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The effect of rudders, props and propwalk 

outdrive 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 and anticipate what will happen before you start a maneuver.

The good news is that all boats tend to work on the same basic principles, so once they are 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;

  • The boat moving through the water
  • From the propeller (propwash)
  • 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.

     

     

     

     

Rudder hard over and engaging ahead, throws 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.

     

     

     

     

When turning 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.

     

     

     

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.

     

     

     

     

Propwalk

Single screw - 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 on single engine boats.

     

To find propwalk - run the engine astern with the boat moored. Prop wash is more visible one side than the other. Propwash to starboard = stern kicks to port when going astern.

     

     

     

 

Propwalk can be used to advantage during tight turns to help swing the stern, especially when astern gear is engaged. Its effect is taken into account during reversing.

     

     

     

     

     

Propwalk - Twin Screw

Most twin screw boats use contra rotating propellers. Each propeller is placed either side of the centre-line and is outward turning therefore rotating in contrary directions. When both engines are used simultaneously they cancel out propwalk and the boat drives straight.

 

Twin-screw boats use propellers positioned at either side of the stern. They allow excellent maneuverability when used independently or together.

     

     

     

 

  • 1) Port engine in forward gear drives the bow to starboard…
  • 2) Starboard engine in forward gear drives the bow to port…
  • 3) Port engine astern drives the stern to starboard…
  • 4) Starboard engine astern drives the stern to port…

     

     

Outdrives

Outdrives are a steerable leg with a prop on the end. They direct power in the required direction because the wheel steers the leg and the throttle adjusts power. Steering is greatly enhanced when the prop is turning.

 

     

Duoprop configuration has two contra-rotating propellers on the same leg, allowing greater performance and the reduction of propwalk.

    

 

Twin outdrive legs are placed closer to the boats centerline so the props stay in the water at speed. Being close to centerline they do not exert the same pivoting force as twin shaft drives and so outdrives require steering to enable an effective turn.

     

In close quarters and going ahead, if you want to turn the boat to starboard; wheel to starboard and go ahead on the port engine. In reverse, to turn the stern to port, steer to port and go astern on the starboard engine.

     


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