Autorotation under sail
Normally the propeller is designed to turn slowly in the opposite direction due to the “bias” that we integrate into the trailing edge of each blade by sanding a small section of the back of the profile. This operation gives a very low angle of attack to the flag blades so as to generate a small rotational force in the desired direction.
The problem we face is that we have to take into account the extreme events encountered by big seas – swell surge, propeller emersion – and these events actually happen – even if they only concern a limited number of our customers.
Without bias, any violent movement of the ship can orient the blades in force in an abnormal position. In this case, the blades force, for a short time, the mechanism that supports them to turn briefly in a direction that compresses the spring. The propeller then turns in reverse and behaves like a fixed-bladed propeller without feathering.
The moment of mass of the shaft/inverter assembly may be sufficient not to allow the propeller shaft to react fast enough to prevent compression of the spring.
Any navigator considering an ocean crossing is concerned about the usefulness of the bias machined in the propeller profile. The solution is to engage the reverse gear to prevent the shaft from turning. Lofez to reduce the speed of the boat until the shaft stops, then engage the gearbox, normally in reverse, to lock the shaft.
It is also possible to engage the reverse gear before the start of the rotation of the shaft.
It is risky to engage the reverse gear while the propeller shaft is rotating because the moment of rotation of the propeller has the effect of compressing the spring and actually engaging the rear rotation – the very result that we seek to avoid.
The propeller once stopped then remains in flag – but with the constraint of bias to cope with the situations mentioned above. The “bias” that we insert at the appropriate location of the trailing edge of each blade is therefore intended to cause the propeller to rotate in the direction where it is not likely to compress the spring in reverse and in fact turn into a fixed blade propeller.
We have to deal not only with the extreme events described above, but also with the degradation of the leading edges caused by rope windings and even paint drips that can apply significant forces to a profile. It is enough to realize the loss of lift caused by the simple accumulation of a little ice on the leading edge of an aircraft wing to understand this phenomenon.
The rotation induced through the trailing edge only starts when the speed exceeds six to eight knots – depending on the shape of the hull, the drift, the load, the state of the leading edge and the water nets that the movement of the boat passes over each blade at that precise moment.
In grip the propeller remains in flag but the blades have a very closed angle of attack with a very limited increase in drag. The Kiwiprop design, characterized by the feathering of each blade individually, creates a drag that always remains inferior to those of the folding propellers. The blades of these are interconnected by a gear system and which always have a larger projected area, resulting in a higher drag in real conditions of use.
So be sure to clutch the propeller before it rotates itself. All manuals confirm that a rotating propeller creates a significant drag. In addition, this solution reduces wear due to the individual oscillation of the blades induced by the angle of the shaft when the propeller rotates.
The best example of this phenomenon is the smooth descent of a helicopter in autogiration engine cut. For maximum lift, the blades must rotate as fast as possible, which is equivalent to the maximum drag of a rotating propeller. It is actually all the rotational energy that is left in the wake. Thus, contrary to popular belief, a propeller locked in grip usually has less drag than a rotating propeller.
Currently the propellers are evenly divided between 50% left rotation and 50% right rotation because the saildrive headers, which today represent some 40% of the world propeller market, are all left-turned.