Feathered keel

[Lafayette Bruce]

While studying foiled rudders, I read somewhere that a real no-no was a blunt trailing edge.  Only makes since that turbulence and resistance would result.  Then why is the trailing edge of the keel so blunt?  About 1.5" wide with a radius all the way down the back.  Has anyone done any modifications to the keel edge and seen improvement? 
I figure they are made from the factory that way because they are easier to handle with less risk of damage.  But once on the trailer or in the water nothing "should" be hitting that edge.  You don't hear of anyone running aground in reverse.
Would like to know your thoughts.
Lafayette Bruce

[MacGyver]

It is actually because the materials used to make that part cannot form that type of edge without being very fragile.
In a mold.process this would be even worse as the laying of the glass wouldnt run that sharp of a edge.

They spray the gelcoat and then do the glass next.

Mac

[Billy]

I've ran aground in reverse  

We were camping one night with our stern to the beach in about 6 feet of water. As the tide was going out, the back edge of the bottom of the keel would slightly pound the sandy bottom as waves rolled in.
I went into deeper water immediately and reset the anchor. But that wasn't the 1st and I'm pretty sure it won't be the last.

[NateD]

What I found in making a foiled rudder (http://cpyoa.geekworkshosting.com/forum/index.php?topic=5072.0) is that the "cleaness" of the trailing edge is probably just as important as the bluntness. Theoretically a blunt trailing edge does create turbulence, but so long as the edge ends cleanly, it isn't as bad. A wide rounded trailing edge with a few dips/bumps caused a lot of vibration in my first attempt with the rudder. My gut says fairing out the trailing edge of the keel isn't going to get you any measurable speed benefit. The extra drag created by the increased wetted surface area would offset any tiny gains in efficiency. However, it would move your center of lateral resistance astern, which should help with the weather helm slightly.

Just for fun, here's a picture of the trailing edge of the CP23 keel.

[Bob23]

Correct me if I'm wrong, but moving the center of resistance astern would increase the weather helm, eh? When I built my wood foiled blade for my 23, I moved the leading edge of the rudder forward about 2 inches. This, and the foiled shape really lessened the weather helm. My big mistake was making the blade too wide. Maybe this winter I'll experiment with some different shapes.
bob23

[Lafayette Bruce]

Hmmm  Don't think I have ever beached her stern too.  Since I am on lakes, tides are not an issue, although passing powerboat wakes can cause some bobbing.  I could roll the bottom edge up so there is not a sharp corner.  Trailing edge would not have to be knife sharp edge, maybe 1/4" wide.  Existing trailing edge is more like 1.5+ inches wide and just seems like a turbulence maker to me.  Extending the keel by a few inches (thinking 4 or 5) is only a small percentage of the overall keel length and it is not moving like the rudder.  Still think it will make that much weather helm difference?
Lafayette Bruce

[NateD]

Correct me if I'm wrong, but moving the center of resistance astern would increase the weather helm, eh? When I built my wood foiled blade for my 23, I moved the leading edge of the rudder forward about 2 inches. This, and the foiled shape really lessened the weather helm. My big mistake was making the blade too wide. Maybe this winter I'll experiment with some different shapes.
bob23

I believe that was because you moved more of the surface area of the rudder forward of the pivot point, which increases leverage while underway (the water rushing by helps to push the rudder to the side). Thinks of it the opposite way, think about moving sail area. I can tell you on my 23 that sailing with just the main in heavy air leads to very strong weather helm, unroll some jib and things get much better. Moving the center of effort forward reduces weather helm. So the inverse side of that would be moving the center of lateral resistance aft.

From the Wikipedia page on Center of lateral resistance (http://en.wikipedia.org/wiki/Center_of_lateral_resistance):

The fundamental cause of "helm", be it weather or lee, is the relationship of the center of pressure of the sail plan to the center of lateral resistance of the hull. If the center of pressure is astern of the center of lateral resistance, a weather helm, the tendency of the vessel to want to turn into the wind.

I'm no expert and I could be wrong, but I think that means extending the keel aft would move the center of lateral resistance aft, which would reduce weather helm.