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.

 

Well, pun intended: The rudder IS broken.

The construction is Stainless steel on the contact points: lower bearing and connection flange.
The rest is plain steel and plywood, coated with glass and polyester.

The plywood is de-laminating and the steel parts are pitted.

So I'm looking forward to re-fabricate the rudder.
Since I have more skills and experience in steel than composites,
I am planning to build it out of stainless steel sheets, lasercut
and weld them together.

Unfortunately the original shape creates a bend in two directions.
That's pretty impractical when using sheet metal. So a shape that
is more or less a trapezoid, is way easier to build, without going to
the English-wheel (that I don't have).

I guess it needed some explanation, why I am walking down this rabbit-hole...

I think I'll need some math to distribute the curved surface to a trapezoid,
while keeping the same performance. Or just eyeball it...?

If anyone has a hint about the math, I'd really appreciate it!
I assume that I have to map out the surface in context to depth,
and distribute it to the other shape while keeping the overall
surface pressure distribution effect the same.

Am I in the ballpark?

 

It's rotten. That's all.

I am going for NOT adding any effective area! Sry for the very rough sketch.
So if I do my math correct, there should not be any higher loads.
And the parts I want to reuse are in perfect condition.

 

Sorry, I could not follow your verbal technical drawing...
Can you post an image just for clarification please?

And I totally agree on the "impact capacity" a rudder should have!

On the other hand, a wood rudder will shatter if if hits the ground
really hard, a steel one will transmit more shock forces to the hull...

Compromises, compromises...

 

Don't modify the shape.

 

OK, so it needs to be rebuilt or replaced. I did not understand what do you envision the change in shape is going to improve in performance. The round trailing edge is more protected in a grounding. Was there a problem with the steering response? Otherwise it is good as it is. Reducing the diameter of the pipe would also require reducing the bushing at top and bottom. If the trailing edge of the keel is concave, it will cause more turbulence unless you change the radius of the keel. Bending a round pipe is easy. Boats of that era had flat sided rudders, which is easy to replicate in steel. Keep it simple.

 

Hopefully less drag. The change I described is about the section "B",

Totally agree, as I posted earlier, I am going to round off the tip.

No issues with steering response. The design change is more about ease of fabrication.
The diameter of the main tube remains unchanged, only in section "B", in the prop area,
I was thinking of reducing drag by making the overall profile thinner.
The original rudder is NOT flat sided!
It's a almost spherical shape. Recreating that would require a sheet that is curved in 2 directions.
That requires English-wheel work, which I don't have.

Hope that clarifies some?

 

Thinner is not necessarily less drag. However, the main function of the rudder is directional control. Less drag may give less control. The rudder functions by deflecting the water flow.

Spherical would be a ball. It hardly seems possible. I think you are making a simple project into a very complicated one. To start with, your boat is not a high performance racer. Steering control is the main issue. A half circle can be bent in two direction without too much effort. The center gets welded first, and the rest is bent one bit at a time.

 

I agree. Build a new rudder out of steel if you prefer but keep the same shape and design. Keep it simple.