Locking the prop

I usually sail with the prop locked (by putting it in reverse gear - fixed
2 
bladed low tech prop on a sail drive). This avoids the annoying noise of
the 
gearbox turning over and is something that a lot of sailors do. However,
the 
handbook for the Volvo MD 2030, says to leave it in neutral when sailing, 
and several people have told me that I am significantly increasing drag by

locking the prop. What do the URS gurus recommend (Please don't recommend
a 
change of prop, as I want to stay on topic.)

-- 
Duncan Heenan
(Speaking personally)