Is Matte Faster?

I agree that riblets reduce drag though a different mechanism than golf ball dimples. My understanding is riblets modify the structure of turbulent boundary layers and can reduce the local skin friction. I found a recent paper on Drag reduction by riblets but have not completely read it. It appears that the mechanism(s) by which riblets reduce drag are the subject of current research.

 

At the other end of the scale, rowing and kayak hulls are also not allowed to
have any hollows in their waterlines. I presume that's to stop people from
trying bulbs, weird shapes like Ward's Optimum Symmetric Ship, or tubercules.

 

You are an evil genius!

 

Ok, you want photos, here you go. Happy?

Riblets are actually well defined in the literature. They are fine parallel grooves, typically triangular in cross section, aligned within 15 deg of the streamwise direction. For USA 17, the grooves were on the order of 50 microns wide, which is why you don't see them in the photos. You can feel them and there's a squeak as you run your fingernail across them.

The attached figure* shows the reduction in turbulent skin friction as a function of the spacing of grooves. s+ is a nondimensional distance that is scaled by the skin friction to account for the differences in speed and downstream location. The faster your boat, the finer the grooves need to be. There is a range of groove sizes that results in about a 5% decrease in skin friction. As the grooves are made smaller, the surface effectively becomes smooth. As the grooves are made larger, the skin friction increases, and large grooves have more skin friction than a smooth surface because you're basically increasing the wetted area. So not just any surface texture will result in reducing the drag.

* From MICHAEL J. WALSH & JOHN B. ANDERS, Jr., "Riblet/LEBU research at NASA Langley," Applied Scientific Research 46: 255-262, 1989. Kluwer Academic Publishers.

 

Hello Tom,

This is very interesting stuff. Can you explain a bit about how riblet size would work on a typical "fast" sailboat, like a 30 foot sportboat or production trimaran?

With a boat like this you could be sailing at <5 knots in light air, 7 knots upwind, and more than 20 knots at times downwind.

Can one size of riblet produce less drag across the entire range? Would having a riblet of a certain size decrease drag in one condition while increasing drag in all others? Would you have to tailor your riblets for the expected conditions?

Thanks.

 

This is all hypothetical, since RRS 53 says, "A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer." But for the 33rd AC, the Defender threw out RRS 53, and riblets were allowed.

s+ is a kind of Reynolds number. The definition of s+ is
s+ = s*U/nu * sqrt(Cfx/2)
s = riblet spacing
U = local flow speed outside boundary layer
nu = kinematic viscosity
Cfx = local skin friction coefficient as a function of x (streamwise distance)

Cfx is approximately
Cfx = 0.370*(log(Rex))^(-2.584)
Rex = local Reynolds number, Rex=x*U_inf/nu

At 5 kt and 8 m (26 ft) from the bow, the optimum riblet size (s+ = 15) would be 195 microns. At 10 kt, the optimum size would be 103 microns, and at 20 kt 54 microns. Closer to the bow, at 1m, the optimum size would be 5 kt: 156 microns, 10 kt: 83 microns, 20 kt: 44 microns. So it's not going to be possible to have one size that covers the whole range.

You don't want to be too big, so it's good to look at the maximum size for break-even on the drag, which is around s+ = 25. For 20 kt at 1m from the bow, that's 73 microns and at 8m it's 86 microns.

Since you don't increase the drag by being on the small side, you might pick 70 - 80 microns as a single size to use for the entire hull. At slow speed near the stern, the surface will be essentially smooth. But you ought to get some drag reduction over much of the hull under most conditions.

 

What ??

All it looks like is a excuse to cover up a bad paint job and give it a technical name then collect the money and run !!
sorry i will stick to smooth and shiny and nice looking .

 

Thank you. This is what I expected.

 

If you are really interested in the effect of riblets and other longitudinal
grooves, there are several papers in the excellent journal "Physics of Fluids".
There was a flurry of activity around 2009-2010.

The state-of-the-art (and mention of contradictory findings) of reduced drag
due to air bubbles, surface coatings and other techniques were mentioned in
the Resistance Committee's Final Report to the (latest) 26th ITTC, 2011.

 

Another recent publication:
"Turbulent boundary layer measurements over flat surfaces coated
by nanostructured marine antifoulings",
Experiments in Fluids, 2012, pp. 1431-1448.

 

Hi Everybody,

After reading a topic regarding friction coefficient, mentionning ITTC 57 , Grigson....., I am still confused to decide which friction coefficient would be relevant for a standard beach cat hull, but also for a standart centerboard or rudder blade (with around 150 mm chord lenght)

Do you think the Cfx mentionned here by Mr Speer could be the answer?

Thanks in advance

Regards

EK

 

I doubt that cfx will be the best. Hopefully they have improved since Date
and Turnock used it in their paper:
"A study into the techniques needed to accurately predict skin friction using
RANS solvers with validation against Froude's historical flat plate
experimental data",
Ship Science Report No. 114,
March 1999.

In a couple of days I will put up a short report in this group showing
comparisons with the latest skin-friction lines and CFD computations.
I started out sneakily by asking what "engineering accuracy" meant to
various people, but nobody offered their opinions of resistance or
skin-friction.

I can definitely say that the Hughes line is ready to be buried and should never be mentioned again.

 

Thank You very much Léo,

I am looking forward to read your post.
I saw the topic about "accuracy", but I am not qualified to participate, Do not want to polute with dummy questions sorry.



Cheers

EK

 

Leo,

perhaps you should have been more specific about that topic. years ago I have worked in aerodynamics and for both a military contractor and a commercial aircraft company. For the defense contractor they had to make certain minimum guarantees, but could also market their design with both top speed and maximum range and payload capabilities. Determining drag accurately was reasonably important to meet these minimum design guarantees. Typically if they were not met the customer, air-force, navy, marines, etc. would still buy the aircraft since much money had already been committed to the full scale development, and it was still better than what your competitors could accomplish, and far better than what they are replacing.

In the commercial aircraft market, you had to have a certain minim number of orders before they would commit the billions in their own money for a full scale development of a new model. And the accuracy of the range and fuel consumption is very critical since if you sold $3 billion worth of aircraft before you even flew one of them, it could bankrupt the company with the contractual penalties if your estimates are not met. The airlines commit to a new model based on how well their aircraft can generate revenues for their business. Accuracy becomes critical to your survival as a company for both the customer and the manufacturer.

While accuracy is important in a military aircraft, it is not the primary driver. In fact to get the new fighter or bomber sold, it is up to the engineering staff (of which I was one) to be as reasonably optimistic as you can in your estimates. You always over promise based on how well you can justify your estimates. these estimates are reviewed by military and civilian employees of the government to verify they are reasonably justified, but when a disagreement arises it is a matter of marketing your engineering capabilities and convincing them that your "new" (and proprietary) state of the art methods are valid (even though no one else in the business can duplicate your results). Once the military contract is signed, way too much money is committed by the time they discover several years (or many years sometimes) latter that your "new" methods were not really any more accurate than the old ones. but the military at that point is committed to taking the aircraft anyway, and it is always a much better performer than their old aging fighter or bomber they are supposed to be replacing.

But an airline is banking on being able to deliver cargo and passengers at the cost promised by the manufacturer, and with a competitive airline market, it could also mean the difference between loosing lots of money, or making a profit (many airlines, if they make a profit at all, are only 1 to 2 percent ahead of their expenses). So the contract requires refunds of money if an a new design does not deliver as promised. So the accuracy of the drag calculations becomes very very important to both the manufacturer and the customer.

To similar circumstances, two very different considerations for making very accurate drag estimations.

And despite the very large investment in supercomputers by the government to develop new military aircraft, I remember hearing our chief CFD programer say out load that no one can estimate drag accurately. Particularly when you consider the very unusual shapes of military aircraft have taken over the last 20-30 years, you have nothing to compare these new shapes to so you can make "adjustments" to the drag estimates.

The issue that a company like Boeing faces is very different, all of their new designs are refinements of their old ones. New technology engines and materials do not really change the shape of the aircraft that much, so they can use their old estimating methods, adjusted by similar size and shaped aircraft already flying, to refine their drag estimates to a level that would not be possible with a completely new shape (and I can tell you that the shape of many of the new military aircraft are not friendly to aerodynamics at all).

So in either case "engineering accuracy" for drag estimates is very different thing for these two very different markets. I am sure the same issue apply to navy vessels vs. commercial shipping demands. The accuracy is only good enough to get the contract signed in one case, the other is much more critical for the financial survival of both the customer and the manufacturer.

Take your pick.

 

Rough or smooth !!

i think its is more to do with the speeds you exspect and get from you boat . a sail boat sanding with what ever i can understand water against water so to speak ,but high speed ON smooth to super shiny and even waxed to have as small amount of friction as is possible . Fareness of the hull would have a big part to play in all these things as well ,smooth flowing lines with no abrupt shapes . powerboats with steps to get separation and introduce less wetted surface but again fareness would have a lot to do with that as well .