Squaring the trailing edge of a daggerboard

I am in the process of building the daggerboards for my 46' cat and have recently learned that it is OK the square off the trailing edge of a foil up to 5% of the chord since this part of the foil is in turbulent flow.

This has interesting implications because I have always been concerned that a sharp trailing edge would be more prone to cause damage to the case in event of a collision.

On the other hand, as I recall, the rudders on my Hobie Cat and the fins on my surfboard would hum at speed if they weren't sharp. Also, I have never actually seen squared off foils. So before I take a power plane to my daggerboards I thought I would ask if anyone has had experience with squared off foils and if they felt that performance was affected.

Thanks,
Mike

 

I agree that super-sharp trailing edges are a pain (literally!) and prone to damage. You can sensibly make the trailing edge 3mm wide.

To reduce hum what you can do is make the cut at 45deg rather than at 90deg

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I'd say a limit of maybe 5mm, rather than 5% of the chord. If you're using a NACA 10 foil then a 5% trailing edge would mean your trailing edge is half the thickness of the board.

On a 500mm chord board you'd have a 25mm thick trailing edge.

I can't see that working too well.

 

Most of the NACA sections you would use for daggerboards are not sharp at the trailing edge. The 0012 section, for example, is 0.252% thick at the trailing edge. The 0012-34 section is 0.24% thick at the trailing edge.

If you cut both those sections off at the 95% chord length you would be around 2% chord thickness at the new "trailing edge". That sounds pretty hefty.

 

5% of the chord is effective and relatively standardized.

Don't put a chisel tip on it, though it may decrease some hum (marginally). A clean, squared off trailing edge services two issues. One is the delicate trailing edge, which in a dagger is prone to damage in a strike or grounding. Second, it provides a clean release of turbulent flow at the trailing edge.

Yep, in a perfect world all foils would have razor sharp trailing edges, but they're very easily damaged, difficult to repair and can drag along eddies. Clipping the trailing edge removes much of this from the equation.

 

Not sure what design offices you talk to, but the people I know prefer to follow the correct foil sections. In the world I live in 5% chord is not any standard, and would actually never be used.

No, they would not. If you look at the foils I mentioned above, commonly used for keels/boards/rudders, the trailing edges have designed thicknesses. They are not razor sharp.

 

Real world experience and extensive foil testing show that it is perfectly acceptable to lose that trailing edge of the foil. But if it is cut off as a flat you will get alternate vortex shedding from the two edges and an increase in drag and vibration. This is prevented by reducing the width of the flat as much as practicable by chamfering the two edges back to a 45 degree wedge, round all the corners and it will be fine and much stronger than the original trailing edge.

Truncated racing foils should supposedly be faired back close to a 30 degree wedge which is undiscernable in testing to the original foil.

 

Mike,
do you know of a site where I can see the full sections of the foils you describe?

 

I don’t think you won’t find any website on this. And I’d like to put this into some perspective for interests sake.

If you read the ship science reports, transactions and papers you also need to keep an objective eye on the idealism of the testing regime and the target vessel type.

When we compare the lift-drag polars for foils they are inevitably from smooth calm water tests in ideal conditions (or ideal CFD simulations). In reality hydrodynamics is not as predictable as aerodynamics, a boat operates with degrees of pitch yaw heel and leeway in water of varying degrees of turbulence. This renders a lot of idealist foil design and accurate reproduction invalid. This is why optimized drag bucket modern foil shapes often perform very poorly in the real world.

You will see studies of drag increase with TE truncation (such as presented in Principles of yacht design), these are for a sharp cutoff, even then the drag ( and Karman vortex properties ) will be entirely dependant on the Reynolds number angle of attack etc relative to foil planform, section shape, tip shape and hull interface.

The fwd 1/3 of the chord is far more important in its accuracy of reproduction and surface roughness even then we are into that last little % that gets all the effort in the racing fraternity, In my view a slightly better sail shape or trim has at least an order of magnitude improvement over chasing an ideal TE or (even an ideal LE) on a foil.

Here’s a thought:
For foil performance over a wide range of Reynolds numbers you would get a far better overall response from an articulated TE on a foil of any section shape remotely close to a 00xx. That really alters the lift drag curves over a range of Reynolds numbers and varying leeway angles. These sorts of innovations have been seen before but were promptly killed off by rating-rule committees.

 

I agree fully with Mike Johns. In essence unless you dry sail your boat (like serious race boats do) there isn't a lot of point going too fancy with the section shape. And the forward part of the foil (and hull) is more important than the aft part.

Over 35 years ago I made an International Moth daggerboard with a 30% trailing flap. It was fun to sail sideways. In 1979 I drew a 40ft racing cat that had similar foils, the idea was that the boat would close reach at very high speed, yet still point high. There is still lots of scope for experimentation.

Finally remember that monohull sailors can only go faster to windward by pointing higher, they can't go faster through the water when the wind increases like multihulls do. And that makes a huge difference to the way you cut/trim sails (and by implication, foils) and also to the way you race, or even just sail fast.

So what may work for monohulls isn't necessarily the best idea for multihulls.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Daggerboard with Flap

Richard:

Did you ever race with this daggerboard? I can see some possible tactical advantages, but maybe not better speed.

Over 35 years ago would be the early 1970's or late 1960's. I was wondering if our paths ever crossed. I raced Moths from 1961 through 1969, mostly in Florida, but several times when the Nationals were in New Jersey & a couple of times when the Worlds were in Europe.

Doug Halsey

 

No we never met, my Moth sailing days were 1972-78, but mainly 72-75

You are right, the trim tab didn't make my Moth go faster, but sailing sideways, even slowly, had advantages when I understood at a mark or were just to lee of someone. Usually my rival would be so amazed as I sailed sideways to windward past his stern that he'd slow down as well and I'd get ahead.

I still think it is something worth trying, after all a catamaran has lots more stabilty than a Moth so can close reach at speed.

Richard Woods of Woods Designs

www.sailingcatamarans.com