So far as I am aware, there is no gameplay setting that sets the radar to off by default, unless there's a mod out there that makes that change that I don't know about, which is likely
I understand your desire to have it off by default to maintain a higher degree of stealth, but from experience i can't say it makes much difference. I tried running with it off in the stock game but still got chased down by aircraft, so I just quit trying to hide that way, and besides, surface search radar is so nice to have anyway, I just had to turn it back on.