Pro - Wing Symmetry - This is the HUGE pro... so correct me if I'm wrong... because I depend on it below. It is my understanding that because its on the bottom of the hull, the wing acts like its on a wall of symmetry (as compared one that has to go through the water surface). In my simplistic mind... the ventilation can not get to the rudder! THUS - Its aspect ratio is effectively doubled. If I am correct, then the rudder can be make with an AR = 2 (benefiting structural) yet still act like an AR = 4 (benefiting fluid-dynamic). ???????????????
Pro - Structural - So here is the back of the napkin analyis using fiberglass tow. I'd probably do a finite element analysis. Besides strength, I'd want to check resonant frequencies of the structure (including the boat). Wouldn't want it to flutter itself to death at 30 knots! 
- Rudder area - I'm not sure if I'm at the point of determining proper rudder area needed to efficiently support lateral loads AND have some extra lift available to actually turn the boat. So I'm assuming your numbers below already do that. At AR=4, you have: 0.7m * 0.7m/4 = 0.12 m^2 (190 in^2)
- Assume rectangular plan view. Although some kind of eliptical or tapered plan view would benefit both structural and fluid-dynmic issues.
- New rudder dimensions @ AR=2. Length = 0.5m (20 in) Chord = 0.25m (10 in) Thickness = 0.025m (1 in)
- Deflection using simple beam analysis. I get a lateral deflection of 8 mm (0.33 in) at the rudder COE. The tip will be more than double that. Too lazy to calculate it... even if it is... say 25 mm, that doesn't seem like too much deflection to me.
- Bending strength safety factor is nearly 6!
Pro - Fiberglass - I happen to have a spool of fiberglass that I don't consider out-of-pocket.
From: willoughby_rick <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Fri, August 13, 2010 3:01:27 AM
Subject: [harryproa] Rudder Ventilation
Dennis
It is poets day here right now and my thoughts are already onto the weekend activities.
I have been thinking about the difficulty of managing rudder ventilation. Fencing is one method and I have another idea to try without using fences but with the talk about mast bearings I started to think about through-hull rudders.
If you use rudders 0.6 to 0.7m deep they should only need to be about 0.1m chord length to develop the required power. They will certainly turn you if they cannot completely cancel leeway. With that in mind I was thinking you could build a well into either end of the hull that goes from bottom to deck that can accept a drop-in rudder bearing block.
I designed something similar to this for a drop-in pedal drive leg although in this case the well was within the cabin:
http://picasaweb.google.com/adventuresofgreg/ExpeditionBoatBuilding?feat=flashalbum#5378098100766812562
The block that takes the shaft
tube is moulded in chockfast that sits inside a matching CF well that is higher than the waterline. This shows how neatly the block matched the hull:
http://picasaweb.google.com/adventuresofgreg/ExpeditionBoatBuilding?feat=flashalbum#5377035108790569554
And from inside the cabin:
http://picasaweb.google.com/adventuresofgreg/ExpeditionBoatBuilding?feat=flashalbum#5378098468295589570
You would make rudders as blades that slide down into a slot within a swiveling tube. That tube would be supported by bearings in a drop-in block that mates to the well.
A rudder that is ventilating has about 30% of the lift of one that is not ventilating. So there is big benefits in avoiding ventilation.
A well in the hull may actually simplify the rudder mounting and reduce the bits that need to hang off some mounting point. It reduced the forces at the bearings because they are as close to the blade as you can possibly get.
The forces
on even a small blade like this are not trivial. If you ever manage to get to 20kts each of them will be capable of generating 2400N. The force will ultimately be limited by your righting moment but it still ends up being significant at the rudders Maximum lift occurs at 6 degrees so not much room for wobbling rudders with floppy supports.
You may never get to 20 knots but if you do not design for it in the first place then you could break lots of things in a thrilling way before it all gets robust enough.
Rick
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