View Full Version : Negative slip
27th January 2009, 09:39
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..
27th January 2009, 10:41
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!
27th January 2009, 13:16
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.
27th January 2009, 13:33
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........(Smoke)
27th January 2009, 15:53
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.
27th January 2009, 16:10
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.
27th January 2009, 16:46
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 .
27th January 2009, 18:51
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 "!!
27th January 2009, 21:33
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.
28th January 2009, 11:34
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.
29th March 2009, 18:00
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!
20th May 2013, 18:12
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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