Under sail with the engine stopped, does a fixed blade propeller create more drag when it’s locked or when it’s allowed to freewheel? The answer is unequivocal: There is less drag when the propeller is allowed to rotate.
Scientists at MIT and at Strathclyde University agree on this. Michigan Wheel Marine, the propeller manufacturer, agrees. It used to be contentious, now it’s a fact.
So, we know we get less drag with the propeller rotating but there are other arguments for and against allowing the prop to turn:
Noise: The rumble from a rotating propeller can be quite intrusive, particularly if you’re off watch in a stern berth. Some people can’t stand the noise whilst others find it interesting; they like to judge the speed of the boat by the level of noise.
Energy recovery: If you want to run a generator off the shaft it has to turn – simple.
Wear: Where there’s motion there’s wear and tear, if not damage, to drive train bearings and seals.
Gearbox damage: One way to stop the propeller turning is to sail with reverse gear selected but it may damage your gearbox.
It seems that Yanmar became so concerned at the number of requests they received for clarification on the best practice for their engine/gearbox combinations that they issued a directive: The gearbox must be in neutral when sailing or your warranty will be invalidated. If you want to stop the shaft use a shaft brake, they say, not our gearbox.
I have to admit that I sailed for many thousands of miles with my Yanmar 3GM30F in reverse gear to stop the shaft rotating and I never had a moment’s trouble.
If you have a Hurth/ZF gearbox you must not select forward gear when sailing forwards. Or reverse when sailing backwards, obviously. Apart from that, use the gearbox in reverse to lock the shaft or let it run free, it’s up to you.
With a Borg Warner Velvet Drive transmission you can do what you like with the gear lever, the prop will rotate anyway. You’ll need a shaft brake to stop it.
On some gearboxes damage can occur because the engine needs to be running to provide lubrication; with splash lubrication there isn’t usually a problem, so check the manual.
It boils down to this: If you are obsessed with squeezing out the last fraction of a knot under sail you need to let the prop freewheel. You’ll be happy to accept any wear and tear on your cutlass bearing and you’ll issue ear plugs to those that find the noise is keeping them awake. Or you’ll fit a folding prop.
If you’re worried about wear or can’t stand the noise you’ll want the shaft stopped and whether you do that by using the gearbox or a shaft brake will depend on your gearbox manufacturer’s advice, and whether or not you’re going to heed it.