legions of fiercely competitive regatta sailors have tried the coarse surface gambit and very soon got out some fine grit sandpaper to recover from the mistake. Go to any yacht club where Beer Races are about to begin. Observe the swimmers/scuba divers, who are busy cleaning pre race, boat bottoms so carefully. Interestingly, it is pretty clear that waxing the bottom is a mistake. The surface needs to be "wettable" for best result. Smoothe matt finish surfaces in the RMS 400 range ,or better, is the order of the day. Towing tank tests also support the smooth surface argument.

The surf board guy is all wet. (Pun intended.)

 

we aren't talking scuffing up the surface we are talking texture. We honestly wont know unless someone tries it. Its funny but this is one area that the techs often get wrong. hell mythbusters proved the golf ball texture works very well on a large scale.

 

Leo,
I found the reference in, "Resistance , Wave-Making and Wave-Decay of Thin Ships, with Emphasis on the Effects of Viscosity" pg 1-5 Lazauskas - U of Adelaide AU. April, 2009 .
" ..... recent attempts to characterize wave wakes have met with considerable disagreement within naval architectural community, .... adding to the uncertainty and confusion, there is a lack of data from far-field full-scale measurements that would allow validation of mathematical prediction techniques [63], and the little data that does exist is often very poorly documented."
Sounds like many opinions is just as good a guess as any other guesses.
Mostly though, it seemed to me that the paper implied that breaking skin friction ( along the stern sections) was of relatively high importance.
I don't know who the writer is.
I think I interpreted the comments correctly, although it was highly technical.
Anyway, I hear this about texture on surf boards and golf balls, where slick surfaces are said to be less slippery than some textured surfaces.
I have been like most other sailors, trying to get the bottom super slick, and have used ablating paints, with inconclusive results.
Now I'm just wondering ???
Bill PKS

 

Viscosity anyone ?

Mr. Messabout,
I know that a dirty bottom with barnacles and weeds is slower ( been there many many many times),, but is this the same thing?
I also saw someone was using a texture on a mast, saying it would smooth wind flow. ?? ( I 'spect that would be very hard to isolate.)
As water is the more viscous fluid, what is happening?
Bill PKS

 

Ok, then that was me. And, no, I don't think you interpreted it correctly.

I don't even know what is meant by "boundary layer surface tension".

Cheers,
Leo.

 

Stern Release.

Leo,
Well, Good to talk to the expert.
This started with trying to figure out how to end the stern of a sail boat, from observations that a long counter seemed to pull up a stern wave (producing drag), and a below the WL Cutoff produces suction and or a wave some distance astern ( implying use of power.)
( Rightly or wrongly, I reached some conclusions with Messabout's proddings.)
After the quote quoted , your paper seemed to imply that breaking the water away or releasing the water along the stern area of a ship was important. I interpreted ( evidently incorrectly) that to mean releasing surface tension, friction, or suction was important.
That lead to recollections and considerations of boundary layers, and the Surfboard and Golf ball examples.
Anyway, this seems to be an interesting consideration and I'll appreciate your comments ,,, ( Although stern conditions for a 1000' post panamax ship, would be far different that for a 44' sailboat. )
What does texture do?

Bill PKS

 

Increase of surface roughness ("texture", "dimples") is only useful in specific flow conditions. Imagine a sphere with a smooth surface, placed in a flow. At low speeds (Reynolds number below ~4e5, based on diameter), the flow will be able to keep attached to the profile about 70 degrees each side from the stagnation point; from there on, it is detached and creating a wide, turbulent wake. In this speed region, the coefficient of drag (Cd, based on the projected surface of the sphere) is ~0.5.

At the critical Re = 4e5, there is a sudden change in the energy distribution within the boundary layer, permitting the flow to stay attached past the sphere equator until about 110 degrees from the entrance point. In this mode, the wake is much smaller, and will consume less energy; the Cd drops to as low as ~0.08, slightly rising as speed is further increased. At Re = 6e6 it is up to about 0.19. Still much lower than for the subcritical speeds.

Now if you are interested in a reduced Cd in the subcritical range, an increase in surface roughness can reduce the critical speed (but not the critical Cd). For instance, if you create a sandpaper surface with a grain/sphere diameter ratio of 250e-5, the Cd dip comes in at Re = 2e5. The low Cd range is restricted to a smaller range, however. At Re 1e6 it is up to 0.34; ie far worse than with the smooth surface at that speed! The figures are fairly similar for a 2-d body, say a mast or a flagpole.

The golf ball goes from Cd=0.5 at Re = 4e4 to 0.25 at 6e4. If you introduce rotation to the body, the surface roughness will reinforce the flow circulation around the ball or rod. This creates a transverse force; the "Magnus force", resulting in a slicing ball, which is the main reason to have dimpled golf balls! The same goes for a football, the number of pads, and how they are sewn together is critical and strictly regulated.

So, to make it short: manipulating surface roughness can give a gain in certain circumstances, but only in a very restricted range, with major negative influences in other speed ranges.

 

It is important from the point of view of making certain calculations agree better with observations. It remains silent on whether it is actually better or worse hydrodynamically.

As far as roughness goes, heed Baeckmo's advice.

All the best,
Leo.

 

Surface tension and the boundary layer are really two different things. Surface tension applies where there is a boundary between the water and air, but except at the waterline, the rest of the hull is fully wetted and there is no surface tension acting on the flow.

I can imagine that for something like a surfboard, lots of air bubbles are brought under the board and if these stuck to the surface of the board they would act as roughness elements. In that case, it might be advantageous to do something about them. But I don't think bubbles are a significant factor in the flow around a typical sailboat.

Surface roughness does affect the boundary layer. Frank Bethwaite, in his book "High Performance Sailing" describes experiments where they towed two nearly identical dinghies from a cross bar to measure the difference in drag between them. Bethwaite investigated surfaces sanded with different grits to see what the effect of texture was. He concluded that the smoother the surface was, the less drag. There was no advantage in surface texture.

 

Baeckom, Tspeer,,
Very lucid comments.
So, is the stern wave under a long counter pulled up, or pushed up?
Does aerating the flow at a Cut off Stern reduce Stern wave making?
Or is this about hull displacement forward of the stern, and there is no "suction" or "connectiveness" between the water flow and a smooth bottom.
Thanks,
Bill PKS

 

Leo,
Interesting..
Obviously, the observations are of the Real Conditions.. and I understand that hydrostatic formula developed to explain the real events and to forecast what will happen if this of that changes by calculating the other.
Is there some unexpected Real Condition you have observed that occurs around the stern differently from what's calculated, or do you see it as more a matter of the complexity of the conditions?
I think my point is... if "break away" helps to better explain the actual observations, is there some other issue in the hydrological mix?
Thanks,
Bill PKS

 

Both, Bill.

There are many different models for what happens just behind the stern. Of course, they are all simplifications. IMO, three common misconceptions are:

1. That the hollow behind a fully dry transom stern can create waves. I contend that it doesn't, because, unlike a solid surface, it cannot sustain a pressure.

2. That the boundary layer trailing behind a stern tapers to a cusp and that this "appendage" is a wave-maker. It isn't like that. The wake of the hull boundary layer diffuses and spreads out over the free surface.

3. That we have adequate models of turbulence. Nobody does.

There are many others I could find, some probably even due to me.

All in all, we are magnificently ignorant!
Leo.

 

Keep in mind most long counter sterns are on sailboats with relatively heavy displacements for their length. I'm no expert on hydrodynamics, but I don't think a stern wave can be "pulled up" by a counter stern. Any evaluation of a sailboat design feature should consider the handicap rule it was designed to exploit. Counter sterns were thought to enhance speed for a given measured length under the International, Universal, CCA, and RORC handicap rules.

Also, I would never put a textured surface on any of my surfboards ...