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Bruce King's Favorite Keel and Rudder Profiles NACA????

Doug177

Member III
Just wanted to know what profiles Bruce used most often on Keels and Rudders?

From a quick measurement it looks like the profile on my E35-3 rudder about 6 inches down from the top is:

Chord = 24 inches Maximum thickness about 4.125 inches about 1/3 of the way back (8") from leading edge which works out to an NACA0017 Section. This seems just a tad fat.

Doug
 

Kenneth K

1985 32-3, Puget Sound
Blogs Author
I haven't seen the term NACA airfoil since back in college. I think it was NASA's early attempt at adapting airfoil design to the "new" computer era. For any chord line, max camber and location of max camber, you could have a computer spit out the possible pressure gradiants for all the possibilities of those three variables.

The NACA stuff, from what I know, was all based on airfoils, using the density of air as part of the computation. I don't know, and I've never read whether the theory would still be valid for foils in water, or if it was common for naval architects to use NACA specs.

But at first glance, with the much greater density of water (and the much slower speed in use) it would seem like you could get "more bang for your buck" by using a thicker foil, as the water would likely stay attached at higher angles of attack (compared to air). Also, due the much greater chances of hitting foreign objects in the water, a thicker foil would do a lot for added strength. So, it kinda makes sense that marine foils would be thicker than air foils in general.
 
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Doug177

Member III
Ken you are right....they are designed for low speed airfoils

The NACA shapes used in sailboat keels and rudders are the low speed ones used for "air" foils. I don't know why, as our speed at 7-9 kts is really slow.
Maybe it is because they work in the thicker fluid (water) too. It could be that the America's Cup folks don't share data. I really don't know why airfoil sections are used.

From reading around the web, it seems like most sailboat designers use NACA0012'ish sections for keels and the fatter NACA0015'ish sections for rudders. The fatter foil is used for rudders as they are turned quite abruptly when steering and fatter keeps the flow attached and going for a longer period of time before stalling out.

I have an S2 7.9 that I race quite a bit and the keel is NACA0011 with the rudder being NACA0015. The 7.9 has a rather narrow sharp entry with a big butt

The Keel Section's from Computer Keels for the E35-3 are NACA0012 (actually 11.7) so that is a bit fatter than the S2 7.9. Perhaps because the 7.9 is more easily driven.

I have not found anyone who does optimized sections for rudders, so I have no point of reference. I think rudder sections are important however and should not be an afterthought.

It could be that the true racing fanatics out there don't talk much about go-fast modifications. Sigh.

Doug
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
"Nice Fins" in 1988

Here we are at a haul out in 2013.
Notice the elliptical rudder and keel. To my untrained eye it looks sort of like the wing section of a Spitfire!
Points well, is fast, and has minimal cross sectional area to bother or impede the water molecules.
OTOH, I admit to not understanding the whole foil discussion all that well.
:nerd:

The 3 blade prop is inefficient under sail, I know.... but everything's a compromise....
 

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Doug177

Member III
Loren...The eliptical does look sexy...underwater Spitfire

A good basic discussion of foil sections is at the link below:

http://www.fastcomposites.ca/site/m...ing_wp_cron=1544371105.0147399902343750000000

Perhaps the thicker rudder section on the E35-3 was for strength. It does seem very strong.

I have been trying to balance out my boat starting with above the waterline and then looking below.

Just getting the mast to have the recommended one inch of rake aft at the masthead has been something of an ordeal.
I have had to move the step all the way forward in it's slots and haul the mast all the way back and touching in the mast partners at the cabin roof.
With all that, I am just barely perpendicular. The next step might be notching a bit at the partners. No room for wedges at the aft edge, that's for sure.

The PO's had the mast raked forward. That was just a bit too avant-garde for me.

Doug
 

supersailor

Contributing Partner
Doug,

I have a 34-2 which shares the hull form with the 35-3. I have had to do some repairs on the leading and trailing edges. The glass covering the foam on the rudder is actually quite thin. The bulk is foam to give it the shape the designer wanted. Don't use the rudder as a depth sounder unless you want to do a bunch of repairs. Should you ever need a new one, Foss Foam can make one for you.
 
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Kenneth K

1985 32-3, Puget Sound
Blogs Author
From reading around the web, it seems like most sailboat designers use NACA0012'ish sections for keels and the fatter NACA0015'ish sections for rudders. The fatter foil is used for rudders as they are turned quite abruptly when steering and fatter keeps the flow attached and going for a longer period of time before stalling out.

Great info Doug! Didn't know that.

I have a 34-2 which shares the hull form with the 35-3. I have had to do some repairs on the leading and trailing edges. The glass covering the foam on the rudder is actually quite thin. The bulk is foam to give it the shape the designer wanted.

To Bob's point: My rudder didn't survive the 500 mile trailer trip unscathed. At least we think it happened during shipping--neither the surveyor or I saw it during inspection. The glass skin over the foam is thinner than you'd think. That's probably why Ericson recommended covering the rudder with a sunshade during haul-out. I did see rippling of the glass skin on the sun-side of the rudder when I left it uncovered on-the-hard.
20 (4).1.jpg 22 (2).1.jpg

So, we changed the shape of the leading edge slightly, but at least I don't worry about splitting anymore. Sails fine.
 

bgary

Advanced Beginner
Blogs Author
I don't know what profile Bruce King used, but I know that in the early 80s many of the designers discovered the beauty of the NACA work, especially the "four digit" series.

Doug Peterson was a leader in this area. He dove deep into the math, and (IIRC) used profiles 0012 and 0018 a fair amount. Maybe 0009 as well.

Somewhere I've got a link to a calculator where you can put in different parameters (span, chord, sweep angle, etc., plus NACA profile) and it'll model how the keel will perform. Kinda cool.

Bruce
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
I recall hearing that Ericson had a test tank in which they tested all their models before finalizing production plans. Apparently the triangular Delta keel that's so common on our boats was determined to be faster than other shapes. I've been puzzled as to why no other builders have used them to my knowledge.
Frank
 

bgary

Advanced Beginner
Blogs Author
...triangular Delta keel ...puzzled

As they got better at modeling the performance of different shapes, they found that a longer "root" (the top of the keep where it meets the hull shape), the more drag there was. So the later designs (both Peterson's forward-raked trailing edge, and the later elliptical plan-form) were attempts to maximize lift while minimizing root-drag.
 

Christian Williams

E381 - Los Angeles
Senior Moderator
Blogs Author
Here's a thread from Sailing Anarchy (the sailing forum that never grew up), which contains Bob Perry's usual attitude on things, and -- because SA actually has a core of brilliant and knowledgeable dudes --may amuse whilst shedding light, or sparks.

It ain't rocket science (well, OK, it can be). A DC 3 carried loads and flew slow with its big fat wings and got the job done. An F-14 has sexy svelt wings but falls out of the sky oops all of a sudden if the "wind" dies. Sex appeal in foils is overrated for my purposes.

http://forums.sailinganarchy.com/index.php?/topic/124211-shaping-a-keel/
 
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