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KFP - Prop Walk

Prop Walk - or the tendency of a vessel to move it’s stern sideways when engaging reverse and before it has way on – is not caused directly by a side thrust from the propeller but by the wash off the propeller blowing back over the hull and in effect creating a lifting effect to one side from the asymmetric flow from the propeller over the hull.

All propellers have a “ race “ that exits from the rear of the propeller in whatever direction it is operating. This is then deflected by the rotation of the propeller transferring energy into this race away from the centerline of the shaft.

So – the main determinant of “ Prop Walk “ ( in fact more correctly termed Hull Walk ) will be shaft angle – the steeper – the more it blows back over the hull, the shape of the hull, the position of the propeller relative to the hull and of course the volume of water and speed at which it flows back over the hull.

In addition some shafts are offset slightly so they can be pulled easily without removing the rudder and this can of course make matters worse – or better – depending upon which side the shaft exits.

Reflect a moment on this analysis – if this wasn’t the case and prop walk was coming directly from a side thrust off the propeller – the stern would swing just as violently in the other directions when engaging Ahead. Yet this does not happen.

Any extra “ Prop Walk “ in reverse from changing to a Kiwiprop is simply a function of the greater reverse thrust and resulting greater flow over the hull delivered from the three or 4 blades which automatically lock into a fixed position at maximum pitchº in reverse. Typically this will be much more pronounced vs a folding propeller which typically has poor reverse thrust – and this little flow over the hull.

Saildrives have virtually no “ Prop Walk “ as the shaft is horizontal and this minimises flows across the hull. This again shows the impact is from the hull shape and shaft angle – not the propeller itself.

Thus there is nothing from a propeller design perspective that can be done to eliminate or even reduce “ Prop Walk “ – it is a particular vessel design issue.

What is important from a design perspective is to generate as much reverse thrust as possible at low engine rpm to ensure the vessel gets way on as quickly as possible and then obtains steerage from the flow over the rudder.

We are confident the Kiwiprop delivers this functionality particularly well.

Prop Walk is not something we often see reported as an issue by users.