Twin Screw Walking

Professional Mariner Forum
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I fully understand that there are many ways to skin a cat. I am fairly confident I could walk a twin screw vessel to starboard, however I’m getting lost in the weeds, is there a “More right” way to do it? Across a range of source material I’m finding conflicting information. Do twin screw ships handle differently than twin screw tugs? Are you more likely to find Inboard or Outboard turning screws on one vs the other?

I made this question, and based my answers off two sources:

Which of the following will walk a tug to starboard

A. Hard left rudder, port engine ahead, starboard engine astern
B. Hard right rudder, port engine ahead, starboard engine astern
C. Hard Right rudder, port engine astern, starboard engine ahead
D. Hard left rudder, port engine astern, starboard engine ahead

In reality, It could be A or C right?

From the available source material I’ve got two (tug) books saying: Inboard is most common, Hard left rudder, port engine ahead, starboard engine astern to go to starboard. I’ve also got 2 (one and a half, really) saying outboard is more common/efficient. Am I misunderstanding something?

Relevant source Material:

The direction of Rotation of the Propellers is important during this maneuver. Usually Outboard turning propellers are more effective than inboard turning screw when moving the stern laterally. A ship Maneuvering Starboard side to a berth uses the Starboard Screw ahead, and the athwartships component of the force created by this clockwise turning propeller walks the stern towards the Dock. The Port screw is used astern, and since the left handed screw turns clockwise when going astern, the athwartship component of that screw’s component also walks the stern to starboard. These forces complement the opposing fore and aft flows from the two propellers- the starboard propeller operating ahead and the port propeller operating astern in this case - that are minimizing fore and aft movement of the ship and also driving the stern to starboard towards the dock.

~ #MacElvey p.240

Inward turning propellers have the advantage of better propeller efficiency and enhance the tugs ability to “walk” or move laterally.

~ #Slesinger p.125

… If the operator wishes to move the tug laterally to starboard, the port engine is engaged ahead, the starboard astern. The rudder is turned to Port to flank the tug to starboard. … This is also where the advantage of inward turning propellers come into play. In the flanking configuration, the paddle wheel effect of both propellers turning in the same direction assists in this maneuver.

~ #Slesinger p.128-129

In general, to walk a boat to Starboard, put the port engine ahead and the starboard engine astern. while brining the rudder to hard left.

~ #livingstone p.5

The outward turning configuration is used most commonly with two fixed pitch shaft lines because of the smaller wake generated by the water flow directed outwards, as well as their greater turning capacity on the spot. , and counter effects combining to give the maximum efficiency in twisting movements on both shaft lines.

~ #HerveBaudu p. 60

The inward turning configuration (of twin CPP) is the most efficient for maneuvering with twist, since the counter effect is greatest in reverse motion with water flow directed to the hull. Nonetheless, outward turning configuration is the more efficient, since water flow directed outwards creates a broader wake and the water wall generated by the low speed is easier to exploit, with over pressure on the rear rather than low pressure on the inside.

~ #HerveBaudu p.62

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A or C would be the way to go. Most of the tugs I worked on, if not all, were outboard counter rotating.

However some boats just can’t walk due to their hull design or configurations.

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I did a few relief jobs on a twin-screw landing craft one time. First time getting away from the pier I just used a spring to get off the pier but the deck hand informed me that the 'regular" captain would walk it out.

I couldn’t live with that so the next time I tried it. Went with “A”, figured it’d be a regular twin-screw twist to stbd but with the rudder shifted to get the stern off.

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A is generally the way to approach it, especially with inboard turning wheels, but as with everything else it can be a lot more subtle than just “hard over away from the walk and twin screw the bow towards the walk”.

Sometimes you have to work the rudder a bit to maintain the walk and keep the bow from over running the stern, sometimes the walk is really more of a “shopping cart” type maneuver where you’re leaving the rudder hard over to port but switching the throttles between a walk and a pivot to keep your lateral motion going. Sometimes you need a lot more initial turns on the engine pulling astern to really use the prop walk before you can put the power on the go-ahead engine to counter the stern way. Every boat is different, but it’s rarely a set it and forget it situation with the controls.

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But how do you do it on a z drive

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Why, you push the joystick right :stuck_out_tongue: Port drive hard over port and running ahead (pushing the stern to starboard), starboard drive slightly less over to port starboard and running astern will do it.

The walk of a twin screw boat is greatly influenced by drive center offset and steepness of the bottom ahead of the screws. Generally, alternative A is favored by boats with large drive center offsets and relatively flat bottoms. If the bottom is steeply angled ahead of the propeller, water pressure on the hull bottom becomes a dominant force, and the boat is pushed away from the screw running astern. Likewise, if you have a large drive center offset, you can’t put much power through the screw running astern, and the dominant force becomes the rudder action.

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I wish I had learned to run a z drive :expressionless:

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Had the misfortune to drive a vessel with inward turning propellers and single rudder. The best advice came from a previous skipper that advised see which way she wants to move and help it. A container vessel with two outward turning props and centreline rudder gave many masters and pilots grey hairs.
With independent rudders the 804 series of AHTS vessels were a joy. Rudders 30° inward, bow thruster,
and using one engine ahead, one astern and away she walks.
My single try at driving a Z drive was using just one engine at a time imagine yourself in a small dinghy with a tiller to a transom mounted outboard before progressing to using two engines.

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That was how I learned on an old Western Geco seismic boat turned floatel. Split rudders make it a cake walk.

Now as for z-drives, I’ve had the displeasure of having to “crewboat” one at a rig for an “emergency lift” and I’ve had to come to a dock with no bow thruster… you will earn some gray hair fast with both those scenarios if the drives don’t rotate fairly rapidly. It can be done, but damn do you get a mental workout as conditions change.

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You generally want inboard turning wheels. It can be done with outboard turning wheels, but you need big rudders and hope the wind is blowing the way you want to go. Might not be so bad with flanking rudders, but I have never operated a boat with them.

Engine forward and rudder opposite the direction you want to walk. Think of it as having both wheels spinning in the direction you want the boat to go. The wheels will pivot the bow and the rudders walk the stern. Don’t automatically assume it’s hard over either. You have to play with the rudder angle and engine speed constantly adjusting both.

On the big supply boats we generally just pivot and use the bow thruster to walk these days and almost all of them are either outboard rotating or Z-Drives with to much hull to fight.

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For tugs…“A” for Inboard turning wheels to walk STBD. “C” to go to port with inboard wheels as well. Balance the ahead engine to keep you in place, it will be the stronger of the two. Work rudder/engines if bow or stern gets ahead of one or the other.You will find the sweet spot. Have no idea how deep draft ships react. Never had the luxury of a bow thruster as well. Light barges (Around 4-8’ draft) would walk with not a lot of wind. Loaded barges at 33/34 'draft did not walk nearly as well. Just used the assist tug. Mississippi River current was easy besides the different power of it. Docking head into the current without walking was one of my easier jobs. Point in, point out,done. Loved handling the rigs around the docks. Quite frankly,after almost thirty years, did my time. I do miss that part of it. Not much else except the great crews I had.

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Just had a look at mine (4th edition) - The text says “walks the stern towards the dock” . The illustration on that page shows a twin-screw ship, the inshore engine ahead and the offshore astern with a tug assist forward countering the twist and pushing towards the dock.

It’s relying on prop 'walk" to move just the stern - not walking in the same sense as moving the entire vessel sideways with the props and rudders.

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I have to say that I am bewildered by some of these posts describing how to “walk.”

The post that said: it’s usually more like parallel parking a shopping cart was one of the better ones.

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The degree of difficultly is going to vary according to the vessel and situation. If it’s just a matter of getting off the dock it doesn’t matter much if the bow gets ahead of the stern a bit.

I’d assume that a landing craft, given its flat bottom, is going to have a relativity high 'walkability" rating.

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My experience was on supply boats, deep sea tugs and ships as a pilot for 30 years.
Supply boats is where I learned about walking. I worked a lot on a boat that was 180’ long, twin screws and no thruster. It was a bitch because they expected you to do all the maneuvers that the boats with a bow thruster could do, but it was infinitely more difficult without it. (A) in your example was the way to go. Ahead and astern to twist the bow, rudder to slide the stern toward the dock. More rpm astern or she would creep ahead as others have noted. The shape of the bow makes a difference too. A sharp vee shape was less effective than a more spoon shape. Burton hulls were admired for their walking ability.

Deep sea tugs usually had a sharp vee bow but were not as long as supply boats so the engine twist was effective. I never paid much attention to inboard or outboard turning propellers. Didn’t want to overthink it.

The vast majority of ships are single screw. The only twin screw ones that would actually walk were the old Mini Lines ships and ones like that. Most small ships had bow thrusters and it was just a matter of twin screwing in the direction you wanted the stern to go with the rudder directing the wheel wash of the ahead engine to push the stern in the direction you wanted her to go.

And, as Sea Eagle pointed out, current is always a big factor. It doesn’t take much current at all to overcome the walking dynamic. With the vessel facing the current, bow to starboard will walk you to starboard. Bow to port will walk you to port. You just need to keep yourself lined up with the place on the dock you want to spot her. Does the same with the current on the stern. It’s something to be aware of if you have to back down a narrow channel where there is current.

Lastly, when maneuvering a ship in ballast, a strong wind on the bow or stern will make her walk. You’ll think you’re golden with the wind on the bow instead of abeam and blowing you sideways, but get that wind to one side or the other of a high bow and you’ll be moving places you might not have wanted to go. That’s why ships move around at anchor so much. They’ll get the wind on one side of the bow and walk sideways until the anchor checks them up and makes the bow swing around to catch the wind on the opposite side, then walk the other way until the anchor checks them on the other end. Repeat.

Have a safe new year. Regards.

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The two main factors are how much traverse thrust the boat (or ship) can develop and how much lateral resistance of the hull.

Next environmental forces, amount of windage area, ease of use of the helm and engine controls.

On a boat equipped with Hamilton jets, lots of horsepower and good controls can be walked without much issue.

Once the throttles are set it takes one hand on the helm for heading and one hand on a throttle for fore and aft.

The amount of traverse thrust depends upon how far apart the throttle are split. If / when the speed sideways needs to be adjusted the throttle split can be increased or decreased as needed.

Then back to the helm / single throttle adjustment.

Assuming the maneuver is not being done in marginal conditions.

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Thank you.

I think another good point to be made is that the real power of walking a twin screw vessel isn’t necessarily purely lateral motion but being able to crab with some way on/ control your headway.

Once you can control your way in a walk, and maintain a constant heading by balancing the throttles and rudders you have a a very powerful tool to deal with cross currents or wind.

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Many mariners have forgotten what it’s like to learn a new skill.

Learning to walk a boat is similar to learning to ride a bicycle, Once the basics are learned the motor skills become part of long-term memory. When the boat starts to creep ahead for example the correction becomes automatic.

Once that happens learning the various subtleties is no more difficult then learning the basics was.

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I have the basics and I’ve seen a lot of variations. I find that I need to get the “feel” of any new boat/barge. Once I have the feel it becomes almost automatic. The only thing I need to think about is how to use the wx and current to my advantage.

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The other important element is feedback. Feedback is why it’s quicker and easier to learn how to drive a car then run a boat.

If the control forces are high and the environmental forces are low the boat will respond well and will be quicker to learn.

Learning with a tug and barge is going to take longer and be more difficult than learning on a landing craft.

Problem with the landing craft were the controls were difficult to use and not much power.

The catamaran with the jets on the other hand had good controls and lots of HP. Didn’t matter that much what the wind and current were doing.

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