Mariners who attended advanced shiphandling courses or have been doing a little bit of digging in books treating ship behavior have some knowledge of the term “added mass.” Simply put, the added mass is a quantity of water that the ship carries along with her when she has headway or sternway. If you have to stop a ship of 40,000 tons displacement, the astern-moving propeller must work overtime to overcome the ship’s inertia plus two volumes of water: the volume ahead that wants to keep going and attracts the bow in a partial vacuum; and the volume astern, the following fill-in flow racing forward towards the poop. The term “added mass” here is rather appropriate, describing the general effect that the phenomenon represents to the mariner as it acts evenly on the whole ship.
Today z-drive tugs and Azipod technology have made building up of significant sideways movement possible. Most shiphandlers are aware of it. But what is new for many of us is that the effect described for fore and aft motion also exists laterally when a ship develops sideways motion. For our own purposes, until some nautical authority decides otherwise, we will call this water movement “the ship-generated sideways current.”
Let’s study more closely how this current is generated. A ship floats still in calm waters. Let’s pretend that God feels that day like the master of all hydrodynamics and pushes our ship sideways. The volume that the vessel was occupying moments ago has to be filled with surrounding water. That water can come from four possible directions: forward, aft, under the ship or from the open side.
The easiest path for the water to travel and the movement that has the most influence on the ship is the flow coming from the open side (the port, if the vessel is moving to starboard). On the opposite side, the water is pushed away. The water by the ship’s ends and close to the turn of the bilges is moving rapidly around the bow, stern and bottom of the vessel. The water close to the parallel body is moving mainly with the ship.
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1. Vessel is pushed laterally.
2. Current sensors positioned along bow and ahead of ship.
3. Currents form along ship’s side as it moves laterally.
4. A thrust forward is applied.
5. As the bow moves into quiet water, currents exert force on the stern, turning ship counterclockwise.
To see a video of the effects of these currents go to: www.cpslc.ca/en/pilotage/pilotage-techniques (Virginia Howe illustration)
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Let’s suppose that, held again by God’s hand, a beam from heaven is placed against the shipside and blocks the motion of the ship. We suddenly eliminate all ship’s momentum. Next step, we lift this restriction and observe the vessel closely: If the ship remains stationary, we can forget everything about the notion of ship-generated sideways current, but if the ship resumes any kind of lateral motion (as experimentation on small scale models has shown), it is because an external force is still acting on it. The only possible explanation is the movement of the water impinging on the hull on the port side and attracting it on starboard side. When you are discussing movement of water in a general direction, then you are discussing current.
The word “current” here is more relevant than the term “added mass,” because in some circumstances that movement of water will affect the two ends of the ship with different intensities, having a direct impact on the ship’s heading — just as a ship entering a protected harbor from a river stream will be affected by the different speeds of the surrounding water at her bow and at her stern.
Here is how the ship-generated current could affect the ship behavior in some undocking procedures. Your ship is tied up on its starboard side. You want to move off the dock, turn 180° and head in the opposite direction. There are a few ways to do that. Here is one: You pull the ship briskly some distance from the dock with tugs and then start the rotation with a kick ahead hard to port and the forward tug pulling. What actually happens by leaving the dock laterally with a certain speed: the ship generates a sideways current. As the ship gathers headway with engines ahead, the bow gets out of the sideways current while the stern is still affected by it. The torque will tend to turn the ship to starboard, against the actions of the tug and the rudder. You will sooner or later succeed in achieving your goal, which is to turn the vessel 180° to port, but you will need to use considerable force. Chances are you will think, “Something just doesn’t feel right here!”
Now the same situation, but instead of turning to port after the lateral movement, the first action will be a kick ahead with hard-to-starboard wheel. By going ahead, the bow will get out of the current while the stern will keep on being carried by it. You will start your rotation with the help of the surrounding water movement instead of working against it and the vessel will more naturally achieve its 180° swing.
A few years ago, I had to direct the departure of a small ship moored on its port side, with the current coming from the stern and the wind pushing towards the dock. The plan was to open the stern and use one tug tied up at the accommodations to keep the stern opened and proceed with sternway until well clear of the dock. When I asked how the vessel would cant with the engines astern, the captain said: “You cannot really predict what she is going to do.”
Well, in the middle of the maneuver, I couldn’t have agreed more. After we left the dock, the efforts of the tug to pull the stern toward the river were giving the opposite results. There was no such effect as stern seeking the wind either. It was the bow that was seeking the wind. I was positively puzzled, totally confused.
Understanding as I do now the effect of the ship-generated current, it is rather simple to explain what happened. The pulling action of the tug made the ship create a sideways water movement. With the astern engine movement, the poop was extracted from the turbulent area. The bow, on the other hand, was always under the full influence of the sideways current and wanted to keep going in the direction of the tug pull, creating a swing: bow to starboard.
I have recently recorded a real-scale experiment with a 45,000-ton tanker. A sideways speed of 1.1 knot was developed with two 5,000-hp z-drive tugs pulling full. The effect could be felt with a kick ahead and stop. It was not spectacular, but present. For the record, the completion of the turn after tugs restarted to push for rotation was really quick.
Although it can be significant in some circumstances, the concept of ship-generated current has its limitations, the most important being the size of the ship. If the ship is very large, the tugs will tend to be too weak to create a sufficiently important sideways motion; also the ship’s engine will be too weak to get the ship partly out of the generated current field before it dissipates.
The uneven resistance of the surrounding water is the principal reason why a ship adrift orients herself beam to wind. Let’s start blowing on a ship having her stern 45° from the wind. The wind will produce headway and sideways drift. As it moves ahead, the bow floats in undisturbed water, while the stern floats in water which is “carried along” with the movement of the drifting ship. The more resistant medium ahead will create a torque that will bring the ship more broadside to the wind. As the ship turns further in the wind than beam to, she will lose her headway and start to move astern due to wind action. The phenomenon described above will be reversed and the ship, after a few back-and-forth movements, will find an equilibrium heading, roughly beam to wind.
This uneven side resistance similarly plays a major role when handling high-windage ships at low speed in high winds. Such a vessel (i.e. containership or car carrier) with the wind on the port quarter will have a tendency to sheer to port. In confined waters, if the control of the ship is lost, it will be hard to override this phenomenon even with the propeller astern. The effect will be sustained until the headway through the water is noticeably reduced.
The generation of sideways current when a ship is pushed sideways, producing uneven resistance to lateral motion forward and aft, has greatly contributed to keeping alive the myth of an off-centered pivot point shifting position whether the vessel has headway or sternway. The ship-generated sideways current could be a big part of the answer to many cases of ghost currents or mysterious ship behavior. Next time you handle a ship and start to wonder about a seemingly unexplainable sheer, assess if you have produced significant side momentum and study how the water movement generated could have played a role in the change of heading. What looks like random behavior could actually be a very consistent vessel reaction every time similar conditions arise.
Capt. Hugues Cauvier is a river and docking pilot on the St. Lawrence River. He holds the certificate of master mariner. His work over his 28-year career includes serving as mate and master on chemical and petroleum tankers.
