RC Yacht Keel

[mojounwin]

Hi Guys,
I recently brought a 750mm R/C yacht She gets upto about 4-5kts but the big problem I'm having is after tacking or when I hit some chop the boat loses so much height because at the slower speeds the keel is unable to produce any lift. I'm thinking of reshaping the keel to increase lift at the slower speeds. What NACA profile would people recommend? Also should the rudder be a simlar profile?

Cheers
Mike

[JesperW]

Ruddes experience much higher angles of attack and thus are more sensitive to stall. Most people do not use the same profiles for rudders because of this.

I would look more at area, aspect, taper and sweep than esoteric profiles in your situation (given that your current keel has some reasonable profile like a 4 or 6 digit std type). 2D predictions of drag buckets and such are notoriously inexact when they hit real 3D waves...

The fastest RC keels use 6% to 7% thickness/chord ratio sections. You might consider a Sails etc.(Graham Bantock) fin -you can check Great Basin in the US. There are some individuals making excellent carbon sections -you might find reference to them on the AMYA site(www.modelyacht.org) or google Lester Gilberts site. Bantock uses proprietary sections designed espescialy for rc model yachts. Because the narrow sections are so much less drag they are used on both the rudder and fin.

[markdrela]

I recommend airfoils which are currently used on the latest RC glider tails:
http://www.charlesriverrc.org/articl...t-airfoils.htm

The HT13 or HT14 airfoils should work well for a small RC sailboat keel. The HT12 should be ideal for the rudder.

These airfoils will operate properly no matter how low the Reynolds number is (no laminar separation, drag is low, lift curve remains linear, etc). So they will still work OK in vanishingly light air when the boat is barely moving.

PS.
If you're hand-making your keel and rudder, these airfoils can be accurately made by hand using the "tangent sanding" method, without templates. Here's the procedure explained, and data for the HT12 (scale the slopes for the HT13 or HT14):
http://www.charlesriverrc.org/articl...egrolite2m.htm
"Allegro-Lite 2m tail shaping data and diagram"
"Accurate shaping of solid balsa surfaces without templates"
"Tail Sanding Sequence"

Thanks for that info, Mark!

[Tim B]

Can you start to eliminate a few issues?

1) you say the same thing happens when you hit some chop, that would suggest the the hull-lines at the bow were too full, or the keel too sharp.

2) you may actually be stalling the sails as well as the keel, especially coming out of tacks, and it may pay to ease the sails as you tack to keep the power on.


If it obviously isn't a hull or sail problem, then certainly change the keel/rudder.

Tim B.