Negative slip - Ships Nostalgia
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Negative slip

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  #1  
Old 27th January 2009, 09:39
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Negative slip

Negative slip

A recent conversation with an ex naval man brought to mind a time when our twin screw vessel left Auckland for Melbourne and after rounding North Cape we took a course well westward before heading down to our Australian landfall at Wilson’s promontory. For most of the three day run down the Tasman sea the distance traveled equaled or exceeded the theoretical distance due to the pitch of the screws, i.e. three days at neutral or negative slip.
The second mate was an avid navigator, living and sleeping the subject and on asking about this event he claimed to have steamed westward to pick up a seasonal ocean current that gave us this ‘free ride.’
I can appreciate this happening in enclosed waters such as a flowing river or a with a strong tide flow in a narrow strait but my companion is very skeptical about this happening over such a distance.
It happened 50 years ago and the second mate may have been pulling my leg so I would be pleased if some of you Navigational experts could comment..

Bob
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Old 27th January 2009, 10:41
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If the ship picked up the East Australian current, the negative slip would be possible as this current runs at two to three knots. However, to take advantage of the current, it would have required a significant deviation from the normal route from Auckland to the Australian coast to take advantage of the current.

In conclusion, it is possible to have negative slip on a three day passage or possibly longer if you do follow one of the advantageous ocean currents. However, in this case, it would be hard to believe that such a deviation would be tolerated by Master or owner!

I was on one ship in the 1980's where the 2nd Mate recorded negative slip for the entire voyage from the Persian Gulf to Jeddah, but that was down to the 2nd Mate having inside information on the engine distance each day and adjusting the ships run accordingly to register negative slip. The poor Chief Engineer couldn't believe it!
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  #3  
Old 27th January 2009, 13:16
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There is no such thing as "negative slip" except as a fictitious entry in Engine Room logbooks. A ship cannot travel further through the water than that which the propeller (or the wind) pushes her. She can of course travel further over the ground if the body of water is moving in the required direction at whatever rate.

Last edited by Lancastrian : 27th January 2009 at 13:34.
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Old 27th January 2009, 13:33
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We had a run heading to Tahiti from Panama on the Rangitane one voyage where the distance made good was further than the old Doxfords had pushed us. Call it what you like but we had an extra 18 hours in Tahiti to show for it.
The Equatorial current was running a "banker" that trip ...... we even topped 19 knots made good one 24 hour run. There was no way the old girl ever managed that in her life!
We even made it into Auckland early!
We carried it almost all the way to the Tuamoto Achipelego and the Fakarava Channel.
Hows that for an old memory! It was 46 years ago........
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Old 27th January 2009, 15:53
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I have sent up a lot of negative slip entries up to the Bridge and Master on the Noon Slips. The ship was usually in the Gulf Stream and or had the wind behind the ship.

The only negative slip I did not believe was on ships with controllable pitch propellers you could never be sure that the pitch was constant. Especially on the home bound leg very morning I would have to lower the engine speed and pitch when on bridge control. The throttles seem to move only when the engine room was unmanned according to the data recorder.

I sailed with one Chief Engineer who had his own Chief Engineer's Noon Report printed on the reverse side he had the formulas and constants printed along with his definitions of each term. His definition for slip was Percent Mate inefficiency. His view was positive slip was due to all of the turns that the mates did not record example turning to avoid fishing vessels, turning to return vessel to course line and the Williamson Turn Drill's. Of course back then there was no GPS or Sat Nav's.

I do not remember any one getting mad at the time the deck department was just glad to get the Noon Report. The Chief would nap after lunch so I would fill out the slip before returning to the engine room with the log book.
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  #6  
Old 27th January 2009, 16:10
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The whole problem with slip calculations is that they compare chalk with cheese. What the Engine Distance should be compared with is Distance Through Water but that is not available, (unless you have a very accurate log, which I've never seen), so the 2nd Mate gives the Chief Distance Over Ground which is all that he can calculate with any accuracy, (on a good day before GPS).
To my mind all these figures which have been religiously entered in logbooks over the years are completely meaningless.

Last edited by Lancastrian : 27th January 2009 at 16:17.
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Old 27th January 2009, 16:46
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On Brocklebanks Mahsud and Maihar we had KaMeWa CP propellors and they always recorded a negative slip .
The process was to take the "K' reading on the OD box and record it in the log. The "K" reading was then checked against a graph which gave the pitch ; this was then used to calculate the distance etc.
The Graph curves were obviously incorrect . We had discussion with the office and pointed out that the log entries were meaningless and a waste of time but were advised to continue doing the calculations .
Some of the Mates would argue that it was because the CP Propellors were more efficient than a fixed wheel which of course was drivel and caused some heated discusion in the bar when we had nothing else to talk about .

Derek
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Old 27th January 2009, 18:51
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What an interesting thread !! There wasn't anything like negative slip in my day - seems it was a technological development by these clever people !
Talking of useless recording of things - do you remember in the 60-70's when you were returning to the UK by plane the stewardess would hand out cards and you had to fill in name, passport number etc. This often involved digging for your passport etc. When I was in the air taxi business we had to get our 8 passengers (or less) to fill them in and I would take them to an office in the terminal at Manchester (MI6) I believe. I once asked the chap what they did with these thousands of cards. He said - "Nothing - we ditch them when they take too much room up "!!
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  #9  
Old 27th January 2009, 21:33
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Thanks for all that learned comment. It comes to mind that the second mate, Jack,I cannot recall his surname, later went on to become the chief examiner at the NZ Marine Department in Wellington so his navigational zeal was real.
Mike, I do believe that performance of the Rangitane, she was a great ship.

Bob
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Old 28th January 2009, 11:34
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Indeed she was Bob.......my favourite ship.

I am trying to dig out a picture I had of her alongside in Kingston Jamaica. She looked a picture that day.
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Old 29th March 2009, 19:00
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I've seen negative slip occur on a day's run, but rarely for an entire passage unless it was a fairly short passage. When sailing northbound along the U.S. East Coast we used to hold well offshore in order to catch the Gulf Stream which, in places, can be well over five knots. On the other hand, while southbound we used to keep as close inshore as possible in order to avoid the current, or even catch a slight counter-current. The captains used to say that you were too far offshore if you couldn't count the bikinis on the Florida beaches!
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Old 20th May 2013, 19:12
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Joined a ship on the N. Atlantic trade in the sixties. Browsing through Reed's almanac I saw a recommendation that when sailing a great circle from North of Ireland to South of Newfoundland one should keep 15 miles North of the track going West and the same distance South when returning East. (Gulf Stream?) I thought it worth a try. Day one - negative slip and derision from the Chief! The same on each succeeding day. Admittedly my sights were the usual North Atlantic variety,( i.e. sometimes the brightest cloud etc!), but I stuck to my guns, and on making a landfall I was proved right! The same happened on the homeward trip, and on several succeeding voyages, except when the weather was more than usually foul.
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Old 26th May 2013, 17:21
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In Bank Line after loading through the South Pacific islands and heading for Panama it was company policy as I recall to head North Easterly (052 (T) if leaving Apia in Samoa), until picking up the Equatorial Counter Current; it was supposed to speed up the passage.
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Old 26th May 2013, 19:09
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I do recall one long passage that we did from Alexandria to Galveston, Texas; which our captain planned via the Straits of Gibraltars, SW towards the Canary Islands, Westwards towards the Bahamas, through "Hole in the Wall" (as we used to call New Providence Passage), and then into the Gulf of Mexico by hugging the Florida Keys as closely as possible to avoid the Gulf Stream. He took full advantage of the prevailing SE "trade winds" and ocean currents, and did everything possible to avoid contrary winds and currents. Our ship, which had a normal service speed of 18 knots, averaged over 20 knots during that passage, and I believe she also achieved a negative slip for the entire passage. It was a good demonstration of what can be achieved, even in modern times with a power-driven vessel, providing a passage is carefully planned to take maximum advantage of favorable winds and currents. I might add that out captain used nothing more sophisticated than a common Pilot Chart to plan that passage.

Last edited by Klaatu83 : 26th May 2013 at 19:15.
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  #15  
Old 26th May 2013, 20:06
Andy Lavies Andy Lavies is offline  
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If you could pick up the Equatorial Counter Current (and keep it) it could make a difference of a couple of days in getting to Panama. I can remember taking sights every hour or so when in the expected vicinity to see whether we were making any easting. Must be a lot easier with GPS but would you be allowed to do your own navigation or is it all dictated by somebody, or some computer, ashore.

Andy
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Old 27th May 2013, 03:02
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Quote:
Originally Posted by spongebob View Post
Negative slip

A recent conversation with an ex naval man brought to mind a time when our twin screw vessel left Auckland for Melbourne and after rounding North Cape we took a course well westward before heading down to our Australian landfall at Wilson’s promontory. For most of the three day run down the Tasman sea the distance traveled equaled or exceeded the theoretical distance due to the pitch of the screws, i.e. three days at neutral or negative slip.
The second mate was an avid navigator, living and sleeping the subject and on asking about this event he claimed to have steamed westward to pick up a seasonal ocean current that gave us this ‘free ride.’
I can appreciate this happening in enclosed waters such as a flowing river or a with a strong tide flow in a narrow strait but my companion is very skeptical about this happening over such a distance.
It happened 50 years ago and the second mate may have been pulling my leg so I would be pleased if some of you Navigational experts could comment..

Bob
Negative Slip seldom, if ever, occurs in reality.
Where negative slip is recorded it is usually because the Chief Engineer has used the observed days run in his calculation (ie distance between noon sights (or GPS) over 24 hour period) when he should not have done so . !!!!!!!
The slip should be calculated by comparing distance travelled by the propeller with the distance travelled through the water - not over the ground (sea bed). ["Through the water" distance is measured by the ships log - mechanical, electronic, or pressure type.]

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  #17  
Old 27th May 2013, 07:07
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The actual situation was that the ship had steamed from a defined point A to a defined point B over several days and the recorded prop revolutions x actual pitch during that time amounted to a lesser distance. Obviously ocean currents and or windage played a part.

Bob
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Old 27th May 2013, 07:28
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Quote:
Originally Posted by spongebob View Post
The actual situation was that the ship had steamed from a defined point A to a defined point B over several days and the recorded prop revolutions x actual pitch during that time amounted to a lesser distance. Obviously ocean currents and or windage played a part.

Bob
Thank you for that.
It is just as I have already indicated, the distance between the defined points A & B were obviously used in the Chief Engineers calculation. That distance should not have been used, as it incorporates the effects of current and leeway.
Slip is determined (or should be determined) by comparing propeller distance with the log distance, measured by the ships log.
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Old 27th May 2013, 08:48
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I am not going to argue over the definition of "slip" but when I was CE I was required to record the difference between the distance traveled by the vessel and the theoretical distance traveled by the engine revolutions (pitch x engine revolutions)' this we called slip, I relied on the Second Mate for distances covered. On many occasions I remember recording negative "slip" so how were the various Second Mates measuring the distance covered. One other point "slip" measured using distance over the ground could be used by Head office engineering staff when arguments arose over a vessels service speed and also indicte that the ships bottom needed cleaning.
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Old 27th May 2013, 11:28
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Quote:
Originally Posted by WilliamH View Post
I am not going to argue over the definition of "slip" but when I was CE I was required to record the difference between the distance traveled by the vessel and the theoretical distance traveled by the engine revolutions (pitch x engine revolutions)' this we called slip, I relied on the Second Mate for distances covered. On many occasions I remember recording negative "slip" so how were the various Second Mates measuring the distance covered.
Quote:
One other point "slip" measured using distance over the ground could be used by Head office engineering staff when arguments arose over a vessels service speed and also indicte that the ships bottom needed cleaning
.
Hmm, I see.
I don't disagree with your comment in quotes.
That doesn't make it right though.
Slip is as I have clearly outlined - determined by comparison of propeller distance with distance "through the water" - not over the ground.
With respect to the 2nd Mates distances - it seems they have not included the log distance along with days run in their noon chit to the Cheng - perhaps because the log was not working, or maybe even, not streamed.
When you were Chief Engineer was that with a Ship Management Company ?
It sounds a bit like it to me.
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Old 27th May 2013, 17:31
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Originally Posted by oldman 80 View Post
Negative Slip seldom, if ever, occurs in reality.
Where negative slip is recorded it is usually because the Chief Engineer has used the observed days run in his calculation (ie distance between noon sights (or GPS) over 24 hour period) when he should not have done so . !!!!!!!
The slip should be calculated by comparing distance travelled by the propeller with the distance travelled through the water - not over the ground (sea bed). ["Through the water" distance is measured by the ships log - mechanical, electronic, or pressure type.]

Using your stated definition of slip ( with which I do not disagree; although most definitions use the term actual ships speed but fail to indicate how this is to be calculated )then one can Never have actual negative slip as it is impossible to have a propeller with more than 100% efficiency .
It is also something which would only have to be calculated from time to time as a measure of hull condition .The calculation would depend on a ships log of either Sal Log type with pitot tube or a Doppler log .It would not matter that the log had n error as long as it was constant . With a clean hull good sea conditions the slip could be calculated for a particular vessel and used for comparison at a later time to evaluate hull condition .
In my company we were require to make the calculation daily ( which as I said before was a useless piece of information ; in particular with the C.P Props where bye the actual average pitch for the day was a best guess using a chart showing pitch )
On our older ships not fitted with in hull Logs the only time the Taffril logs were streamed was in decent weather so I am sure the bridge "actual speed " was by calculation by observations or dead reconning which made the calculation of slip even more useless due to errors in observation and the effect of tide wind and current .
All I know is that it took me an extra couple of minutes in completing my log before going for a pint before lunch .


Derek
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Old 27th May 2013, 18:17
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Originally Posted by Derek Roger View Post
Using your stated definition of slip ( with which I do not disagree; although most definitions use the term actual ships speed but fail to indicate how this is to be calculated )then one can Never have actual negative slip as it is impossible to have a propeller with more than 100% efficiency .
It is also something which would only have to be calculated from time to time as a measure of hull condition .The calculation would depend on a ships log of either Sal Log type with pitot tube or a Doppler log .It would not matter that the log had n error as long as it was constant . With a clean hull good sea conditions the slip could be calculated for a particular vessel and used for comparison at a later time to evaluate hull condition .
In my company we were require to make the calculation daily ( which as I said before was a useless piece of information ; in particular with the C.P Props where bye the actual average pitch for the day was a best guess using a chart showing pitch )
On our older ships not fitted with in hull Logs the only time the Taffril logs were streamed was in decent weather so I am sure the bridge "actual speed " was by calculation by observations or dead reconning which made the calculation of slip even more useless due to errors in observation and the effect of tide wind and current .
All I know is that it took me an extra couple of minutes in completing my log before going for a pint before lunch .


Derek
Ah well, yes of course, the variable pitch prop (Nautical) would definitely be another "headache" when it came to slip calculations.
In aviation it is known as a constant speed prop, as opposed to a fixed pitch prop. Ships of the air, are somewhat different, although in many ways they are not. There are many " parallels " between the two.
A Good posting Derek, - so much of it so true.
With respect to the other guy and management company tactics (William H) all I can say is with regard to the condition of the ships bottom, it was easier to just look at it rather than calculate it. Today, with the availability of mini underwater ROV's, (some very reasonably priced,) that should present little in the way of any problems at all.
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Old 27th May 2013, 22:08
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Originally Posted by oldman 80 View Post
Ah well, yes of course, the variable pitch prop (Nautical) would definitely be another "headache" when it came to slip calculations.
In aviation it is known as a constant speed prop, as opposed to a fixed pitch prop. Ships of the air, are somewhat different, although in many ways they are not. There are many " parallels " between the two.
A Good posting Derek, - so much of it so true.
With respect to the other guy and management company tactics (William H) all I can say is with regard to the condition of the ships bottom, it was easier to just look at it rather than calculate it. Today, with the availability of mini underwater ROV's, (some very reasonably priced,) that should present little in the way of any problems at all.
Agreed ; short dive over the side will reveal all .
Regarding propellers however there is a misconception that all ( solid cast ; for the want of a better word ; are fixed pitch ) Propeller design is such to my understanding a very a very complex science and most modern designs of ( solid ; not controllable ) props have a variable pitch i.e It is not a constant pitch as in the case of an Archimedes Screw ; the pitch by design changes from the hub to the blade tip .
The correct terminology for a Prop . which has the ability to move its blades from Ahead to Astern or anywhere in between is a Controllable Pitch Prop.

My interest in the subject is of local interest here in Rothesay New Brunswick . In 1927 a local man Wallace Turnbull invented the first controllable pitch Prop for Aviation . His original Prop is on a wall in our local airport . I was however also called a Variable pitch propeller and it may well have been a fixed pitch that was controllable .
Therein lies the dilemma of what is the correct name or lack of understanding of the science .
The same applies to aircraft as to marine applications .

Look forward to the views of the experts .

We shall not fall out over the issue ; I will leave that for "The Ashes "

Regards Derek
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  #24  
Old 27th May 2013, 22:22
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No Derek. If it can`t be moved its a fixed pitch prop.
It will have a different angle of attack, blade thickness, aspect ratio along its length, but it is fixed.
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Old 27th May 2013, 23:05
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Originally Posted by Rocket_Ron View Post
No Derek. If it can`t be moved its a fixed pitch prop.
It will have a different angle of attack, blade thickness, aspect ratio along its length, but it is fixed.
So you are saying that all cast ( I e immovable blades are fixed pitch and cannot have a varying pitch from the root ( Hub ) to the tips ???)

Cheers Derek
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