Let's have a look at the word Donal is all upset about...

chal·leng·ing ( ch²l“…n-j¹ng) adj. 1. Calling for full use of one's
abilities or resources in a difficult but stimulating effort.

So, Donal is basically denying that sailing in force 6-7 conditions can't be
difficult and stimulating. That's rather sad. I find that even force 5
conditions can fit the definition of "challenging" at times.
I'd say force 6 is challenging and force 7-8 is very challenging!

Capt RB