The effect of rudders, props and propwalk - (Sail)
The effect of rudders, props and propwalk on boat handling.
The key to effective boat handling is to learn what your boat wants to do and watch what it’s telling you. Then you can work with the boat rather than trying to make it conform to your will.
Only then will you be able to anticipate what will happen before you start a manoeuvre. The good news is that all boats tend to work on the same basic principles, so once learned; they’re with you for life.
Steering is only effective when there is water flowing over the rudder. Water flow over the rudder is gained in three ways;
1) The boat moving through the water
2) From the propeller (propwash)
3) When a stream of water is running over the rudder such as tide
Yacht rudders have a large surface area and are able to steer at very slow speeds. Motor boat rudders are smaller and may require propwash for effective turning.
The rudder hard over then engaging ahead will throw propwash over one side of the rudder blade, diverting the propwash and turning the boat.
When going astern, prop-wash does not flow across the rudder. Steering relies on water flow gained by motoring astern.
In tight spaces, turn the rudder first then apply power. The prop-wash from the propeller deflects off the angled rudder and turns the boat in a smaller space. ‘Gear before steer’ makes for a larger turning circle as the boat will travel forwards before turning.
Propellers are designed to push the boat forward through the water and pull it backwards when astern is engaged. Props are ‘handed’, either left or right hand - referring to their rotation in forward gear.
Because of a props rotation it may also make the stern ‘walk’ to one side. This action is called propwalk and is more prominent when going astern.
To find propwalk - run the engine astern with the boat moored. Prop wash is more visible one side than the other. Propwash to port = stern kicks to starboard when going astern.
Propwalk can be used to advantage during tight turns to help the stern swing and its effect is taken into account during reversing.
Article Published: September 28, 2010 10:13
Article Updated: June 29, 2012 10:40