Updating a keel-hung rudder

Dear fellow Boaters,

I am currently refitting a Endurance 35 Ketch built in 1984 in Ferrocement.
Unfortunately the keel-hung rudder got too salty after 40 years, so the plywood
core of it delaminated and is soaked in salt.
On the bright side, the connection of the rudder to the shaft and the pintle
where made out of stainless, so I will be reusing them. But the rest of the
backbone was built with mild steel, so it got pitted badly.

Long story short, I have to build a new rudder. Having done that a couple of
times before, I opted for a ribbed lasercut stainless design for the construction.

So here comes the question:
I stumbled upon this image, showing the difference in shape of keel-hung rudders over time.

My rudder has more or less the first shape. So, I was thinking of modifying the shape to the
second image. But unfortunately I have found no information about how to go on about this.

First thoughts on this:
-Increasing the surface area will increase drag and steering forces.
-Distributing out the surface to where it has a bigger effect during sailing (lower),
will reduce the surface in the area of propwash, thus reducing maneuvering abilities
in close quarters and low speed.
-A straight trailing edge would make it simpler to design and fabricate.

So the question is, how to go about the surface distribution, in order to create a positive
effect in both situations. I assume that it is contradictory, but I hope that someone has
been confronted with this problem before and could share some insight on the procedure.

Also, the shape of the prop-aperture leading edge is an area where i think I could improve.
The original design has a bent 60mm diameter tube in the leading edge. I assume that I can
decrease this diameter, while keeping a round profile on the edge.
The question here is if this would really result in an advantage, as this area has no laminar
flow during sailing, due to the turbulence created by the resting prop. Reducing this diameter
would make the fabrication way more complicated than keeping it that size.

I am really looking forward to hearing your input and thoughts!
Best regards, MM.

 

Just to clarify what I am thinking of in terms of shape, here's another example of a
modified rudder shape.
This is the original shape:

And this is the modified shape:


And here's the preliminary design:

 

Not one single reply...?

That's kind of sad.

 

Extension of the lower rear angle make it more damage prone . If the boat reacted poorly to the helm, enlarge it.

 

Really two things
Did it steer poorly before…if yes then adding area can help
But adding the area will increase the loads on those reused parts….will they still be strong enough?

 

The rudder on my 16' plywood dinghy was built like that. It is ridiculously weak for a boat that size. Find an 6' length of 3" x 3" steel bar and attach it to the bottom of your keel. Let it stick out the back about 6 inches. Bore a 41 mm hole to take a 40mm shaft, and weld that into a 40mm heavy wall pipe. Connect the pipe pieces together with at least 25mm worth of plate(s) that has 10 inches minimum on the flat. The rudder on my 38'er had just under a 4" tube, but it didn't have a bottom bearing. Seriously, those are dingy-sized gudgeons. You want to be able to drop the back of the boat 2' onto concrete and have all the damage be to the ramp.

 

Is there a problem you are addressing? If it work well before, I recommend not fixing what ain't broke.