A little knowledge is a dangerous thing. I have (only) a little knowledge - so caveat emptor applies.
Propellers create drag under sail, whether locked or spinning free, unless they are locked in an aperture. Simply stated, whether the prop freewheels or is locked, you are creating drag.
Iíve read opinions that the locked prop actually produces less drag than the free-wheeling prop. On the other hand, Practical Sailor published a summary of a study they sponsored at MIT a few years ago - Relative to freewheeling, they concluded, though test lab limitations prevented full testing of freewheeling props, that less drag would be created for any given fixed prop if it freewheeled rather than held fixed. Motor Boating
& Sailing has published similar results
I donít think itís quite that simple. Itís likely that he relationship of the propís characteristics (pitch, diameter, blade count & shape/area), the boatspeed, and other factors will determine the actual drag ratios (between fixed & freewheeling) in any given application.
Your log will be the best judge
of whether leaving the prop to free wheel
has more or less drag.
Your shop manual will be the best judge
of whether leaving the prop to free wheel
might cause damage to your gear box (etc).
When the propeller
is allowed to freewheel some of the force propelling the boat is transferred into the rotational motion of the propeller
(and shaft), thus creating drag at the propeller, and unless you can disengage the driveshaft at the aftermost shaft seal
, at all the bearings between the propeller and the transmission (and for that matter, at the transmission itself).
On the other hand, if you lock down the propeller, you are creating a vortex behind the propeller caused by the water
being forced to follow the blades and continuing in a helical pattern after leaving the blades. Also, because the water
will not be flowing past the propeller at an angle of minimum slip, you will be creating a low pressure area aft of the blades, effectively pulling back against the forward motion of the vessel.
Dependent upon the shape of the hull
below the waterline, the effects of drag can be reduced by up to 50%, if the propeller is in a vertical position when sailing. With long-keeled yachts where the propeller is located in a propeller well between the keel
and the rudder
, a vertical twin-bladed propeller causes very little or no drag. This is not the case with three bladed propellers. These can not be hidden behind any projections and can cause a decrease in top speed of 10% or more. If one does not want to compromise, one must dig deep into one's pocket and invest in a folding or feathering propeller.
The transmission on the Yanmar
engine using the cone clutch
device should always be left in REVERSE. Apparently leaving it in forward still allows the propeller to spin the shaft against the transmission. They went on to say that when you start the engine while sailing (remember your trans is in reverse) that you should also START it in reverse and then shift into forward (the water pressure on the prop makes it difficult to shift into neutral or forward while under sail).
A number of major gearbox manufacturers have dispelled an earlier unfounded fear of gearbox damage due to improper lubrication while freewheeling. However some gear box manufacturers do not advise leaving the prop to free wheel as this can cause mechanical problems within the box (overheating due to lack of lubrication).
Ie - From ďVelvet-DriveĒ:
It has been determined by tests and practical experience that all Velvet Drive marine
transmissions call be free-wheeled without risking damage in sailing or trolling applications. Caution should be taken to be sure that proper oil
level is maintained prior to freewheeling as well as normal running. Freewheeling one propeller of a twin engine boat at trolling speeds will not cause damage to the transmission connected to the freewheeling propeller.
Extended periods of free-wheeling at high speeds may cause the transmission to overheat; therefore, it is recommended that transmission sump temperature be monitored and free-wheeling discontinued whenever 230 degrees F or 111 degrees C is reached.
Since the hydraulic transmission needs hydraulic pressure to shift the transmission, moving the shift lever will have no effect. Even if the transmission was in gear before the engine was shut off, it will not be once the engine is stopped because there is no longer any hydraulic pressure. The pump also lubricates the gears and bearings. With no pressure, there is no lubrication. The only way to stop the prop from turning is to put a brake on the output shaft.
I lock my propeller (Reverse Gear) when sailing. Iím not certain whether this results in more or less drag on my boat - but I do feel that it places less strain on my drive train.