Yes Sean, it's most important to know what the maker has to say. Some "gear" boxes are asked for a specific gear direction for oil
flow purposes. It ensures the flow is being sent into small oil
galleries to buricate bearings correctly. Hydraulic boxes are a little different again. Some can't be spun without the engine
running, as there is nothing flowing between friction plates. Some can, because the friction plates don't get forced together when the engine
isn't turning. And so on and so on. So it really does depend.
Some boxes allow you to free wheel
for a certain number of hrs before they need to be engine run. Some can free wheel
with no regard to engine at all.
As to the load from the propellor, it's a difficult one. It depends on the design. So props will stall when not turning and will "drag" through the water
with little effort. Some, the drag will be too great and free wheeling is best (if it can safely be done). I have been doing a bit of work on the free wheeling of the prop part. Sometime back, several of us talked about this issue here and I don't think we came to a rock solid answer. Well here is what I found.
When a prop is free wheeling, it actually has significantly more "bite" in the water. It is acting much like a helicopter blade does in auto-gyration. this is because, as the prop is spinning, it is also cutting the water in a hydrodynamic way. Thus it has a greater "grab"
in the water. I am purposely calling this grab and not Drag at present. It is possible the more correct term is Drag, but I am trying to show this as a different "result" of pull. If the shaft/box can spin freely, then there is little drag
, as the energy that would normally be considered a drag, is turned into rotation. A certain amount of loss is experianced. How much is dependant on how much force it takes to spin the shaft/box. So a spinning prop is possible better than a locked shaft as far as Drag is concerned. But it also depends on the box as far as free wheeling safely is concerned.
As for folding props, that is a difficult one when you get to boats with Hp as large as mine. A standard folding prop is not particuly efficient, especially in reverse. The geared props are a little better, but it depends on makes as to how much better. The feathering props are possibly some of the best around, but very expensive and complex. I dunno, I get along just fine with my big three blader and Velvet drive box free wheeling.