Ship Behaviour while going astern


#1

Hey All!

I was wondering if someone would point out any paper out there which discusses how a large ship behaves when you turn the helm as you go astern on the engines.

I was under the impression that it really does not make any difference and always used to put her midships when going astern, but have recently have come across many senior masters who say otherwise.

Any experiences from you chaps out there?

Cheers,
Velu


#2

Your post said Engine[U][B]s[/B][/U] which to me implies twin engines. In this case it often would be usually be correct to center the helm. If you have a single engine and a right hand propeller then things are different. The stern will of get a kick to port as you start and so right helm will offer some correction to this. Followed by just a little right helm to keep her straight as you get sternway on. This is just a generic answer and you really have to get a feel for your ship to find out what she needs.
As for reference material, the best I can think of right now is Shiphandling for the Mariner by Daniel H. Macelrevey. This book is an absolute MUST in my opinion.


#3

[QUOTE=velu;25119]Hey All!

I was wondering if someone would point out any paper out there which discusses how a large ship behaves when you turn the helm as you go astern on the engines.

I was under the impression that it really does not make any difference and always used to put her midships when going astern, but have recently have come across many senior masters who say otherwise.

Any experiences from you chaps out there?

Cheers,
Velu[/QUOTE]

Depends. Whenever you operate astern propulsion, the minute you start moving, your pivot point is 1/4 the length of the vessel from the stern. That’s not very much leverage, even with a stern thruster, better option is steering with a bow thruster or tug. If you’re a twin screw vessel, you can offset the power and walk off of centerline one way or another, but not really turn without the help of twin screwing, thrusters, tugs, or a combination - wind and current can act as an extra thruster depending on the situation (I rather have something to work against most days, because you can make it work for you)
Certain situations will make rudder your best option when operating astern. If you were dockside, bow facing downstream, with a pretty stiff current on the stern, you can turn your rudder outboard, back into the current (which might just keep you into the current), and you’ll walk right off the dock - good technique in the Mississippi River around April-May with the snow melt high current season.

Hope that helps.


#4

In my experience, the behaviour of any ship needs to be learn from direct experience, since the interplay between hull, propulsion, rudder and the environment is unique.

Having said that, there is some guidance available in books and around the web. One example is http://www.fas.org/man/dod-101/navy/docs/swos/cmd/ms/Ms4-1/sld018.htm.

All the best!

Alexandre


#5

Every vessel is different, indeed, but the physics never change and so you can assume certain behaviors will be found (all else being equal of course).

On a single screw vessel, assuming a right hand wheel, the vessel will have a tendency to swing its stern to port at first, as the pivot point is amidships when the vessel is dead in the water and the transverse thrust of the propeller causes the stern to move laterally when turns are first applied. Once the vessel gains astern momentum, the pivot point moves toward the stern, and as anchorman says the most effective way to steer will be by applying thrust at the bow using a bow thruster or a tug. As the vessels’s astern speed increases, the rudder becomes effective enough that you can counter the transverse thrust produced by the propeller and also steer the stern to starboard if you wish. This behavior of course varies from vessel to vessel, depending upon numerous factors, but a single screw ship can be successfully backed to starboard. You can also “back and fill” by using the rudder and ahead engine commands to “kick” the stern one way or another without taking too much way off.

A controllable pitch (CP) propeller on a single screw ship is a whole different animal, though. A CP ship will tend to back to starboard as the propeller rotates in the same direction at all times. Most, but not all, CP ships have right hand turning wheels. CP wheels are also notorious for having much less backing power than fixed pitch wheels, so you must be careful.

A vessel’s trim also affects the way she steers. A single screw ship in ballast will be more challenging to control moving astern as the wetted surface area of the rudder is much less, she has much more windage, and is much lighter than when in a loaded condition. Single screw vessels also back into the wind, so you must be careful to consider that when trying to steer while backing. In those cases it is best to plan your astern movements with the wind in mind, if you wish to be successful.

A twin screw vessel is best steered astern by using different power settings on each engine, or setting the engines up in a twist and using more astern power in conjunction with bow thruster inputs… The rudders on a twin screw vessel will also be of use at higher speeds due to the same sheering effect that a rudder on a single screw ship has, although the smaller surface area of twin screw rudders can make this effect less noticeable. Some newer large twin screw ships (tankers, in particular) have independently controllable rudders that can be set at different angles, allowing some very creative use of engines and rudders when moving astern.


#6

Doug,
I disagree about the CPP usually backing to Stbd. In my experience as a pilot, the overwhelming majority (90%) of variable pitch ships back to port/bow to stbd. In 9 years, I’ve piloted very few where they back to stbd. I believe the rest of your post was well said.


#7

I’m actually glad to hear that information. Both CP boats I’ve run had right-hand turning wheels and backed to starboard. It sounds like in your experience they behave more in line with a fixed pitch right hand wheel. The most important thing I should have said was that with a CP ship it is essential to know which way the wheel is turning before you maneuver astern!


#8

Doug,
You are correct. It is one of the first things I ask when coming aboard. In an aside, a classmate of mine from college was the guy you relieved before your fateful trip on the Blue. He said nice things about you when he got home.
Regards


#9

Wow!

Thanks for all the input guys. I really do appreciate the time you guys put in here. The truth is that all i need to do to resolve this is go up on bridge and give a good astern kick. But the truth is that with the tigh running they have us on, it become quite difficult to experiment at sea these days. I know with the varied experience out here, I was sure to get some results.

I missed out clarifying that I was looking at Single- right hand screws.

Doug - When you say that you can steer the ship to Stbd at higher astern speeds, would you give the rudder to stbd or port? Am still a bit confused about that part.

On the whole I understand that since the pivot point shifts astern, its pretty much meaningless what I do with the rudder at slower astern speeds & the ship will behave more based on the other factors.

On another note, I think ship handling is becoming a bit of an lost art on guys from bigger ships with all the tugs and pilots. Every time I come to the US, I have 3 - 4 tugs tied up. I think even if I tried to turn back, they’d still drag me to the berth with their leashes!

Cheers & thanks all,
Velu


#10

There are some very good studies of ship motion out there . . But you know what? . .

Not only is every ship diffferent, the behavior of a particular vessel can change significantly based on factors which can change in real-time . .

Part of my current employment involves trying to learn predictave ship behavioral modeling from a couple of the best Hydro guys in the world, and you would be amazed how often their response to an analysis of a particular situation is " Well I really wouldn’t trust those numbers, as real-world observations will vary by up to 50% in comparison to our predictions"

The big 2 are:

Squat - Always the #1 point of argument between those who predict bahavior and those who observe behavior (Truth is squat is almost unpredictable to any high degree of accuracy)

Backing - Always #2 when it comes to debate over behavior . .


#11

Velu,
With Sternway and Starboard rudder the stern SHOULD cant to Starboard. It should follow the rudder depending on all the endless variables but generally a single fixed pitch Right Handed ship will back SOME to the rudder. A deeply loaded ship or environmentals throw this concept out the window however.


#12

[QUOTE=velu;25221]Doug - When you say that you can steer the ship to Stbd at higher astern speeds, would you give the rudder to stbd or port? Am still a bit confused about that part.[/QUOTE]

When you were a little kid did you ever put your hand out the window while driving with your folks and angle it up and down? If you did, you learned about lift and drag, perhaps without even realizing it.

The same thing happens with a rudder passing through water. When you angle the rudder across the stream of water, high pressure is created on one side and low pressure on the other. The rudder, like your your hand in the wind, wants to find equilibrium and so moves toward the low pressure side. Because it happens to be attached to a ship, it thus drags the rudder post and the rest of the ship with it. So if you’re backing a ship and angle the rudder to starboard, the stern will sheer off to starboard as well. Same thing to port, but keep in mind the movement will be more pronounced to port if you have turns on (assuming once again a right hand wheel).

If you’ll allow me to stray off topic a bit, you can do the same thing with the hull of a ship while working ahead against a current or wind. Again, think high pressure and low pressure on opposite sides of the hull. Use nature to your advantage. You can move the whole ship sideways if you turn the bow across the current or wind and use kicks ahead and rudder. You can find the spot where your ground speed is zero and you’re moving sideways. The current gives you the added advantage of allowing you to use power longer to keep wash flowing over the surface of the rudder, and it also keeps the pivot point forward so you have a longer lever arm to work with. You can be stopped relative to the ground yet enjoy all the benefits of your pivot point being forward.

It is true what you say about the dying art of shiphandling. In my world (<1600 tons) DP has very nearly killed the art. As a hands-on kind of boat handler it just about breaks my heart to watch a guy (or girl) dock a boat on DP. I’m just glad I’ve had the opportunity to learn on and work on “old school” OSVs and tractor tugs. If DP ever shows up on tugs I think I’ll hang it up. I guess it is a good thing I’ve got a shoreside gig these days. :slight_smile:

In the deep draft world with the advent of tractor tugs and the consequences pilots potentially face for not using them, there is less opportunity to use just the ship’s power and anchors to get the job done. In remote ports you’ll still find pilots using just the ship to get the job done. I loved to sit on the dock at Pohnpei for example and watch them bring in the container ships without tugs, in screaming winds sometimes. It is a joy to watch.


#13

One of the places you still get to see some degree of real shiphandling is on the St Lawrence Seaway and Welland Canal. The old lakers don’t have bow thrusters and even the salty’s often don’t. I had one pilot up there who still dredged the anchor to get her alongside. A real pleasure to watch and learn.


#14

[QUOTE=dougpine;25235]
It is true what you say about the dying art of shiphandling. In my world (<1600 tons) DP has very nearly killed the art. As a hands-on kind of boat handler it just about breaks my heart to watch a guy (or girl) dock a boat on DP.
[/QUOTE]

WOW. That’s kind of a big no no where I come from. Sad. Handling the vessel on manual is one of my biggest joys at work, and I take every opportunity.


#15

I spent 3 months last summer riding lakers - Old fashioned “Shiphandling” is still an absolute requirement for advancement - If you can’t [B][I][U]really[/U][/I][/B] drive a ship, you won’t be advancing. It’s quite impressive . .


#16

An interesting dialog. As a pilot of deep draft ships for many years I agreed with most of the comments. I’ll add two comments.

For CPP vessels. Most but not all have Left hand turning engines, this is an attempt to mimic the fixed right handed propellor action which pilots and masters are accustomed to, when going astern. IMHO CPP is much more difficult to maneuver. Not as responsive and noticibally less effect going astern. Basically I hate them.

When trying to turn a single screw RH ship around in a confined area I found the following useful. Go half astern with the rudder 20 right. You will notice much more turbulence on your stbd side. If space allows get a big ball of this turbulence to work up the stbd side almost to midships. Stop the engine and let her drift. The ball of turbulence when it reaches the bow will push on the bow to port. After the engine has stopped the transverse thrust will stop working the stern to port and the rudder ‘may’ work the stern to stbd. When you have stopped gaining in the direction you want, go half ahead with hard left rudder. Kill the sternway and begin to gain headway with a strong swing dveloping to the left. If you haven’t got enough room to complete the turn, go half astern again. Leave the rudder hard left until you lose headway then put the rudder 20 right again. Repeat until you’ve got the job done. You will find that the ball of turbulence on the stbd bow really helps, especially on a loaded vessel.

All the previous comments regarding knowing your vessel’s characteristics, the vessel’s trim, draft, wind and current are very true. These “other” considerations are at least as important as the mechanics of the work.


#17

Another tactic in a confined space with a RH fixed pitch wheel is backing and filling to starboard, taking advantage of the transverse thrust that accentuates the stern’s motion to port when you’re backing. Once you get the stern swinging to port with ahead turns and hard right rudder you keep it moving that way when you’re backing. You can leave the rudder hard over to starboard the whole time, as you only use kicks astern to keep her swinging. The rudder won’t really come into play except when you’re working ahead.


#18

Can’t you just push your bow up against a conveniently moored ship and pivot yourself around at 1/2 Ahead?

Oh yeah - I was supposed to call you . .I just got back from Newport Beach - You should have seen the weather . .


#19

[QUOTE=Captain Electron!!;25698]Can’t you just push your bow up against a conveniently moored ship and pivot yourself around at 1/2 Ahead?
[/QUOTE]

I guess, if this conveniently moored ship made of marshmallow.


#20

[quote=Captain Electron!!;25232]There are some very good studies of ship motion out there . . But you know what? . .

Not only is every ship diffferent, the behavior of a particular vessel can change significantly based on factors which can change in real-time . .

Part of my current employment involves trying to learn predictave ship behavioral modeling from a couple of the best Hydro guys in the world, and you would be amazed how often their response to an analysis of a particular situation is " Well I really wouldn’t trust those numbers, as real-world observations will vary by up to 50% in comparison to our predictions"
[/quote]

My real-world observations while backing a large car carrier when the wind is blowing vary from my predictions by about 50%. However I have found so far that I can mitigate my ignorance with a heavy hand on the bow thruster control.