The Next Rudder Blade

I was in a hurry when I made my rudder, so I just threw something together that I thought looked right. 

And now I have a really nice piece of cedar that Godfrey was going to use for his rudder, which he gave me in exchange for some glassing I did for him:

As you can see, it has a big chunk out of it. G was making a balanced rudder, so that nose sticking out would go under the boat, making steering easier.

But my rudder has to be straight down, as the skeg extends straight down from the transom. So, what do do. A really nice piece of wood, the time and sometimes the energy... So I went back to read—really read, this time—the article about rudder blades (pdf file) that I just skimmed last summer.

Now I understand that the outer dimensions of a correctly foil-shaped blade are: Thickness of board = 12% of its Length (of chord, or fore & aft section). So to really know how much of that rudder to use, I only needed to see how thick the plank is. It's 1¼" thick, so I calculated that the blade needed to be 10.42" wide. Which worked out well:

I just sawed a line parallel to the after edge and 10.42" away from it, cutting off that nose that I couldn't have used anyway. John's bandsaw.

This blade is 6½" longer than my current blade and 1½" narrower. According to the author of the article, J.R. Watson, the foil shape will add a performance boost to the rudder. He says:
"A section that has a faired airfoil shape is preferred over one that is flat with parallel sides. This is because the airfoil shape produces lift when moving through the water, thereby counteracting the sideward forces exerted by the sail rig. A flat section produces less lift and at a great expense of drag, slowing the boat and making it more difficult to steer."

Using the above-linked article for reference, I made my own NACA 0012 foil shape to those dimensions in my Illustrator program, to print onto 8½ x 14" paper, which will then be glued to foam board, cut out, and used as a template to get the foil shape right. Or more-or-less right.

The author says that solid wood is good for blades, but plywood laminations are not, because the layers tend to shear apart. And he says that cedar is the best. So I'm making a good start. I'll glass it after it's shaped, of course.