Whale fin Rudder Modification

MrBill_FLL · May 29, 2013

http://www.otherpower.com/images/scimages/2637/leading_edge_tubercles.pdf

walt · May 30, 2013

Finding, you’re a good guy to try this since you have been racing your boat already and know how it did against other fast boats before you do the mod.

I’ve seen forum posts on this idea for maybe 15 years now and this idea hasn’t really shown up on racing boats but who knows The paper above cited an actual lift to drag advantage for whatever Reynolds number they were using - but as far as I can tell, this was ONLY a computer simulation, not actual real test data.

The whale that uses that fin needs to be very maneuverable for how it captures its prey Very maneuverable - fins that can generate lots of lift quickly. That implies to me that it needs to operate at a high angle of attack without stalling. Makes me wonder if the main benefit of the tubercles is an increased stall angle.

If your rudder is stalling all the time (results in a roundup), well, the tubercles would probably be a good idea although there are other ways to solve that such as just a good quality foil shape and sail balance.

I wonder if anyone has actually measured (not a computer model) an increase L/D with that type of foil? You also have to be careful of what Reynolds number (which I barely understand) they did the measurement at as it would need to be reasonably close to what Reynolds numbers sailboat foils operate at. The computer model is certainly interesting but if it actually did have those benefits, I think it would have shown up in the racing sailboats 10 to 15 years ago as the idea has been around for at least that long.

walt · May 30, 2013

http://perryboat.sail2live.com/yacht_design_according_to_perry/2011/10/rudders-lets-start-here.html

A note from this link was interesting:

http://www.tda-ri.com/applications/sailboatrudders.html

As expected, in “straight line” sailing there was no measureable difference. This alone was important; to assure that the BLEF rudder does NOT result in increased drag at small AoA. However in tacking and gybing the helmsman’s comment was “she tacks very nicely, quicker and smoother than usual”. Unfortunately, there was insufficient time remaining to get the BLEF-modified rudder measured and accepted for use in the Louis Vuitton Series, so BMW-Oracle Racing had no choice but to use their original CLEF rudder. As an ironic postscript, after spending many millions of dollars to design, fabricate, assemble, test, and sail two IACC racing yachts, during their initial elimination series against the Italian Luna Rosa team, the BMW-Oracle boat stalled not once but twice during pre-start maneuvers, badly losing the start both times, and going on to be eliminated from the America’s Cup competition.

Got me wondering about one thing regarding dagger board Macs.. A few years ago a guy who was racing a 26M would note that in very light wind, he would lose some ground to other sailboats during tacking – he would slide sideways, they would not. Foil lift (needed by the dagger to get around in a tack) is proportional to velocity square so at very low velocity, you can need higher angle of attack to generate lift since velocity is so low. The high aspect dagger foil is great for L/D.. except that it probably stalls at a lower angle of attack than a keel boats lower aspect foil – and why he had the trouble in very light wind rounding. Maybe a tubercle high aspect dagger would be a good solution here? Note that with the swing centerboard on the 26S, we can also solve this problem by raking the centerboard back and essentially turning it into a low aspect foil. But the centerboard boats are just not as fast in the first place.

Doc_holiday · May 30, 2013

If nothing else, it looks cool. So what will you do now? Just rough up the rudder and wrap fiberglass over the new part to make the new foil? I know adding some leading edge to the rudder has been popular by itself. Looks like a fairly easy thing to do. By all means keep is posted. I need to re-glass my rudder some day and if I have to do that, I might add the leading edge if its not too difficult

walt · May 30, 2013

From the same link as above, it looks like these guys do mainly think the advantage to this is a resistance to foil stalling without any bad effects L/D - ie, the foil can be operated at a higher angle of attack without stalling.

Also, while the use of BLEF features on keels and daggerboards is unlikely since these items always operate at small AoA where BLEF shows no advantage relative to CLEF, the same is definitely NOT true of rudders. Under nominal “straight-line” sailing the rudder is normally at either zero or small AoA. Here, a BLEF modified rudder would provide no benefit, but would also provide no disadvantage. However, when tacking or gybing, especially in close quarters when racing it is often true that a substantial change in direction is needed rapidly. It is not altogether uncommon, even for world –class helmsmen to stall the rudder during such maneuvers. Consequently, the use of a BLEF-modified rudder may prove beneficial.

I been sailing a 26S for a long time but don’t race it except for the "drag racing" that occurs when any two boats get at all near each other.

I have a now about 6 year old Rudercraft rudder which I believe were CNC milled from a chunk of plastic so are accurate. This is the only rudder I have used on the boat but one of the things I believe people liked about this rudder is that it was less prone to stall compared to the original rudder. I personally have never really thought I had a problem with it stalling - which seems to be the problem that Tubercles solve.

Finding, since you race, do you ever think you have a problem with your rudder stalling? Maybe the problem always occur at very low boat speed and I just don’t know about it because I don’t really ever sail in very light wind (just not something I care to do that much - I’ll start the outboard first).

Justin_NSA · May 30, 2013

Doc_holiday said:
If nothing else, it looks cool.

I agree

I'm like Walt. I have only ever used the IDA rudder on my Mac

walt · Jun 1, 2013

FYI, I’m glad you’re trying this and I’m looking forward to what you think after trying it out.

I mentioned Reynolds number (RN) in another link and it’s not something I really understand but what it does is allow you to relate what happens between for example a small scale model foil and the larger full size foil. RN somehow takes into account the size of the object, the speed it travels at and the viscosity and to relate results, the RN numbers should be ballpark.

Why bring this up? I don’t believe I have ever noticed a problem with the rudder craft accurately foiled rudder stalling at any sort of boat speed except for when I have just the main up in very high winds and then I’m a little more prone to roundup from the rudder stalling. But the situation of stall that were discussed here were always in very light wind where the boat speed is very low.

I only slightly know enough about RN number to bring up the concern.. but that is that the bumpy leading edge that works for the whale in the feeding frenzy might be operating at a significantly different Reynolds number than a much smaller sailboat rudder that is operating at probably less than 2 knots. What works at one RN doesn’t always work at a different RN..

Don’t know... but like I said, good experiment and good luck!!!

walt · Jun 1, 2013

The attached picture is from http://perryboat.sail2live.com/yacht_design_according_to_perry/2011/10/rudders-lets-start-here.html

These guys probably spent a bazillion bucks refining that leading edge shape. How close do you think yours is to this?? Looks like its still mostly something like the Naca 0012 (or whatever rudder craft uses)

vizwhiz · Jun 1, 2013

Just thought I'd chime in a little to help explain Reynold's Number from my engineering background. RN is, from our perspective in boats and water, a relative indicator of the turbulence in the water. Low RN's indicate laminar flow. RN's reach a "transition" zone where the liquid movement is beginning to break up from being laminar, all the way up to "not quite fully turbulent". The numbers above that indicate turbulent characteristics and the higher the number, the more turbulent the flow or movement is.
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That said, let's include a little background, then define laminar and turbulent briefly...
Conceptually, water moving over a stationary surface (a rock in a river) exhibits the same characteristics as if you moved the surface thru stationary water (a rudder thru the lake), so everything is discussed in terms of the liquid moving.
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The concept of viscosity is thrown in, as mentioned above, but water is not very viscous, so it doesn't have a lot of impact on what we're doing. Viscosity is defined as the liquid's resistance to shear. If you think of a liquid as being made up of individual particles like marbles or b-b's, but magnetic ones that stick together, then it might help. The stronger the magnetism, the higher the viscosity. So while you might be able to slide your hand thru a bucket full of liquid that has low magnetism between particles, like water which is low viscosity, the higher the magnetism, like syrup or molasses, the harder it will be to move your hand thru...the particles don't like to be separated (shear) to allow your hand to pass between them.
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So now think of a slow moving river of water, low magnetism b-b's, flowing along. All the particles kinda line up and move smoothly next to each other, not really rubbing or moving relative to each other (one is not slowing down and another speeding up and another going sideways). They all just move together...this is laminar flow. Imagine that they flow along this way until they come to your rudder. Now they have to separate (which they don't like to do) and go around the rudder, and then meet back up on the other side.
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Our foil type rudders like the water to stay laminar. As the rudder is turned, it creates an angle of attack, like a wing, which creates lift and pulls the boat that direction. For any given airfoil, there is a point at which the water can't separate, get around the rudder, and join back up on the other side in the nice clean lines it was flowing in, and when this happens it is called separation or we like to call it turbulent flow. When that happens, the rudder loses lift, and the boat stops turning. The energy in the water ends up being wasted as drag instead of lift, and we say the rudder has stalled. It is the same phenomenon as an airplane wing stalling due to the combination of high angle of attack for the speed of the wing.
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So the low wind speed, which results in low boat speed, means you cannot turn the rudder as far before the particles of water get turbulent instead of flowing in a laminar fashion and the rudder stalls. You don't actually stall the rudder any easier, just at a lower angle. The lower speed also means less energy available for lift, thus lower turning speeds, which is not really the fault of the lower angle, just all goes together.
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I hope this has helped a little.
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So the airfoils used for the rudder are a big part of this, some being better at lower speed and some at higher speed, some lose laminar flow (separate) at lower angles of attack, some higher. I would guess that our boat rudders were originally designed, and replacements are too, for optimum performance at hull speed (or close to it) with less concern over low speed performance. Improving low speed performance might require a specialized rudder design that might negatively impact the higher speed performance.
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It will be very interesting to see how this whale fin rudder works out!!

walt · Jun 2, 2013

when this happens it is called separation or we like to call it turbulent flow

There is a paper by Tom Speer whom used to be involved with land sailing (how I know about him) and is now the Oracle racing team aero and sail project leader and if you can tolerate technical papers, its interesting to read (the pdf link below). This paper is mostly dealing with wing masts (and is applicable to rotating mast used on the M boats) but it also talks about the different types of flows that we care about.

In the paper, there is attached laminar flow, attached turbulent flow and separated flow. Both attached laminar flow and attached turbulent flow produce lift. Separated flow does not - this is stall. The variables where these flow occurs is influenced by the Reynolds number - why is we care about it here.. For example, most of the flow on our sails (if not stalled) is turbulent attached flow. The tubercles may be nature’s way of inducing turbulent attached flow. Airplane wings will sometimes have "turbulators" that induce the flow to turbulent early as it is less prone to separation (stall). http://en.wikipedia.org/wiki/Turbulator

Tom Speers web page http://www.tspeer.com/ Oracle Design Team, Aero & Sail Project Leader

The link below is the interesting page I wanted to point out
http://www.tspeer.com/Wingmasts/teardropPaper.pdf

(below is snipped from the above web page)

At some point, the boundary layer can’t maintain this smooth state of affairs, and eddies start to appear. This is known as the transition from laminar (the smooth flow) to turbulent flow. A turbulent boundary layer is much thicker than a laminar flow, because the eddies surface, and throwing them some distance away fromthe surface. They are also bringing some of the higher velocity air from outside down closer to the surface. This higher velocity air gets slowed down, naturally, so this causes the “skin friction” of the turbulent boundary layer to be higher than the laminar boundary layer. But the turbulent boundary layer is not all bad, as you’ll see.

But a laminar boundary layer is much more fragile with respect to flow separation and stall than a turbulent boundary layer. So we want to maintain a fair amountof laminar flow so as to keep skin friction low, but we want the flow to be turbulent as the air slows down heading back to the trailing edge. By using the turbulent boundary layer’s ability to slow down more, we can use higher velocities up front and get more lift. A laminar boundary layer is a little like driving on ice You don’t dare go too fast because you can’t slow down quickly. A turbulent boundary layer is like driving on wet pavement - you’ve got better braking, so you can go faster without breaking loose.

walt · Jun 6, 2013

Hard to tell and I could be completely wrong.. but the leading edge always looks to blunt to me. I wonder if it should be something like a Naca 0012 shape that "modules" the chord lenght??

If your lucky here, you will have the L/D of the Rudder Craft CNC milled foil but more resistance to stall. As I said, I could be completely wrong.. but cant help but to think the blunt leading edge would suffer on the L/D

???? Who knows..

Whale fin Rudder Modification

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