8 knots is pretty slow for a 40' boat, I've done that numerous times with my 25'WL boat! Even hit 11.1 water speed. Not sure what my over the ground was that day, with a upwards of 2-3 knot current, 14 knots in a 28' on deck boat, equal to a C28mkII! You has 12 more feet! hmph!
that was before the "here hold my beer watch this moment!