About active sonar...

[ppk]

I'm starting a ''what if'' campaign with the XXI in 1939, with the goal to seek out and sink as many capital ships as possible. My boat has a sonar, which I've never really used before so I have a few questions.

1- How accurate are the range estimates? When trying to figure this out by myself I found out that a first ping will give a relatively 'far-off' estimate (in testing, I got 2300m for a merchant I had ranged visually at 1500m). Then sending out multiple pings seem to drive the range down (by as much as 1000m) until it seems to stabilize closer to what I've eye-balled. So how accurate is all of this? Do I have to ping everything 10-20 times before I can consider it good enough?

2- Kinda related to the first one, but how accurate are courses plotted from sonar ranging alone?

3- Merchants seem to be completely oblivious to it, but will it spook DDs / capital ships?

4- What's the difference between ''Estimate range to target'' & ''Precise range to target'' commands you can give the sonarman? There's only a single ping button I can find when I take his seat and they seem to produce similar results.

That's it for sonar-related questions... Two random ones now :
a- Where can I find a fleet carrier on patrol? Is there a scripted event where I'll find one?
b- What's the max safe speed for the XXI's schnorchel?