Rudder Shape. Building a Better Rudder

Elliptical was all the rage for racers in the 80s to 90s. Now we're seeing some extreme shapes with tails.  Is it fashion of function? Maybe both.

First, defining the project for my boat and the problem:

• I don't want to replace the rudder with a transom hung upgrade. I like the swing down cassette for aesthetic reasons (I'm weird), the transom would require modifications to mount one, it would interfere with the boarding ladder, and there is nothing mechanically wrong with the rudder or cassette.
• I have added an anti-ventilation fence to help with high speed ventilation. It helped, but there is still not enough control in some sea states. This was designed and speced by the designer of the boat, Ian Farrier.
  
• Although the rudder cassette is OK, I am mindful of not adding much more bending moment. The bearing carrier is DWV PVC, is sunburned, and can't take much more. I have replace the bearings. I also navigate some very shallow waters. I do not want to increase the draft. [The rudder in the F-24 MK1 is mounted in a cassette at the transom that is rotated up with a rope to lift it out of the water. When lowered into possition it functions like a spade rudder.]
  
• The rudder feel is very light. In fact, the rudder is so over balanced I can often let go for a minute upwind in good conditions. She just goes straight. I'm calculating ~ 22% balance, and I think something in the high teens would make more sense.

The core of the problem is that the F-24s have low volume amas that are very prone to driving down off the wind. If there is a steep quartering sea, they like to bury, followed by the stern lifting and the boat immediately rounding up. When the bow buries it becomes like a new centerboard forward, moving the COE and making it impossible to turn and bear away. Monohulls and catamarans do this, but it is many times worse on a trimaran, in part because of the fine amas and in part because of smallish rudders. 90% of the solution will always be balancing the sail plan, and bearing away preemptively and surfing away (really fun, by the way), just before the bow buries, but a little more control would be nice.  I really don't care about racing speed or class rules, but I do like a boat that is fun to drive.

As I said, adding a fence helped. Now I am considering changing the rudder plan form slightly. The Spitfire elliptical profile has proven strengths, but within the constraints (length and ventilation near the waterline) there is probably room for an upgrade. Ian speced out the anti-ventilation fence I have and he changed rudder designs for other models over the years.

The Dragonfly rudder seems pretty extreme, but they're smart guys. Jeffa uses a more squared off profile. There are many examples and I don't really want to go too far down the design rabbit hole.

 

 

 2024 Dragonfly 28. a pretty extreme tail. This would drastically affect balance and would affect the shaft placement. I'm not interested in creating a heavy helm.

 

 

 Jeffa Rudders cruising designs.

 

 

My current idea. I have not decided how much area to add. Call it option A and option B.

 

This also seems more in line with F-24 Mk II (top) and F-28R (bottom) rudders, which are known to be more effective and less prone to ventilation and loss of control.

 

 

 

 

What about just adding a little area down low and aft? This should decrease the balance to about 17-18%, where it should be, increase the turning force about 5-8% in calm conditions, and perhaps 10-15% in tough conditions when the upper portion of the blade is ventilating.The area is added to the deepest part, where the water should remain air-free.

An apparently successful and slightly more radical F-24 MK1 rudder modification.

I'm pretty good with fiberglass, and I've built paddles and dinghy rudders before. The fairing will take some figuring, templates, and some time, but it really is a small area. Seems like a fun project. I like tinkering.

Thoughts?