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Negative slip

26K views 92 replies 29 participants last post by  Fred Field 
#1 ·
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
 
#2 ·
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!
 
#38 ·
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!
How did he hide the extra miles?
 
#3 · (Edited)
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.
 
#4 ·
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)
 
#5 ·
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.
 
#6 · (Edited)
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.
 
#7 ·
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
 
#8 ·
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 "!!
 
#9 ·
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
 
#11 ·
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!
 
#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.
 
#13 ·
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.
 
#14 · (Edited)
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.
 
#15 ·
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
 
#16 ·
!



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.]

(Ouch)
 
#21 ·
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.]

(Ouch)
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
 
#22 ·
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.
 
#17 ·
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
 
#18 ·
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.
(Sad)
 
#19 ·
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.
 
#20 ·
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
.
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.
 
#28 ·
This works for me.... http://www.propellerpages.com/?c=articles&f=2006-03-08_what_is_propeller_pitch

Re the distance 'steamed' through the water and the calculation of slip.... pre SAL, Chernikeef, and Doppler logs coming into vogue I rarely if ever sailed on a ship where a Walkers Patent Log was streamed as a matter of course. This of course meant that the calculation of 'slip' was a nonsense, often making little more sense than 'Zanzibar Time'.
 
#29 ·
Aye indeed.
I Sailed on a few ships back in the early to mid 1960's where the Patent log was streamed - it was very much a cadets job streaming and hauling it.
And then of course there was the old drum type sounding machine which required you to hold a "feeler" on the wire as you let it run to the bottom having armed the lead, and all that stuff.
We all took it very seriously, in the full and certain knowledge that the Orals Examiner of Masters and Mates would throw us out of the orals examination room if we didn't.
Sorry Cisco - but nostalgia may be beginning to flourish once again, both at sea and in the air, so to speak, - despite your feelings on the matter.
Yes indeed, I feel Nostalgia may be returning at last.
 
#30 · (Edited)
I have checked my very old diary re this voyage to be reminded that it had more drama than a matter of ‘under screwing’
The MV Kaitoa left Auckland 5/11/59 and arrived Adelaide to load oranges 14/11/59, a long time ago.
I am sure that the chief engineer, a competent man, would have calculated his log report in the correct manner. I recall the event of a fast passage was a frequent discussion in the Mess about the ship having travelled further than the propellers screw motion but as most of you say there is a difference between travel between two fixed points and through the water medium when helpful or hindering currents affect the passage.
It is a long time ago and I am not saying that the daily log recorded ‘negative slip’ but the conversation led by the enthusiastic second mate allowed us to witness this unusual assistance from an ocean current. I guess a European Canal Cruiser would experience the same effect steaming down the Rhine but mid ocean, the effects are not often very noticeable or dramatic.

Bu wait, there is more to recall about this voyage. We sailed from Adelaide on Wednesday 18/11/59 for Lyttelton via a southern route through Foveaux Strait and while heading north up the East Coast in very heavy weather we picked up a May-Day call from MV Holmglen, a coastal trader en-route from Dunedin to Wanganui. We were north of her position and on attempting to go about to join a search we rolled heavily and our cargo of cased oranges shifted enough to warn us back on our original course and to produce a list. This was mainly corrected by pumping ballast as I recall and we were ordered to continue on to Lyttelton until more information came to hand.

Only one call was received from the Holmglen and nothing else emerged as to what went wrong. She was later found sunken about 35 km ESE of Timaru with a loss of all the 15-crew members. An empty lifeboat and three bodies were later recovered.
Something was bugging me as I struggled to recall this drama then I remembered that Holmglen’s normal Chief Officer had been on leave in Wellington and was prevented from re-joining his ship due to the bad weather that closed Wellington Airport, a blessing in disguise. He is my Namesake, Bob Jenkins, now in his eighties and retired in Wanganui.

Google
home engels - Shoreline-man.name and roll up


Bob
 
#32 ·
The difference between speed by engine revolutions and a ships actual speed through the water is the "apparent slip" and this is invariably expressed as a percentage of the engine speed in knots.
Should the actual speed be greater than engine speed for any given revolutions there exists what is termed "negative slip" which is found on a ship with a very full Stern profile.
 
#33 ·
There can be no such thing as "negative slip", it is "apparent negative slip". I was Chief Engineer for 30 years and logged the term every day in my Abstract. Got the days run every afternoon from the 2nd Mate. The only use for it from my obsevation was that when the ship required docking, comparing the slip over a long period, there was a much larger positive slip percentage.
 
#40 ·
I was covering all sizes of vessels in my comment, "negative slip" occurs when it is found that the wrong size Prop has been fitted when carrying out Trials covering the range from Slow to Full.
 
#35 ·
Sorry guys but I just can’t resist it.
Post # 31 has stirred something inside me.
I am minded of that day January 29th 1993, when I was lined up on runway 14 Mackay, in Queensland Australia awaiting clearance for take off.
Beside me sat the CAA Examiner of Airmen, (Mike) who was conducting my final flight test for my Multi Engine Command Instrument Rating. As I sat there making final adjustment to the pitch controllers for each propeller and monitoring each manifold pressure , just awaiting that clearance from the tower, I remember relating some comments to the examiner about propeller pitch, slip, and Admiralty Coefficients. I could sense he wasn’t completely at ease, as he was looking at me sideways with a sort of “ funny look” on his face.
Then Nigel came through on the headset, (another of Australian Aviation greats), “Echo Delta Hotel you are cleared for take off”. As I released the brakes and rammed the throttles open, my examiner, I suspect, knew he was in for a bit of a ride, and, as it turned out at the end of the flight, so was I.
Wow !
What an experience that particular Two hour thirty minute flight test was.
I was 47 years of age, and had 10 years command experience in very large sea going vessels, yet that was the day, and that was the flight when I finally realised what the very limit of my own personal endurance and ability was.
That’s what they do in aviation, they take you to the very limits of human endurance, push you over the edge, and then pull you back again – in a jiffy, - just like that.
At the end of that examination flight, I stepped from that aircraft, shaking a bit, and saturated in perspiration. The examiner then took me by the hand with a big smile on his face and said, “Congratulations , - You have Passed” .
As a mariner many might think the greatest day of my life would have been the day I was finally appointed to Command a rather magnificent Swedish Built O.B.O. of some 105,000 dwt tonnes.
(she was orange, and grey, with some cream)
However that would not be true. The greatest day of my life came 13 years later, as I stepped from that aircraft in Mackay, Queensland , Australia.
What an experience – was that flight test.
It simply blew me away.
That was Mike – one of Aviations finest – I suggest.
 
#41 ·
Geordie Chief I don't quite understand what you mean in your last post.
As I see it there is confusion about apparent slip, positive and negative. The confusion comes from the way the speed of the vessel is measured ie. through the water or distance relative to the earth. I studied for my "tickets" at South Shields Marine starting in 1964. At that time the head of Naval Architecture was E.A . Stokeo, he had the usual string of letters after his name but the main one indicted he was a member of Royal Instute of Naval Architects. He gives the definition of apparent slip as (engine revolutions x pitch) minus distance travel by vessel and then he goes on to give two methods of obtaining the distance traveled by vessel. 1 speed through water and 2 speed over the ground but he does not say which method is the correct method to use.
 
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