Photo by Pro-Zak
I have always thought tugboats (or water tractors as they are sometimes called) were pretty cool. Maybe it was the Little Toot book I enjoyed back when I was just a little toot myself, or maybe it is all that raw power packed into a small no frills work machine, like a locomotive for the water. Indeed, I read that some tug boat engines were originally designed for locomotives, then had a sump oil pan added to handle the rolling sea. I have recently learned that many tug boats do not use a conventional propeller (or “screw” for you nautical types) for propulsion, but instead use a fascinating system called a Voith Schneider Propeller (VSP) which combines propulsion and steering in a single efficient unit. Developed nearly 78 years ago, the Voith Schneider unit uses vertical blades protruding from the ships hull from a rotating disc. The blades all travel around a common center axis, while each blade can also rotate on its own center axis. The pitch of the blades is continuously varied as they rotate around the circle so that they push against the water in one direction, while slicing though it in the other. This allows the system to produce thrust in any direction, or produce a rotational thrust around the center of the unit. This system allows a tug boat to move in any direction regardless of which way it is facing, and allows it to rotate in place, giving them excellent maneuverability in tight spaces. Keep reading for more details, diagrams, links, videos and more.
Some large ships combine two VSP units working in tandem. These units are also used on ferries which need to align with a dock on both ends of a route without having space to turn around.
Here are some nice pictures and diagrams to aid my awkward description.
This is an excellent animation from Wikimedia Commons:
Watching the animation I can see that the blades actually oscillate back and forth, pushing with one side for one quarter of the rotation, then the other side for the next.
Here is a better description from the Voith web site:
Over 80 years ago this ship’s propulsion system, the only one of its kind in the world was developed by Voith from an idea by the Austrian engineer Ernst Schneider. It allows thrust of any magnitude to be generated in any direction quickly, precisely and in a continuously variable manner. It combines propulsion and steering in a single unit.
This solution is as convincing as it is straightforward: on the Voith Schneider Propeller, a rotor casing which ends flush with the ship’s bottom is fitted with a number of axially parallel blades and rotates about a vertical axis. To generate thrust, each of the propeller blades performs an oscillating motion about its own axis. This is superimposed on the uniform rotary motion.
Blade excursion determines the amount of thrust, while the phase angle of between 0° and 360° determines its direction. As a result, the same amount of thrust can be generated in any direction, making this the ideal variable-pitch propeller. Both variables – the magnitude and the direction of thrust – are controlled by a mechanical kinematic transmission.
They even have a very cool Interactive Voith Schneider Propeller Program
that allows you operate the controls and see the blades in action.
It turns out that the Voith Schneider unit can also be used to provide roll stabilization, as dramatically demonstrated in this video:
Model R/C boat enthusiasts have also begun using Voith Schneider propellers in model tug boats and fire boats. Graupner makes a unit (shown below) , and it turns out Voith themselves have made a pdf document available with various complicated formulas and diagrams for making your own VSP unit, pointing out that it is not possible to simply scale down the design of a full size unit.
Here is a short video of a RC Tugboat using a Voith Schneider drive. It does a good job of showing the extreme maneuverability the drive provides on full size boats as well.
On a related note, Voith produces a full size radio control boat, or “Bow Steering Module” to be used in helping to guide a group of vessels being pushed from the rear. These units can be placed in the front, or middle of the line and controlled from the main pusher boat in the rear.