SeaJay,
I have done a couple of rudder-in-drum designs, one for Project Amazon which worked very well, and one for Saint Barbara which you can see at the following link:
http://www.sponbergyachtdesign.com/SaintBarbara.htm
On Project Amazon, the drums were 1 meter diameter and the chord length of the rudder blade 600 mm. This put the drum axis just ahead of the center of lift on the rudder blade. These drums were expensive to build, in aluminum, with carbon fiber blades (the blades were 3.695 M tall, over 12', and they weighed about 66 kgs, 145 lbs.). The bearings top and bottom were Teflon plastic strips 50 mm wide and wrapped around the inside the cylinder and on the outside of the drum, recessed appropriately. The top flange was a locking flange to keep the drum from moving up and down, and the insides of the mating surfaces were also lined with Teflon strips.
The design of the Project Amazon drums were laid out with the idea that, if warranted, the geometry and sweep of the rudder blade could be modified and still fit inside the drum. That never happened, and they worked fine as designed. There were also two completely independent hydraulic steering systems, each connected to their own drum, and the drums also had a tie bar between them so that only one steering system needed to be operational for both rudders. Should the hydraulics go down, the drums could also be connected by cable to the steering wheel and then be steered by cable. It was a really neat double-redundant system.
On Saint Barbara, my thinking distilled to having a drum only slightly larger than the chord length of the rudder blade. There is some cant to the rudder relative to the drum, so the center of lift is behind the turning axis of the drum. The bearings, as originally designed by the owner, are plastic ball bearings. The upper bearing seems to work fine, but the bottom bearing has some drag and roughness to it. The rudder drum and bearings are going to be rebuilt this winter. As for balance, the rudders seem to work fine.
Saint Barbara's rudder does not have a drum per se, merely a form-fitting cassett with top and bottom flanges that end in bearings. This was the owner's design, not mine. As a result, there is always water in the bottom of the opening as it sits below water level. This boat sails in Lake Michigan, so it's fresh water.
I should note that the owner of Saint Barbara is a University of Michigan engineer, and he delights in conceiving of ways to solve problems, build them, see how they work, and refine them. So I am responsible for all the lines on Saint Barbara, and the owner worked out the structures for himself with oversight from me.
On both Project Amazon and Saint Barbara, the hull is essentially dead flat right in way of the rudder so that the bottom surface of the drum does not cause any disturbance to the flow. On Project Amazon, both the drums and rudders are perpendicular to the hull surface. On Saint Barbara, the "drum" (cassett, actually) is perpendicular to the hull, but the rudder blade is swept aft about 7 degrees relative to the drum. On my new design, the Scandinavian Cruiser 40, we will also have a rudder in a drum. Again, the hull is essentially flat at the rudder, and the rudder and drum, right now, are coaxial and set about 6 degrees aft cant relative to the surface of the hull. However, due to the slope of the hull at that point, the rudder still has an aft sweep to the water. I may rethink this somewhat and make the drum of such a shape that I can alter the sweep of the rudder for adjustable balance.
As for off-the-shelf components for DIY construction, there aren't any to my knowledge. You just have to cobble it together from what you an ingeniously apply. Every rudder for every boat is different, so there is little will to standardize designs and parts. You just have to figure it out.
Eric
