asymmetric daggers

I have a Kurt Hughes 30ft cat stretched to 36ft. After 3.5 years of basically having no life during the building phase we built the dagger boards last and perhaps were not quite as meticulous with them as we were with the rest of the boat. The time has now come, after 20 years of great sailing, to replace the daggers with something we hope will improve our windward performance.
Most of the details have already been worked out but I am stumped with finding the correct foil shape. After many hours of web searching, much of it way above my head, I need someone who has used asymmetric daggers to tell me which foil to use and how to acquire the DXF for it. Since the dagger cases (Kurt calls them trunks) are so integral to the boat that I dare not modify them then the new daggers must fit within 66cm * 8.5cm. Since this website was the starting point of my web search and many erudite contributors gave me great ideas, I hope someone will help out with the final piece of the puzzle. As a retired accountant my engineering knowledge is sketchy at best

 

Asymmetric boards have got to be 3 times the work of symetrical boards, hell of a job. Are there any other cats like yours that even have them let alone make them work?
Every time that you decide to throw in a tack, one of your crew is going to have to tack the boards and they are gonna be pretty big and super heavy

That said, I poached the shape of mine [For a trimaran] by grovelling around under Orma 60s etc. in LaTrinite. Happy days!

 

Thanks for your input Teamvmg. I normally race with a team of 6 enthusiastic younger people and we always have at most 1 dagger down. The new daggers will be foam and carbon with an anticipated weight of 15 kilos compared to over 80 kilos for my present timber and glass ones. One refinement I am looking forward to is the ability to retract them clear of the water which will help to clear attached weed and obviate the need for antifoul paint which is always a very rough surface.

 

a-boards

I am certainly not an expert, but I can relate my experience so far. An a board works best if it is set at some initial angle of attack that helps prevent leeway. My trunks are around 2 degrees toed in, which seems about right. The only local high performance cat that I have measured- a Reynolds 33, also has its trunks set about the same. The foil profiles available are almost unlimited, but most of the ones I have found have a "zero lift" angle of attack that also has to be taken into consideration.
The "easy" answer might be to make the board enough undersize to allow some on the boat tuning. Your trunks will allow about a 12% section, and with the extra lift available from an a-board, you might do fine with a 10% or less, enough thinner to allow for some adjustment. From my limited experience, an a-board can be about 80% of the size of a symmetric board while developing the same amount of lift, and have less drag.
Something like a NACA 2408 or a 2410 might be a good place to start. They are flat bottomed enough to fit in a typical trunk and still have good lift-drag numbers.
B

 

Thanks for your input BruceB, you have opened up a whole new dimension to my thinking as I had not considered altering the AOA. At present my trunks are directly fore and aft as required by symmetrical boards. I suspect that there is sufficient clearance in the trunks for the existing daggers to adopt at least a 2% AOA naturally but now I may be able to pad the trunks to force the toe -in you suggest. There is a local company with NC hotwire cutting to shape the foam blanks but he has asked for the DXF of the foil shape and I cannot find any mention of it on the web pages relating to NACA 2408. Perhaps it is there under a different name but my understanding of technical drawing is zero so I still need help to acquire it.

 

You might get ahold of a copy of..... Bethwaite, Frank; High Performance Sailing, 1993 Waterline Books......for foil insights

Since you are restricted by trunk dimensions you might want to review this....

http://www.3-7class.org.nz/Design_of_my_Farr_3-7_Centreboard.html

 

many foils

Cat2, google "naca 2408 airfoil", or most any other foil series, and you will get an information overload "airfoiltools.com" is a very useful site, but there are many others. Most of them show a profile, and let you download cut files. You can spend days studying the options.
I also used foam blanks as my starting point on my last set of foils, but by the time I added compression ribs and finished covering, bagging and fairing them, I am not sure they saved me much time. The boards did come out pretty accurate, especially the leading third where all the action happens. It is really hard to stall them as a result, and I expect the lift to drag ratio is good also.
Have fun, B

 

A typical asymmetric board is the NACA 63-412.
We have been using it on the C Class for many years.
If you build a double ended molds, you can reverse the laminates and build both boards out of a single set of tooling.
Coordinates and gif renderings are published at the http://www.ae.illinois.edu/m-selig/ads/coord_database.html#N
It's a mother lode of airfoils.
SHC

 

more options

Thanks Steve!
Cat2, The C-class guys are probably way ahead of most people (and their boats) in board and foil development, and knowledge. Pay attention.
I couldn't use a board like the 63 series because I had already built my trunks , a mistake on my part. However, the simpler series foils do work, and more important fit in my "mistakes".
I am building a new set of floats, and my new trunks will be adaptable! The 24 type profiles or something similar will probably be a better fit/performance compromise in your trunks than some of the higher performance foils, and according to some engineer friends, much more forgiving of errors in profile construction. Mine are certainly not perfect, but work well.
On another note, the local Reynolds 33 recently broke of its boards at about 22 kts- and it was not even all the way down. Make yours STRONG!
B

 

NACA 63-412 doesnt seem to be on that site. The 63 series seems inordinately 'fat' to be used for Cat foils.

Any extra info would be appreciated

 

How does one construct such a fine trailing edge in composites, kevlar? carbon micro fibres? or maybe embed a SS strip and file sharp!

 

Sharpened trailing edges are a no-no here. We have a thriving lobster fishery with pots often laid in dense clusters which are impossible to avoid with a boat 7 metres wide. Imagine the carnage it would create if we hit one at 15+knots. We have 30 cm thick foam crash blocks which are regularly called upon to do their job, but they would not help if we had a knife edge at the back of the dagger board. From what I have read the front 30% of the dagger does all the work and the extra turbulence from a rounded or even squared off trailling edge makes little difference. Any input would be appreciated.

 

Low drag and other sophisticated sections have to be accurately made and highly polished if they are to benefit you. I doubt if you'll have the finish on your boards to match a C class cat.

Usually you can finish the trailing edge about 3mm thick, but cut the end at 45deg, not straight across. That should stop any vibration.

I assume you have already upgraded your sails and use the best cloth available.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Cut off the back 50-75mm top to bottom and then slice it again fore and aft, epoxy the two halves back together with a piece of biaxial carbon sheet in the sandwich, reattach sheath and fair.

 

The NACA 63-412 really appeals to me with a proven history on cats. Sadly the 100% width is just too fat for my trunks but 90% works well. Please can one of you very smart people advise if this would be an acceptable option.
I have been overwhelmed by the response to my previous request and I thank every last one of the respondents for their input. Since I am now close to finalising my research I cannot wait to get started on the construction and I will report on the success or otherwise.