According to some tractor tug experts, only the Voith Schneider cycloidal drive is a “true tractor” and z-drives don’t qualify. I don’t know whether that is a distinction really worth making. In any case, the Voith Schneider Propeller system, or VSP, is very interesting but because so few of them are in service in North America almost no one here knows how they work. They’re used for tanker escort and docking work in Alaska’s Prince William Sound, Washington’s Puget Sound, Louisiana’s LOOP facility, and the Placentia Bay terminal in Newfoundland.
Fortunately, Capt. Bill Brucato of the NY Tugmaster’s Weblog has just provided us with a gee-whiz demo site that shows exactly how it works in fine detail and even gives you the opportunity to play around with one a bit. Just click the lower link that says Open iVSP – Interactive VSP Program and have at it. There are also several links on the left sidebar with lots more info about the VSP if you want to learn more.
Thanks, Capt. Bill!
Towmasters: the Master of Towing Vessels Assoc. Forum 
I run V/S boats on Puget Sound. They are indeed a rare breed in North America, and are much more popular in Europe. The term “tractor” tug originated from the pitch levers on Voith boats, as they resemble the track brake levers on farm tractors. Voith’s original term for the boats was “Water Tractor”.
Nowadays, more precise terms are used as there are variations on the theme. These are “True tractor” and “Reverse tractor”. True tractor refers to tugs with the drive units placed forward, like V/S boats and the few which have Z-drive units forward, such as the Chouest escort tugs used at the Sabine LNG terminals. Reverse tractor is used to describe a standard Z-drive, or ASD (Azimuthing Stern Drive) tug.
Voith boats excel at tethered escort work. Their hull design allows for large indirect forces to be generated and used as braking or steering assistance by the pilot on the tanker. The latest generation hull design Z-drives are getting better at it but all else being equal you still can’t beat a V/S boat for tethered escort.
Because they are so flexible over many applications, Z-drives are more popular these days in North America. The primary reason is that their drive units are more efficient and require less maintenance. For example, my boat generates 60 tons of bollard pull and has 5500 horsepower. Another boat in our fleet, a z-drive, has 6800 horsepower yet generates 91 tons of bollard pull. Z-drive reverse tractor boats are also more suited for linehaul towing as they can use their full range of power. The first z-drive boats on the West Coast were built by Western Towboat for their Alaska trade. A V/S boat towing astern has restrictions on the amount of pitch that can be applied and so you won’t see a Voith boat tethered to a barge for more than a local move.
Both types bend the brain when you learn to run them. Forget everything you know about handling conventional boats and prepare to be humbled. Some say Voith is easier to learn than Z-drive, some say the other way around. Once you get the hang of it they are lots of fun to run.
By: Doug Pine on December 1, 2008
at 7:30 pm
Voith maintains a USA site that will be faster than the posted link:
http://tinyurl.com/6bvpl9
The demo application can be run from there or downloaded to your machine.
By: Doug Pine on December 1, 2008
at 7:37 pm
And don’t forget Hvide’s Ship Docking Module (SDM) with 2 diagonally offset z-drives (1 “forward” and 1 “aft”) that is run today by Seabulk in Port Everglades. Plus the 3 z-drive (2 forward, 1 aft on the centerline) rotor tugs of Europe. What will they think of next? How about mini Azipod tugs with diesel-electric power backing up the lithium-ion battery packs?
To me, they’re all simply “tractor tugs”: either cycloidal or z-drive (forward, aft, split, or rotor). The “tractor” signifying that they use a non-conventional propulsion system of some sort and possess maneuvering capabilities that far exceed those of a conventional tug.
By: towmasters on December 1, 2008
at 10:58 pm