Technically this is a DNS and webserver configuration thing:
Pinging opencpn.org [126.96.36.199] with 32 bytes of data:
Reply from 188.8.131.52: bytes=32 time=185ms TTL=51
Means that DNS is set correctly and it's possible to resolve opencpn.org (without www.)
to an IP address. It also means that my ISP is working correctly in resolving it for me...
You should get the same if you do
The webserver config is virtually impossible to check from the user's side, it either works or not (but for opencpn.org is set correctly and www. is not needed)
If it's not working for you I would probably blame your ISP (which is probably going to ignore you if you ask them or will tell you that they do it intentionally "for your own good and safety")
Will try to remember that I have to post the links with www. at the beginning next time...