Centerboard shape

[WALT SCHERF]

Am starting construction of a small daysailor and have a question about centerboard shape. Given two identical boats, one with a flat steel C/B and the other with an airfoil-shaped section, would there be any measurable performance difference between the two?

[N. Scheuer]

If the foil CB has an aspect ratio in excess of two-to-one, and has a quality foil shape and finish, I would say it certainly would make a measurable difference. If it is a pivoting CB with just a quarter circle arc, save your time ang go flat.

Moby Nick

[TomRobb]

Flat blades stall easier than fat foils.

[Ken Hall]

Which paints a picture of me, wrestling the tiller, trying desperately to pull my Fireball out of a flat spin after an accelerated centerboard stall... http://media5.hypernet.com/~dick/ubb/biggrin.gif

Ken, looking for a gallon of propwash Ken

[Art Read]

"Small daysailor?" "Centerboard?" I'd think trying to engineer an efficient "airfoil" shape would be overkill. What are we talking about... a boat with maybe a 4 or 5 knot "hullspeed"? All the work to build in a "airfoil" will add what? A quarter knot, tops? How are you going to design the trunk to work with the compound curves involved? I'd just bevel/fair the leading and trailing edges and call it good. What do the plans show?

[This message has been edited by Art Read (edited 12-19-2001).]

[Ian McColgin]

Definatly go with the NASA cord 45Z4D.

[N. Scheuer]

NASA-45Z4D, Eh? I was thinking just a regular old NASA-0012.

And Art, we're talking hydrofoils here, not airfoils.

Moby Nick

[Tom Lathrop]

Art,

I've spent much time trying to find speed differentials much smaller than a quarter knot in racing sailboats. If there is somthing as easy to do as shaping a centerboard that would gain a quarter knot, I'd jump at it. Trouble is, it won't gain anywhere near that much except in special cases and that would be mostly in maneuvering or VMG and not in straight line speed.

[htom]

Airfoil shape will be better than flat plate (assuming you use a reasonable hydrofoil shape.)

Constructing it so that it can be "angled" in the case for proper angle of attack on each tack will also help, but it's a PITA to arrange and remember to do on each tack. Usually the top (in the case while the board is down) is shaped like, a pair of triangular wedges, bases together:

/\
\/

with some means of locking one pair or the other against the sides of the case.

The improvements should be measurable and independent of each other, if you want to do one or the other but not both, and conduct experiments.

[Scott Rosen]

You guys . . . The issue isn't speed. It's performance. The shape that will provide the greatest lift will give you better windward performance. If a good hydrofoil gives you an extra degree or two to windward, then that's as good as straight line speed if you're racing. If it's only going to be used as a daysailor, you're probably over-thinking the issues.

If I seem to know what I'm talking about, it's just an illusion.

[This message has been edited by Scott Rosen (edited 12-19-2001).]

[Art Read]

Okay, okay... Maybe I'm old fashioned. But I defy anyone to "prove" that the airfoil, (sorry...hydrofoil) shape they put in the centerboard on their home built daysailor has added a tenth of knot boatspeed or improved windward performance 2 degrees. Seems to me the time would be better spent getting the hull fair and keeping the damn trunk from leaking! Then again, I'm building an eighty-five year old, gaff rigged design "anachronism". Treat all "advice" according to it's source! http://media5.hypernet.com/~dick/ubb/smile.gif

[This message has been edited by Art Read (edited 12-20-2001).]

[bainbridgeisland]

Folks are pretty much on the right track here but let me clarify a few principles.

The most important shape is aspect ratio. A deep narrow board is far more efficient than a shallow wide board.

Another important consideration is the board area. Too much means excess drag. Too little means excess leeway.

Taper and sweep angle are next. Most boards have incorrect taper and sweep ratios. The wrong combination means extra induced drag.

Least important is the board cross section. For a relatively thin board, say thickness is 1/10 the cord (fore and aft width) or less, even a lab has a hard time measuring the difference between a flat plate with rounded edges and an NACA foil. Thus efficiency is not usually influenced too much by the cross sectional shape of the board.

[fair&fair]

Ok, my 2cents. A foil section will be much more effective at preventing leeway than a flat plate. Bainbridge is right in suggesting that aspect ratio is of great importance when considering a centerboard or dagger board. My thought about this daysailor is to just make something that seems about right, and it should do fine. As far as building a foil section goes, it really is not very hard at all. The offsets for NACA foils are in 'Theory of Wing sections' by Abbot and Van Doenhoff. Once the offsets you want are determined, then make two templates of the section as it applies to your board (ie you must know the chord size of your board) and set them up on something flat. Take a laminated board that you've made from something like mahogany and place it between the half foil sections (one at the top and one at the bottom). Now just make a router jig with two long slats that will follow the contours of the foil jigs as you run the router back and forth along your board. A good shape can be achieved this way very easily.

[mmd]

An anecdote to possibly enlighten: A good friend and boatbuilder has an eighteen foot Swampscott sailing dory (a lovely creation; thank you John Gardener)that he maintains in pristine condition. When first built, the rudder would vibrate quite strongly when boatspeet approached her maximum - enough to make the whole boat hum at about 30-40 Hz. The rudder was built of 3/4" ******ed ply with the leading and trailing edges merely rounded over. To fix this fillings-rattling vibration, he took a power planer and shaped the rudder into a rudimentary foil shape with rounded 3/4" leading edge tapering to a rounded 1/4" trailing edge. The boat is now quiet at all points & speeds. No measurements of speed differentials was taken, but just the quiet indicates that all is better in the water. I suspect that this observation is applicable to a fin-type centreboard, as well.

[bainbridgeisland]

fair&fair

Theory of Wing Sections is a great book. I have used it a lot, but you have to be careful with it. While I agree with you that the faired, NACA, section shapes will produce better lift to drag ratio, my point was that the improvement is very slight for thin sections.

A great example is the Olympic Finn. The metal centerboard on these boats is not even close to what most of us think of as an airfoil shape. Yet, altering the centerboard to be "airfoil" shape would not improve performance. The reason for this is that the plate centerboard is too thin. What little shaping it has is already pretty good.

To improve the lift of the Finn centerboard, it would need to be thicker. The thicker board would need more shaping, i.e. "airfoil" shape. However, along with the extra lift more drag would be created. Also the area of the board would be too big, adding unneeded wetted area. Thus the total shape of the centerboard would need to change to be as effective as the metal board.

The question is, at what thickness does "airfoil" shaping become significant. The answer must be on a general basis. For a racing boat, anything thicker than %8 of the cord length (the fore and aft length of the board) needs some significant shaping. For a cruiser or day sailor %10 is a reasonable limit. This is a judgment call so other opinions are OK too. The calculated value for one particular boat was: For a %10 board with a simple taper on the back and a radiused front, speed loss would be in the order of 1.7 seconds per mile compared to the "airfoil" shape when beating.

[N. Scheuer]

I sail a fiberglass Shearwater yawl. The leeboards that it was built with are fairly crude by any standard, being 1.5 inches thick, with a .75 radius on the leading edge, a 6 inch taper on the trailing edge, and a .375 inch flat along the trailing edge.

Another Shearwater, which sails a bit faster than mine for a variety if reasons, also beats with a VMG that certainly appears greater than the 2 degrees specified by Art Read, above. This boat has a pair of owner-built assymetrical-foil leeboards having cord sections derived from a NACA 0009 foil, as near as I can tell.

I am in the process (greatly protracted due to procrastination, as well as the press of other concerns) of making a new pair of leeboards.

Moby Nick

[johnw]

Some classes, such as the Cotuit Skiff, allow both wood and tin boards. In the Cotiut class, the boats with the tin boards were reportedly faster. This may have something to do with the fact that the centerboard case could be narrower, producing less drag from that source. And as for humming appendages, that's a real mystery. Lasers are a one-design with a high-aspect rudder with a foil section, yet some hum and some don't. The rule is, if it hums, change something. I think a tacking daggerboard would be easier to build than a tacking centerboard. You make it wider at the back than at the front on the part that stays inside the case, and the water pressure tacks it for you.

[ahp]

The humming/vibrating centerboard or rudder blade is no mystery. If the trailing edge has a radius rather than a sharp edge, it will shed vortices in a very repeditive fashion, alternately from each side. It is called a Prendl vortex sheet, named after a German hydrodynamisist. It makes the centerboard or rudder vibrate. The frequency is proportional to speed and inversly proportional to the radius. There is a family of commercial flow meters based on this principal.

Art

[TomRobb]

The galvanized steel cb on our old daysailer vibrated at a certain speed (as well as being a pretty fair sonar transducer.) It takes energy to make the noise. The energy comes from what little horespower the sails make. Dealer's choice http://media5.hypernet.com/~dick/ubb/biggrin.gif If you don't care, don't worry about it.

[bainbridgeisland]

[QUOTE]And as for humming appendages, that's a real mystery.

Actually there are 4 possible reasons for a hydrofoil to vibrate and make noise.

Flutter – When a hydrofoil is already vibrating, it induces the surrounding flow to vibrate as well. The vibrating flow can either increase or dampen the vibration of the hydrofoil. This phenomenon can add to a rudder or centerboards problems.

Galloping – Owing to stall, a side force can amplify vibration of the hydrofoil until friction forces are balanced. For rudders and centerboards, galloping tends to be a transient problem.

Buffeting – This is turbulence induced drag. This one is actually somewhat common on boats. It means that turbulence from other parts of the boat cause the rudder or centerboard to vibrate. A good example would be a propeller in front of a rudder, or a propeller aperture in front of a rudder, unfaired deadwood in front of a centerboard and so forth.

Vortex induced oscillation – This is the big one. This phenomenon is usually the biggest contributor to unwanted vibration of hydrofoils. Von Karman, a famous fluid dynamicist, discovered the principle in the 1920s, or maybe 1930s. The common term is Karman vortex street.

So, what is a Karman vortex street? As velocity increases, symmetrical vortices at the tail of the hydrofoil become unstable. The vortices alternately grow and detach from opposite sides in a periodic manner. Since the shed vortices do not immediately dissipate energy, they create a meandering wake called a Karman vortex street. When the shedding frequencies of the vortices match the natural frequencies of the hydrofoil, then resonance will occur. In other words, noise and vibration problems will be accentuated.

Ok, but what is a vortex? We are talking about little whirling eddies here. In the case of the Karman vortex street, they spin opposite directions from either side of the hydrofoil. This is what causes the wake to meander.

The vortex shedding frequency is directly proportional to the size of the hydrofoil. The constant of proportionality is called the Strouhal number after the physicist who discovered the principle. For a cylinder, the vortices collapse on the cylinder, causing tremendous vibration. This is why your anchor line vibrates in high current. The Strouhal number for a cylinder equals frequency times diameter divided by velocity.

But a thin, flat, plate oriented parallel to the flow, operates somewhat differently. First of all, the vortices collapse in the wake, behind the plate. The Karman vortex street is not a result of flow separating from the body, like the cylinder, but is caused by an unstable wake. Vibration in this case is from impulses transmitted up the wake. A centerboard or rudder would lie somewhere between the flat plate and the cylinder. However, for most centerboards and rudders, the vortices collapse onto the hydrofoil.

What causes the flow instability that initiates a Karman vortex street? This is a bi-ig subject. Flow instability has the most far-reaching significance of any phenomenon in fluid dynamics. Trying to simplify here: speed, angle of attack, elastic stiffness of the hydrofoil and vibration of the foil are common causes of flow instability. Unfairness of the foil, or non-symmetry in just the wrong place is also a common cause. Even the finest, smoothest, most symmetrical foils you have ever seen can have this problem at times.

How do I stop the noise? If the cause is a Karman vortex street, the answer is simple. Plane a flat on the back of the foil. The flat creates a tiny amount of suction. The suction stabilizes the flow just ahead of the trailing edge. Also, the sharp corners on each side of the hydrofoil that were created when you planed the flat, tend to nail down the initiation location of the vortices. This way they can’t move forward and aft so easily, further reducing the chance of vibration.

If this doesn’t work and the cause of vibration is a Karman vortex street, the flow instability is probably due to some large unfairness or design error in the foil. For example, a highly raked aft and highly tapered foil will stall easily. It could then develop galloping problems. Flutter could weigh in at this time. And then the vortex street develops and so forth.