Twin Daggerboards Setup

On Open 60s, Mini Transat and Volvo 70s boats how are the asymetrical boards setup as far as the longitudinal angle (toe in/toe out) off the cl?.
It seems from pictures that they are towed out ie the leading edge is angled out, this would mean that the boat would have to sail with quiet a bit of leeway angle, is this the case or not?.

 

I don't have a factual answer regarding particular current boats, but I'm guessing there must be an optical illusion at work in the photo you're looking at. If I were designing such a boat I'd be toeing the daggarboards in at least 2.5 degrees, perhaps more should more be suggested by the heeled waterlines of a broad sterned hull.

Could the rake of the boards or their orientation to the gunwales be causing your eye to be deceived?

It could be that most of the foil would appear to be oriented fore-and-aft, and that the asymmetry would appear to occur mostly at the trailing edge as a consequence of foil selection. A designer might experiment with a daggarboard that twists slightly to reduce drag at the hull/board juncture, but I think it would result in too much angle of incidence near the tip, so I doubt there's much advantage to be gained by it.

 

Guest,
On the Wind of Change 40's I gave the boards 2 degrees of toe-in, and asymmetrical foils. This allows them to sail upwind with no (percieved) leeway. I cannot think of any reason why you would want to toe them out.. ;-)

Steve Baker

 

Hi,

I am currently working on my final year project (yacht design study). Could u give a value for the designed CL of the daggerboards? A value of angle of attack of 2° with a small value of leeway means that the daggerboards are working far from their optimal L/D ratio (around 7.5°). So the required area must be quite important. = more drag. I surely miss a point somewhere.

THank u

 

i was not logged in for the last post

i d like to add:

At sailing heel angle, the waterplane must be distorted (as far as 5° for Mini 6.5). Which will reduce the angle of attack of the daggerboard. (Wind of change must have a low value compared to open 60 or mini, must the angle must still count). So 2° of attack + a bit of leeway (1°) - distortion (1-2°) doesn t give much attack angle.
Thank u

Nico

 

The Wind Of Change boats are fairly wide (at 12' 6" on 40')and light (8000lbs), but have a very balanced waterline when heeled. The boards are acting at their 2 degrees, plus may a degree or two of leeway, making 4 max. The boards are only 18" on the chord, and need only be lowered 4 or 5 feet to achieve the best performance. Upwind sail area is about 1250 sq.ft.
The thing is that you have to look at the boards as reducing leeway, rather than designing them to work with the expected leeway. For the tech junkies who want to know CL, the foil was a 6412. Look it up in your Abbott and VonD
Steve

 

Steve,
I take it that you were refering to a reference work on various foils. I was going to take this up in the dinghy design thread, but why not here? What are some good reference works for various foils? About all I have been able to find so far are 6x and 4 digit series. Of course, none of this would be of any use unless the reference works provide offset tables.

 

Many NACA papers are available at http://naca.larc.nasa.gov/
and for their offsets many are available at http://www.aae.uiuc.edu/m-selig/ads/coord_database.html

Nico

XFoil is very good to test 2d airfoil.

 

NACA Offsets and Data

Saildesign is referring to Theory of Wing Sections by Abbott and von Doenhoff.

The offsets and data for the NACA sections are printed there.

 

Guest has it right. Also nico has the link to an obscenely large collection of foil data But use it carefully, as not all foils that are good in air will work well in water. Look carefully at the application, and think. ;-)
i.e. a foil developed for low-speed fairing on a biplane (or for Zeppelin rudders) will not be a good lifting foil at the sort of Reynolds' Numbers you will see on a boat.
Steve

 

Another useful reference for foils is Richard von Mises' "Theory of Flight". A bit much flight but has some good basic foil stuff thrown in.

 

As an introduction to why you might go about picking one foil section vs. another for an underwater appendage, look at:

Online article "Foil Design Parameters and Performance" by David Vacanti at <http://www.vacantisw.com/foildesign.htm>

Chapter 6: Keel and Rudder Design in Lars Larsson and Rolf E.Eliasson's "Principles of Yacht Design."