Those who have operated a twin engine boat may know that it’s possible to “walk” the boat laterally or sideways. This is of course a huge boon for close quarters boat handling and docking a twin.
So you try it on your boat and it doesn’t walk at all – you might think there must be some magic touch that you don’t have. Well, there is some of that, but also some boats just will not walk sideways, period. It all depends on several factors related to the boat’s geometry and the forces involved. Also, lateral control of a twin is normally a topic for an inboard engine boat – not outboards. It is possible to walk an outboard twin – more on that later.
The Propeller Forces
First, let’s look at of the forces generated by a propeller. On an inboard single engine vessel, there is of course forward thrust from the prop’s discharge current when turning ahead. There is also a lateral (transverse) thrust due to engine torque and underbody flow. This can be thought of as if the propeller is walking along the sea bottom. While turning ahead (forward) with a right hand prop, the force pulls the stern to starboard.
On a twin, the starboard engine normally has a right hand prop and port has a left hand prop, so these forces cancel out.
The lateral force is much more pronounced when turning astern (reverse).
OK, so far so good. Now let’s look at the forces created by a twin in different control configurations. There are many combinations of propeller thrust and rudder position which results in different overall force vectors. These are all in “normal” mode (not attempting to walk).
Study these different images closely and you should be able to decipher what is happening with one or both engines turning in different directions.
Note that in the following configuration, the stern has a strong vector to port and this would be how you would normally spin the vessel in place clockwise.
Then there are the rudders. Each one generates a lateral thrust as a result of water moving across it. When turning ahead, the propeller’s discharge current is directed onto the rudder and that creates the force up the rudder post to direct the stern.
When turning astern, the discharge current is towards the bow, and the suction current does not have nearly the same effect. In other words, the rudder does much more with an engine that is turning ahead (assuming dead in the water).
A larger rudder will generate a greater force and provide more rudder authority.
Note that the graphic of the rudder above is a simplified example of what’s called an unbalanced rudder. Many boats use a semi-balanced rudder to ease the steering force and capture more of the prop’s discharge current.
There is also the factor of the amount of spacing of the two engines in relation to the length of the boat. Since the engines are off to each side of the centerline, the thrust from the propeller creates a twisting moment which wants to turn the boat to one side or the other.
The shorter the boat and the more spacing between the engines, the greater the offset moment. Generally, the shorter twins of 30 to 40 feet in length have a larger beam to length ratio and suffer from this effect.
SO…now let’s look at a hypothetical vessel that is attempting to walk sideways to starboard. To do so, the port engine is placed ahead and the starboard is placed astern. First consider the forces without the rudder.
The combination of propeller thrust, lateral forces, and engine offset are all pushing the stern to port and trying to spin the boat to starboard. So basically, all of the forces are against us. In fact, this is exactly what we would normally do to spin the boat clockwise to starboard. But…we want the stern (and the whole boat) to go to starboard.
We can use the port engine’s discharge current and left rudder to offset all of the other forces. The discharge current from the port engine creates a large lateral force to starboard. If the force is large enough, it can overcome all of the other forces and push the stern to starboard.
Remember that the starboard rudder does not do much with that engine turning astern. The discharge current is all directed forward.
Here’s another view of the forces at play:
The last piece of the puzzle is getting the bow to also go to starboard. This is done with varying amounts of astern power on the starboard engine. That will pull the bow to starboard and result in walking the boat to starboard as well.
A simplified summary of the forces looks like this:
With all of the ahead thrust on port, most boats also make a bit of headway so the net result is walking a bit forward of directly abeam. This can be counteracted by momentarily turning both screws astern and sort of see-sawing to starboard.
So what are the characteristics of boats that walk better than others?
Longer, less beamy hulls and close engine spacing both minimize the offset force, and large rudders that produce enough lateral force to overcome all of the other forces. Lots of boats are built with massive beams in relation to their length to make them more liveable, and thus give up this sort of maneuverability. Those boats just “run out of rudder” when the operator is trying to make this happen.
Outboard (and I/O) Twins
It is possible to walk outboard twins sideways. Some do it better than others. With outboards, the benefits of offset are the opposite – the more spacing between engines the better.
Outboards do not experience the propwalk effect that we see on inboard boats. They also have directed thrust rather than rudders since the entire engine rotates with the helm commands. Unfortunately, they are roughly equally effective turning both ahead and astern (unlike the inboard where the rudder does not do much turning astern). So it is not possible to use the same technique.
With outboards, the goal is to direct thrust towards the pivot point of the vessel. Recall that most boats pivot around a point roughly 1/3 of the way back from the bow while going ahead, and about 1/3 from the stern when turning astern. You’re not trying to pivot the boat – just to push it through its pivot point.
To move laterally to starboard, start with just a bit of left “rudder” and then apply a short burst ahead on the port engine. Just clutch ahead, no throttle.
The boat will move a bit forward and sideways with no rotation. Then shift to right rudder, and apply a bit of astern thrust on the starboard engine. You can think of this as “pulling” the boat through its pivot point.
With this back and forth seesaw motion, you can work the boat off the dock. There will be some fore and aft movement so you’ll need a bit of space for that.
This definitely takes some finesse on the helm, and you’ll need to be lively and delicate with the steering to find that exact point where the force is through the pivot point and not rotating the boat.
Newer boats have fully electronic steering controls and actuators such that both engines can be controlled independently by computer, and those boats can truly walk directly sideways or go in any direction you wish with control by just two fingers on a single joystick.
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Walking sideways takes a lot of practice to learn the nuances of a particular boat. Don’t be frustrated by your attempts, but also realize that some boats WILL NOT walk no matter who is behind the helm. So don’t be too hard on yourself!