Prop walk

What happens

As it turns, a propeller doesn’t only push along the boat’s centreline: it also moves the stern sideways. This is called prop walk. The French call it pas d’hélice, the Italians effetto evolutivo.

Which way it goes depends on the propeller. A propeller is right-handed when, seen from astern, it turns clockwise to drive the boat ahead. With a right-handed propeller:

• going astern, the stern walks to port and the bow to starboard;
• going ahead, the stern walks to starboard, but the effect is slight and the rudder corrects it easily.

With a left-handed propeller, everything is reversed. On a given boat, the direction never changes.

Why it is mostly astern

There are several explanations, which complement each other:

• the propeller shaft slopes downwards, so the blades don’t work the same way on both sides;
• going astern, the water churned up by the propeller strikes the hull and pushes it sideways;
• going ahead, the prop wash passes over the rudder, which can counter the effect from a standstill; going astern, there is nothing of the kind.

The effect is strongest with the boat stopped, at high revs and at low speed. As soon as the boat is properly moving astern, water flows over the rudder and the helm takes over again. Easing the throttle then reduces prop walk and lets the rudder steer. See Carrying way for the speed the rudder needs.

What it changes when manoeuvring

• An easy side and a hard side. With a right-handed prop, the stern comes in towards a quay on your port side by itself when you go astern to stop: berthing port side to is easier. Starboard side to, the same burst astern pulls the stern away from the quay. Hence the advice, when you have the choice, to berth on the side that prop walk favours.
• Turning in your own length. By alternating short bursts ahead and astern, with the helm held on the same side, you can pivot the boat almost on the spot. With a right-handed prop, the boat pivots easily to starboard; the other way, each burst astern works against you.
• Going astern in a straight line. When you set off astern, the stern kicks sideways before the rudder takes effect. When you can, start with the stern cocked slightly the other way (to starboard with a right-handed prop): prop walk brings it back into line. Then ease the throttle as soon as the boat has way on.

Saildrive or shaft drive

On a saildrive, the propeller shaft is horizontal. Prop walk is markedly weaker than with an angled shaft. Some owners describe it as “minimal”. It doesn’t go away, though: every propeller produces some. See What the boat changes.

How to find out about your propeller

• Look for an “RH” (right-handed) or “LH” (left-handed) marking on the propeller, or watch the shaft turn in forward gear.
• Most telling of all: in open water, boat stopped, helm amidships, give a burst astern and watch which way the stern goes.
• On an unfamiliar boat, do this test in open water before entering harbour: a helmsman who doesn’t know the boat is easily caught out when the effect works against them.

Pitfalls

• Picking the wrong side. Berthing or turning on the unfavourable side: prop walk pushes you away instead of helping.
• Too much throttle astern. More revs means more prop walk, not more control.
• Forgetting the wind. Wind, current and hull shape all change the effect. See Wind and windage.