Actually, varying the diameter is useful. If you cannot vary the diameter, there is no way to tune the system, and you will steer a worse course causing more wear and chafe to the
sails, a less comfortable ride and also inefficient
steering. If you are making your own, you should consider 2 or 3 diameters, or attach the lines to the
rudder before the wheel linkage somehow. Also you really must be able to vary the relative offset as well, so that you can adjust
weather helm for broad reach.
In light
wind and closer to the
wind you would be able to use a larger diameter, and downwind and stronger air, a smaller diameter.
On a tiller you can vary the angle of the tiller, or where the lines attach. I know many claim that one position works all the time. But you can't steer as far off the wind with the headsail poled out without occasional backwinding, you won't be able to steer downwind in 3 knots of wind, you won't steer as efficient of a course. It's like saying you can sail on a reach with the
sails sheeted in all the way, sure it works and I see this a lot, but it isn't optimal.