Volvo cone clutched gearbox with fixed pitch 3 blade prop -
We always sail with it locked in reverse if only to stop the noise (sailing is supposed to quiet else should get a power boat
Manual states leave in neutral if a fixed pitch prop and lock in reverse if not - commissioning engineer said to disregard and lock, and if jambs just start the engine (we find that on the occasional time it does jamb that just a quick crank of the engine with no fuel so doesn't even start always frees the box).
Prop designer states that our prop has least drag when locked and I guess he knows - it is a high aspect ratio 3 bladed (long skinny blades) prop optimised for sailboats.
As an aside, the experiments conducted in the MIT lab did not test locked props at all even though there are continual claims that they did (a simple read of the paper confirms that
). The experimentors did make a couple of very shallow assumptions, probably unsound in my opinion, and from those stated that freewheeling props had least drag - at best it was an unsound generalisation.
But in the end the silence of a locked prop wins out for us and all else is irrelevant.
I would have thought that if drag was important enough for someone to fuss over whether locked or not is least drag, they wouldn't have a fixed pitch prop would they