I have had quite some enquiries in the recent years regarding the use of a RHM pendulum for the outboard rudder of a double ender or Colin Archers type of boat. After a short investigation it became clear that the RHM pendulum system cannot easily be fitted on that type of boat.
On a boat with a transom it is quite simple. Axis a-a is the horizontal pendulum axis and b-b the vertical axis. For a normal mounting there is always an arm X available and with a chosen position of the pivot point P there is a suitable ratio of X/Y possible. Furthermore there is no problem for finding a position for the connection points D on the transom. The angle between the restraint lines and the heart line of the boat can be 50 degrees or more. (The nearer to 90 degrees, the better)
This system is very suitable for the outboard rudder versions of f.i. Westerly's, Moody's, Sadlers etc.
I am already sailing with a RHM pendulum for more then 25 years.
For a short impression of RHM (in combination with a USD vane) see : http://www.youtube.com/watch?v=I-2GxsrSmo0
On the double ender as shown on the sketch, the intersection point of the axis a-a en b-b is very near or just on the rudder axis c-c. There is no arm X and so the RHM principle will not work. Because of the narrow stern sections the angle between line P-D and the heart line of the boat is smaller compared with the boat with a transom and hence the forces will be much higher.
Solutions may still be possible, when the axis a-a is lowered and/or the pendulum axis b-b is set further aft. Also the restraint lines can be guided by a mounting so that its angle to the heart is more favourable. It can be an interesting challenge to find a solution.