I have only seen one commercially available fin list its airfoil.
Its from Anders wallin.
It lists NACA 0006 ( 6%T/C)
From what I have read this is a common foil for fins and rudders.
Sailsect lists fins 6 ,6.5, and 7 % T/C , but no airfoils.
Does any one else have Info on airfoils/ sections?
Robert
I think you are after who uses what but you may find this useful. It’s a program called Profili2. It was written for model airplanes but has it’s uses. It has over 1000 aifoils, about 150 are fully symetrical. It has all the graphs and charts that I don’t really understand. It works with Reynolds numbers in our range(for gliders). You can import new sections. You can print sections allowing for skin thickness to make blanks. I costs about $15 as a suggested donation. You need a key from the owner but I don’t think he insists on payment. I think it’s a great tool and if someone understood the graphs it would be even better. Maybe I could even find this “drag bucket” I hear about. Just Google “Profili2”
Don
Thanks Don
I was just looking at it the other day and it looks great, I have used compufoil , but this has way more features.
I wanted to make templates for a discus launch glider wing, but I also want to make templates for fin plugs for making another mold.
I’ve been poking about with Profili on this issue. It looks to me like the optimal technique, at least for the smaller boats, is to use a thin foil (5%) and make the chord (fore and aft dimension of fin in this case) 50 percent wider. This will give you a fin with the same or less drag than a narrower chord with a thicker section, plus it will give you less leeway. With a thicker section, you can afford to use a narrower chord, but the Reynolds number is worse.
I think the crossover where it’s no longer worth it to go thin is maybe around Re=100,000. But then of course you have to decide to favor high or low speed. I suspect favoring low speed makes more sense, as you have more to lose there, and the fin would be a larger proportion of total drag when the boat is not making waves.
Profili seems to like the HT-08, though the NACA-005 or -006 appear to be ok as well. It’s probably best no to use a laminar foil like the 65-009 on the smaller boats. In addition to higher drag, there is a non-linearity around 0 angle of attack which might make steering pretty interesting in some conditions. A fix, which helps many other foils too, is to add turbulation at about 70 percent back. Profili shows reduced drag and linear lift curve near 0 angle of attack with this mod. HT-08 doesn’t need it. For thicker airfoils where the thickest point isn’t very far back, it may pay to turbulate at, say, 20 percent. Unfortunately, I don’t know how low the Reynolds number can be and still have turbulation work. So it’s probably best just to use the thin foil with a wider chord, at least if you can put some carbon or metal in it. I’m also not completely sure how much turbulation is required. Perhaps a piece of striping tape. Probably very much worth trying on existing boats.
This all assumes you can put a decent finish on it.
Thanks for that. Does Profili allow for the increased drag of water? A 50% increase in chord adds a lot of wetted surface.
Boy it would sure be nice if some rich person would finance a shipload of testing at low Reynolds numbers. Any takers, hmm, please? :sly: :sly:
Don
Profili allows you to put in the correct Reynolds number. I am not aware of any “increased drag” not covered by that. Cl (coefficient of lift) and Cd (coefficient of drag) and so on are “dimensionless” numbers which take density into account, so the numbers should be valid, as far as I know. Of course it doesn’t allow for wavemaking, which is a whole other can of worms. But I don’t think fins have much to do with wavemaking. Rudders, maybe a little if they are on the transom, but then I bet the section has little to do with it.
If the Cd of the section is 33 percent lower, then you can afford 50 percent more “wetted surface”. The neat thing is that the Reynolds number goes way up and helps you have the 33 percent lower drag. Obviously it has to be a pretty good section as well.