Foils in scale sailboats

[mholguin]

I'm building a small sailboat. I'm working now with the appendages (keel and rudder). The keel will be aluminum, 1 mm thick, 21 cm long, 4.9 cm chord.

In my first attempt, I used flat corss sections, and I experienced poor performance, specially with the rudder which will stall easily in puffs or high winds. My UN-educated guess is that having a flat cross section, it would loose flow sooner, then stalling and loosing control. Also, the keel had a very small chord, so wouldn't generate much lift (again, I'm assuming, as I have no way of actually verifying this).

I applied balsa to the new keel, and shaped it after a NACA 63-010 foil, max thickness being 10% of the chord, placed ~35% aft of the leading edge of it.

Same line of though is going to be followed with the rudder.

Questions for gurus out there: In such small scales (boat being 65 cm LOA, 12.2 Cm max beam, 2,250 sq. CM SA) is it worth to use NAC profiles fro the appendages? Shall I expect better rudder performance?

My guess is yes, as even when the boats are scaled, water density cannot be scaled down. But, given the scale, should another NACA profile be a better approach instead?

As it is painfully evident, I'm not a designer, but very curious abot how things work. I read about Reynold numbers, but it went over me at around half a million miles over my head. Maybe in 15 or 20 years I might get.

Hope this poses some interest from you guys (it is always good to go back to theory trying to bring some sense to somebody illiterate like me)...

You can find a whole lot of information on the technical aspects of small boats by doing a search for "Lester Gilbert". His site is filled with technical detail
It's been found by people who race small sailboats-above and below one meter LOA that a t/c ratio of 6-7% is best. The sections used by the top guys are generally not publisized. NACA sections should be fine with the thix where you put it-Lester's bound to have more info.

Here it is:
http://www.onemetre.net

[markdrela]

For the rudder I suggest using a low-Re airfoil commonly used on small RC glider tails. The HT08 should work pretty well. Coordinates are here:
http://www.charlesriverrc.org/articl...t-airfoils.htm

It normally has 100% laminar attached flow, which makes its lift properties at small AoA's very nearly independent of Reynolds number. So it will work OK no matter how small the rudder or how slow the boat is moving. For the same reason it should also work well on a keel for a small RC sailboat, especially in very light air.

The HT08 is 5% thick, but can be thickned up to about 6.5% with no ill effects except a slight drag increase. However, the nice low-Re lift properties will rapidly degrade if you go much thicker than 7% or so.

[mholguin]

Thanks for the answers!

Given the recommendations for given foils, Can I assume that benefit will be derived out of using foils for the appendages? Remember, I lack knowledege in design.

Thanks for your patience!

[markdrela]

Originally Posted by mholguin

Given the recommendations for given foils, Can I assume that benefit will be derived out of using foils for the appendages?

Yes. Their 100% laminar attached flow will give the lowest drag possible. They also make aspect ratio optimization relatively simple, since attached laminar flow has very predictable profile drag behavior: Cd_profile ~ 1/sqrt(Re). One can also consider high aspect ratios without worrying about low-Re problems, since there won't be any.

The linear lift response (no deadband or hysteresis) will also give good control properties. Deadband is REALLY noticable on RC glider tails, and the new airfoils are a clear improvement over traditional alternatives like a NACA 0010. On a sailboat this should translate into a more solid heading hold, but I don't know if this will be noticable.

FWIW, simple "flat plate" airfoils with rounded LEs and wedge TEs have OK lift properties, but much higher drag.

[markdrela]

PS

These links describe the "tangent-sanding" technique for shaping the airfoil in wood or other sandable material. This is a good way to make an appendage or a mold plug for an appendage if you don't have CNC or hot-wiring capability.

http://www.charlesriverrc.org/articl.../tail_sand.pdf
http://www.charlesriverrc.org/articl...il_shaping.pdf