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Author Topic: Bollard Pull  (Read 3678 times)

Stavros

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Bollard Pull
« on: December 09, 2007, 12:56:06 PM »

Interesting one this but aremed with a set of anglers spring scales we had some fun today and some very intersting results were had

Odin running 12v 100mm Kort nozzle Robbie 1000motor                                                                            2.5lb bollard pull
Northsider running 18v 4ins prop MMM Motor                                                                                          3lb """""""""""""
Boston Typhoon running 12v 3ins prop Thermo king fridge motor                                                              2lb""""""""""""""
Tid Tug running 3ins 12v Protech 720 motor                                                                                          3.5lb """""""""""

These finding really surprised us for the simple reason we all thought that th Kort nozzle was by far  more efficient than a std prop.OK probably our way of doing things are not exactly text book but it doesn't half give one a good idea on what goes on

Stavros
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bigford

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Re: Bollard Pull
« Reply #1 on: December 09, 2007, 01:37:03 PM »

one would think the 18 volt northsider would pull the most
you started the pulling slow and gentle,not with a hard jerk
how long was the rope? O0
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Roger in France

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Re: Bollard Pull
« Reply #2 on: December 09, 2007, 03:38:46 PM »

Do be aware that many simple spring balances are notoriously inaccurate. They may not even give consistent results from one weighing to another and so comparisons can be unreliable.

e.g. you may think that say, 3 lbs. +/- X ozs.  would compare with 2 lbs. +/- X ozs. but frequently errors are not linear.

You will often experience large variations where the pull is not in the exact same direction. A rudimentary test, say by using a bag of sugar, may not tell the whole story.

Your local Weights and Measures Dept. /Consumer Protection Dept. may carry out some more reliable tests for you if you get them "intrigued". But don't tell them I suggested it.

To me the bollard pulls you recorded seem very low.

Roger in France (former Inspector of Weights and Measures).
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barriew

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Re: Bollard Pull
« Reply #3 on: December 09, 2007, 05:40:59 PM »

Surely to make comparisons between Korts and plain props the only thing to change must be the Kort! Hull shape and motors also have an effect. I think you are comparing apples and pears O0

Barrie
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farrow

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Re: Bollard Pull
« Reply #4 on: December 15, 2007, 10:59:37 PM »

To make true comparisons between kort nozzles and standard props. You would need the same hull and motor and then use max size prop and kort that the motor will take before reducing in revs. Using different hull forms with different motors and size props, I would have thought would not give a true indication of power output for comparason. Like taking a small harbour tug with 1200 bhp fitting it with a kort and comparing it to a large screw tug with 5000 bhp, no real comparison.
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banjo

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Re: Bollard Pull
« Reply #5 on: December 15, 2007, 11:08:15 PM »

Are not "Anglers scales" prone to what the politicians call terminological inexactitudes?

 8)
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BarryM

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Re: Bollard Pull
« Reply #6 on: December 16, 2007, 12:57:58 PM »

Full size practice uses a towrope connected to a bollard via a strain gauge link. Once the power is applied the static bollard pull may be read. However, in such a situation, the screw(s) are rotating in still water with cavitation/aeration reducing the force imparted to the water. Thus the 'free-running' bollard pull when the vessel is moving through the water, is always higher.

It was not unknown for superintendents during static bollard pull trials of a new tug to quietly swing the stern a few degrees either way - not enough to be obvious - in order to bump up the figure recorded on the BP certificate.  ;)

Barry M
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farrow

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Re: Bollard Pull
« Reply #7 on: December 20, 2007, 11:14:56 PM »

In bollard trials I have been involved in, the tug puts the power on very slowly, so as to not cause cavitation, the peak reading being the figure recorded. I have in the past been full ahead looking at devils point 200ft away on the way up from Plymouth sound, the tug was not moving but the RFA using my tug as a bollard to swing round the point and yes after a while cavitation does kick in and then you have to ease the revs.
But getting back to the bollard test, it was always instilled on the masters in the MoD to keep the position of the tug steady to the direction of pull, otherwise the bollard figure would be lower.
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BarryM

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Re: Bollard Pull
« Reply #8 on: December 21, 2007, 04:03:18 PM »

The idea behind allowing the stern to swing away a bit was to take the screws out of turbulence and into clear water. The momentary extra pull achieved looked better on the techspec produced to paying charterers - the MoD never had to show a profit!  O0
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farrow

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Re: Bollard Pull
« Reply #9 on: December 23, 2007, 10:38:40 PM »

I was told that when a vessel swung in a pendulum fashion on a bollard pull the screw or unit was pulled side ways through the water and did not achieve its full potential, as some of the energy was lost in the movement and then the rudder is used to correct and that will reduce the energy from the wash. But never mind, it is not a subject to get beat up over, I think the thread of this discussion is why different model screws were better than a model Kort nozzle. I suspect that the model nozzles are not made to the same spec as the full scale ones are and also in the real world I believe you can get different spec ones.
But I do wish you a merry Xmas and a better New Year to come. A final thought for every one is that a Bruce anchor on a 25 ton winch would certainly overcome a tug with a 27 to 30 ton bollard pull. Merry Christmas.
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cos918

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Re: Bollard Pull
« Reply #10 on: December 24, 2007, 11:47:07 AM »

here a question on bollard pull. If a tug has say a bollard pull of 50 tones what does this mean. Big ships weigh thousand tones or does it mean the the tug could drag 50 tones across land, how does all this translate in what size of ship it can pull. john
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BarryM

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Re: Bollard Pull
« Reply #11 on: December 24, 2007, 12:02:31 PM »

John,

A BP of 50 tons means just that - the vessel is capable of producing a tension of 50 tons in the tow line. The fact that a towed vessel may weigh thousands of tons is immaterial as all that weight is acting downwards. What the towing vessel has to overcome is the inertia of the towed vessel, its frictional resistance to movement through the water and any external influences, e.g. wind, wave and current forces acting on the hull/superstructure. Thus, a small pull can move a large mass.

Imagine yourself sitting on a quay pushing off a small craft with your legs. The craft may weigh a few tons but you certainly don't need to exert anything like that force to get it to move.

Happy Christmas,

Barry M
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farrow

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Re: Bollard Pull
« Reply #12 on: January 22, 2008, 10:14:23 PM »

If it helps you, when I went onto sea going tugs as a junior mate, I was always taught that it was the weight of the tow wire which pulled the towed vessel. There for we always kept the bight of the wire below the surface of the water, never allowing to lift out, as if it did there was always a strong chance of parting the wire. Harbour work yes you always maintain tension on the wire and never snatch it, but then usually the wire is as strong or greater in breaking strain than the bollard pull. I have seen some spectacular breaks in tows, if there is a synthetic part to the system take cover, most wires are preformed wires and usually just drop. In the days before towing winches, most ships would insist on using there mooring warps rather than the tugs tow system, this reduced the towing fees, an often would come down the end of a nylon rope?
Here is another interesting fact for tug enthusiasts, When a tug is engaged by a ship, when its tow is connected, any damage caused by the tug to anything. The towed ship in law is held responsible, as in law it is considered part of the towed vessel, an there have been some expensive claims paid out to other vessels and parts of docks etc over the years.
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BarryM

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Re: Bollard Pull
« Reply #13 on: January 22, 2008, 10:36:22 PM »

The 'UK Standard Towing Conditions' as insisted upon by most harbour/coastal towage companies where the towed vessel is usually at the mercy of a near (or total) monopoly are biased in the favour of the tug operator. For deep sea towage, the owners of the tow have the opportumity to charter on more equitable terms.
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