That is close to what I am trying to say, but not quite. Relating my response to what you wrote, it is not that the propeller turns at 60% of the engine rpm BECAUSE it is the most efficient operating point, but rather it turns at 60% because the torque converter and the load are matched when the system is designed. In other words, operating at the most efficient condition is a consequence of an appropriate design, not a cause.Are you saying that for every 100 rpm the engine turns in cruise, the propeller will turn at 60 rpm because its more efficient for the torque converter?? :think:
If the torque converter does not match the load then the speed ratio under operation will not be at the optimal efficiency point but at some other point, either a higher or lower ratio than optimal. This means that one must pick the appropriate torque converter so it matches the load so the rpm ratio is at the most efficient operating point of the torque converter. It may also mean messing around a bit with the diameter and pitch of the propeller to fine tune the load so everything is matched. In a way it is a lot like matching a prop to an aircraft but a little more complicated.
As far as matching is concerned, one reasonable strategy would be to set the design point so the torque converter operates at its optimal efficiency point under cruise conditions.
Another strategy would be to choose a slightly higher rpm ratio than optimal when the aircraft is cruising. I'm not talking about a much higher rpm ratio because that would get you too far from the most efficient operating point, but rather just enough so your efficiency is down by 1 or 2%. Then when the aircraft is in take off or climb mode the torque converter will be operating at a slightly higher efficiency than if one were to choose the design point mentioned in the previous paragraph.
The best choice would depend somewhat on whether you want to give more emphasis to cruise or take off/climb, kind of like when you choose a propeller.