Asymmetrical thrust

Asymmetrical Thrust, which can be defined as the tendency to laterally move the stern of a ship when reversing is engaged before the boat wanders, is not directly caused by the lateral thrust of the propeller but by its wake whose asymmetrical flow creates turbulence that tends to deflect the hull laterally.


All propellers have a wake that exits in the opposite direction to the direction of movement of the propeller. This wake is deflected by the rotation of the propeller which transfers energy not parallel to the axis of the propeller shaft.


Thus, the main determinant of the “asymmetrical thrust” is therefore the angle of inclination of the shaft. The more the shaft is tilted, the more strongly the asymmetrical thrust applies against the hull. To this factor are added the position of the propeller in relation to the hull and of course the volume of water displaced and the speed at which it flows on the hull.


In addition, some propeller shafts are slightly offset which allows them to be deposited without the need to remove the saffron. This feature aggravates, or mitigates, the effect of asymmetric thrust depending on which side the tree is oriented towards.

This analysis deserves to stop for a few moments. Indeed, if the asymmetrical thrust were the direct consequence of the lateral thrust of the propeller, the stern would also move violently in the opposite direction to the start of the forward gear. Yet this phenomenon does not occur.


In reverse, the increase in asymmetrical thrust that can appear with the installation of a Kiwiprop™ propeller is simply a function of the increase in thrust and water flow on the hull due to the locking of the three blades on the maximum pitch in reverse. This difference is especially noticeable when the Kiwiprop™ propeller replaces a folding propeller which by construction delivers a poor thrust in reverse and consequently causes a low flow of water on the hull.


Saildrive headers produce virtually no asymmetric thrust because the shaft is horizontal which reduces flow against the hull. This remark again shows that this phenomenon is produced by the shape of the hull and the inclination of the shaft and not by the propeller itself.

Thus any research aimed at eliminating or simply reducing lateral thrust by modifying the shape of the propellers is in advance useless, the problem being specifically related to the architecture of the boat.


What is important for a propeller is to produce the greatest possible thrust in reverse at low engine speed, so that the boat wanders as quickly as possible and the flow of water on the saffron allows the boat to maneuver just as quickly.

We are convinced of the superior efficiency of Kiwiprop™ propellers in this area. However, lateral thrust remains a problem rarely mentioned by the users of our propellers.