Rudder and daggerboard profiles.

[Milton Thrasher]

Searching for and testing of CFD code or other simulation software to prove the advantages of scalloped edges on sailboat foils would be a waste of time even if you stumbled onto some such code before sea trials are done. Until the advantages of scalloped edges are proven in actual use, you could not rely on computer simulation results.

No funding is needed for trials of scalloped edges on Laser or Sunfish sailboats. It would be very simple to cut scalloped edges into piecies of plastic and affix them to the leading edges of the foils. The cost would be very minor. There are lots of college sailing programs that have the necessary fleets of sailboats already and students with enough initiative to conduct the tests.

Another test opportunity is with radio controlled model sailboats. There are many clubs that race one-design models such as the American Model Yacht Club sanctioned Soling One Meter Class marketed by Victor Model Products of Downing, CA which can be found on the http://www.AMYA.com web pages with a link to suppliers.

We have many Soling One Design sailors here in Sarasota, FL that race the
Soling models on four different sites several days each week. I plan to contact some of the more venturesome and ask them to try the idea out.

The idea of seeking sponsorship or funding smacks of trying to get government grants for foolish projects. There are too many of those already such as studying the sex lives of African Red Squirrels which the US Government has funded recently.

[tspeer]

There's definitely a role for CFD as well as classical cut-and-try methods. Simple geometries can have very complex fluid dynamics - look at the flow around a sphere - and it takes sophisticated shapes to have simple fluid dynamics - such as laminar flow sections. I think you'd be hard pressed to stumble on a modern laminar flow section design by hand carving boards and rudders. They can really only be designed using computational methods.

The same thing could hold for tubercles. They may have evolved in whales for reasons that have little to do with hydrodynamics. Or it could be that to make them work well requires subtle shaping that would be hard to guess if you didn't understand the real flow mechanisms behind them. Fluid dynamics in general, and it seems to me sailing in particular, is filled with what I call (after Kipling) "Just So Stories" - "explanations" that get handed down and widely quoted yet are completely wrong. A classic example is, "Lift is produced because the air flows a longer distance over the lee side than the windward side."

CFD has the advantage that it gives a very detailed picture of the "why", whereas experiments often give the end result with no visibility as to the causes. The problem, of course, is that CFD always leaves out some of the physics in order to make the problem computationally tractable. If you leave out important physics, then the results won't match experiment.

The best way to make progress is a combination of computation and experiment. The computation can guide the experiment and eliminate a lot of unpromising cases. And the experiment is always needed to validate the computation.

Sure, you could hack away at Laser boards. If the mods are successful, fine, but you won't know if you're getting the most out of the tubercles or not. And if they aren't successful, is it because tubercles don't really work, or is it because you just didn't implement them correctly?

And there are lots of pitfalls with small scale models not being representative of the real thing - remember Mariner?

[sorenfdk]

I agree with Tom. It took millions of years to come up with the tubercle design of today, and who knows if it has reached the optimum point yet?

Better to try different shapes in the computer to try and discover the secrets behind the tubercles and then use this knowledge to try and design a board or a rudder.

[tspeer]

There's a role for both. If the effectiveness of the tubercles is not highly sensitive to their shape, then simple experiments would show them to be beneficial. And, unless you have the software already, that's the cheapest way to go.

But research using CFD is definitely not a boondogle.

[Frank Fish]

Originally Posted by Milton Thrasher

SCALLOPED WHALE FLIPPERS – DESIGN FOR FOILS?

In the August 2004 issue of Scientific American there is an article that discussed the investigation of the hydrodynamic performance of humpback whale flippers. Dr. Frank E. Fish, a biology professor at Pennsylvania’s West Chester University and a specialist in the hydrodynamics of vertebrate swimming noticed that the humpback whale pectoral flippers had evenly spaced bumps along the leading edge.

He worked with fluid dynamics engineer Laurens E. Howle of Duke University and David S. Miklosovic and Mark M. Murray of the US Naval Academy. Howle built 22 inch long plastic fins, one with the scallops and one with a smooth leading edge. In a wind tunnel at the Naval Academy testing the scalloped fins showed advantages, especially at higher angles of attack. The test results were reported in the May issue of Physics of Fluids, Leading-edge tubercles delay stall on humpback whale (Megaptera novaeangliae) flippers). The scalloped fins generated 8 percent more lift and as much as 32 percent less drag than the same size smooth fin, (probably at high attack angles). The scalloped fin resisted stall 40 percent steeper than the smooth fin.

The reported reason for the performance improvement was that the flow coming off the bumps formed pairs of counter rotating swirls or eddies coming off each side of the bumps. The bumps act as vortex generators that keep the general flow attached to the suction side of the fin when there is an angle of attack that normally would cause the flow to separate from the leading edge.

There may well be an application for r/c keels and rudders since they operate in the transient Reynolds number range between laminar and turbulent flow. The current trend for keels is to make them very thin with sharp leading edges, which makes them liable for flow separation at low angles of attack. The scalloped leading edge delays separation and thus improves turning ability without causing excessive drag. The drag of the scalloped fin was similar to the straight fin at zero angle of attack. A scalloped rudder would work the same going down wind, but when the rudder is kicked over for a tack the rudder would create a faster turn while losing less speed through the turn. It’s possible that a million years of whale evolution may have developed a better rudder for us.

Dr. Frank Fish has patented this concept.

Scalloped edges could be added to existing blades for
testing against standard blades on matched pairs of one-design sailboats such as Lasers or Sunfish to prove or disprove the theory.

I am considering trying it on my Irwin 23 with wing keel and high aspect
rudder but without a trial horse it would be hard to judge the advantage.
I would be interested in hearing the results if someone else tries it.



I will post a graphic image showing a pair of plastic fins, one curved edged and one scalloped to be sure that the concept is conveyed correctly.

If you would like to try the design I can supply the humpback tubercle geometry. F. Fish

Originally Posted by Frank Fish

If you would like to try the design I can supply the humpback tubercle geometry. F. Fish

Dear Dr. Fish.

I have read you article on your work regarding the Humpback Whale flippers, which I thinks sounds very interesting.

Can you supply me with further information on your work?
- lift and drag coeffieicnt etc,
I would be very interested to see the humpback tubercle geometry.

Best regards

Steen Worsøe
Technical Manager of the MacArtney Group

[Frank Fish]

Steen, please contact me through my email at ffish@wcupa.edu and I can supply you with the information on the humpback flipper. I would be interested to hear of a possible application.

[Ilan Voyager]

An apparté from an avid reader of this thread:

For those too young, Mariner was a 12 meter JI designed by Britton Chance in the beginning of the seventies.

After a very intensive campaign on models at the test towing tank, it results a very curious shape, like a the belly of a hydropic whale pregnant at least of gemels, and the stern brutally cut. The theory behind the chainsaw cut of the stern was that some water will stay stuck to the stern, thus reconstuting a "phantom" smooth flowing stern.

12 meters JI were very heavy and slow boats with a plumb mine inside the keel. The size of the wave made at hull's limit speed was astonishing, you could see the keelson of the rear part of the boat. It's amazing to see how much money has been spent on boats so inefficient that could be beaten by a couple of kids on a Hobie 16 with worn sails.

The cut stern worked in towing tank but it seems that someone forgot or understimated the factor of viscosity of water, "more" viscous on a scale model that on the full size boat (similar to the bumblebee which can fly with its small wings because air,at its size and reynolds number, is syrup alike).

Scale models are very tricky. That explain why on ships the hydro guys work on very big models, often more than 8 meters long, and may spend weeks correcting the results with the scale factor, born of two spoons of physics, an enourmous bulk of empirism and a pinch of wild guessing.