Thanks for both these replies. Our very limited experience with Delphine is that the rudder doesn't stay down very well by itself - at least, not while we're sailing - so we may have to keep the downhaul fastened until we're absolutely ready for the rudder to come up.
Our experience with the auto-cleat on Miss Fidget has been slightly mixed. The auto-cleat is adjustable, so it can be set to release at different loads and we found that if we had the setting too low, the rudder would sometimes swing up in deep water under the action of waves. We'd know that this had happened because we'd suddenly have significant weather helm! This could be resolved by increasing the release load, but it's then harder to click the cleat back in to place after it has released. However, we liked the peace of mind of the system and it was very easy to pull the rudder down again (we had a 3 to 1 purchase on the downhaul) if it released when it wasn't meant to... I think the rudder downhaul line on Delphine is probably 6mm too, but it's expanded with time and won't jam properly in the auto-cleat.
Best wishes
Gilliane
Delphine, Parker 275, no. 41