I have an open mind on this and never really seen any authorative collection of knowledge that proves it one way or another. Seems to depend a bit on where one looks. But as has been said, as far as aviation is concerned, is well proven in air that freewheeling props of the aviation type have more drag than those not freewheeling. As said by others it possibly depends on the boat, prop and drive - especially when you see so many sail boats with big elliptical bladed "power boat" type props.
But it is not something that I have concerned myself over much in respect to our own boat. We have a 3 blade
18" diameter high aspect ratio prop (on shaft, not saildrive) so the blades are no bigger than a man's hand with an inch or two of wrist attached. The worst I can imaging is that the drag, when locked, is something of the order of being similar to as if 3 men
on board took to dragging one hand each through the water as we sailed along. Not something I would panic about on a cruising boat from a performance point of view - we have no rule
on board about not dragging hands in the water
What has impressed me is just how hard it is to turn our prop by hand and if it is freewheeled it sure absorbs enough energy to get a fair speed up so quite a lot of work being done. Note, this is for a shaft drive.
While I have on a few short occasions tried watching the log comparing with the prop freewheeling and when it is locked, any advantage one way or another was lost in the "noise" of speed fluctuations from waves, wind
velocity changes, etc so I quickly lost interest. Might be different if just ghosting along in little wind
and flat sea and tried the same test, but that is when you use the engine anyway; Is it not?
Which leads to we lock it with the gearbox
just because of the noise
if it freewheels (probably worse on a metal boat especially as we have hard bearings for lifetime reasons on the prop shaft) even though the engine/box manual says to sail with the box in neutral.
I checked this out and was informally told is really no problem locking our box (is a cone clutch
type box) manual really just cos people don't know how to sort out the box jamming in reverse on the clutch
cones. (Note my comment on that applies ONLY to my gearbox
and to me, and I ain't telling you what that is in case yours goes wrong and I get the blame
- ask your own gearbox's manufacturer or representatives whose advice should be followed in my view).
So there you go, all those words and no real answer from me
. Think it must get down to try it for yourself on yer own boat and work from that. And if wanting to lock the prop with the gearbox, follow the manual's advice or that of the manufacturer.
Err, actually there is in my opinion a real answer, if one has reason to need to be concerned about prop drag, it would seem sensible to fit one that feathers, folds or otherwise makes itself low draggy when sailing rather than losing sleep over the lock or don't lock debate. That at much higher capital and maintenance costs of course and that might give rise to an alternative reason for lost sleep