Prop walk is the sideways push a turning propeller gives the stern. It is small going ahead but strong going astern, when the stern kicks to one side (to port on most boats) before the rudder can correct it.
A propeller does not only push a boat forwards or backwards. Because it is turning, the blades bite slightly differently at the top and bottom of each turn, and the result is a sideways push on the stern. This is called prop walk, or paddle-wheel effect.
Going ahead, the effect is small. As soon as water is flowing past the rudder you can steer it out without noticing.
Going astern, it is much stronger. The rudder has little effect until the boat has picked up some speed backwards, so for the first few seconds the stern swings to one side whatever you do with the tiller. On most boats the stern kicks to port in reverse; on others it goes to starboard. It depends on which way the propeller turns.
To find out which way your boat walks, try it in open water, away from other boats:
- Stop the boat with the rudder centred.
- Put the engine astern with a firm burst of throttle.
- Watch which way the stern moves. That is the way it will always go.
Once you know, you can use it rather than fight it:
- If the stern walks to port, come alongside a pontoon port side to. A short burst astern then stops the boat and pulls the stern neatly in towards the pontoon.
- When reversing out of a berth, expect the kick and allow room for it, or start with a little speed so the rudder takes over sooner.
- Prop walk can help turn a boat in a tight space: alternate short bursts ahead and astern, with the rudder held to one side, and the boat will turn almost on the spot one way much more easily than the other.
The common mistake is reversing slowly and expecting the boat to go straight. Use short, firm bursts, and keep the speed down overall.
More in the book: Chapter 8, Under power, and in the marina