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Author Topic: Bollard pull question  (Read 5139 times)

sean Half-pint works

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Bollard pull question
« on: February 22, 2013, 08:54:32 PM »

Hi all, daft question, I have been looking for the answer to for about a week now...

How does Bollard pull relate to the loads a tug can pull?  :P

Any help appresiated!

Sean
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derekwarner

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Re: Bollard pull question
« Reply #1 on: February 22, 2013, 09:41:18 PM »

Sean....try this ....http://en.wikipedia.org/wiki/Bollard_pull ...Wiki seems to have a good explanation for most things  :} Derek
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Derek Warner

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sean Half-pint works

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Re: Bollard pull question
« Reply #2 on: February 22, 2013, 10:07:32 PM »

Thanks Derek, although that's not quite what I had in mind, it was an interesting read!  :}

How does the bollard pull rating (almost put drawbar there, I have spent to long on model rail  :-X ) effect the size of object the tug can move, as the Bollard pull of the Adept (what i am hoping to model) is near 30 tons, obviously even the smallest of destroyers, let alone aircraft carriers ect, weigh a lot lot lot lot lot more than 30 tons...

that's were I am getting unstuck just a little bit confused...

Yet again, THANK YOU and sorry for not being as clear as i should...  :embarrassed:

Sean
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derekwarner

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Re: Bollard pull question
« Reply #3 on: February 23, 2013, 12:27:27 AM »

Sean.....in the same Wiki page there are 50 or so words in blue ...... and the word force is probably one of the best to start with.........
Remember if we stood on the wharf or quay & attempted to push a 100,000 ton vessel away from the wharf  >>:-( we only have to ovecome hull friction in the water  O0
Please also excuse me using words in the Moderators color  %) ...just trying to highlight the Wiki page........
We once went to Harndorf in the Adelaide hills.....& at the German Arms Hotel I asked for a large beer [not half a pint  :embarrassed: ] the waitress asked...."you want a large beer?"...so I outstretched my hands maybe 600 mm
You guessed it........I was then presented with a "yard" glass of DAB beer.............. {-) %%  ...tasted nice though.......Derek
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NFMike

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Re: Bollard pull question
« Reply #4 on: February 23, 2013, 12:38:18 AM »

That's a bit of a length of a piece of string question, Sean. With trains the forces (mainly friction and gradient) are fairly predictable, but with ships (incl. barges, rigs, etc) wind and currents are a large factor alongside simple inertia of course. Plus trains can only go back and forth on the rails, ships can move in any direction - until they hit something, like land.

Boats move fairly easily through the water (even I can get a boat weighing a few tons to move quite easily), but if the wind blows I can quickly find it getting away from me. So the displacement of the ship to be 'tugged' is not actually a big factor.

Towing companies and ports have ways to decide what tugs are needed for a job. I found a formula for required bollard pull in the Port Designer's Handbook, and it contains factors for wind, gust, current, wave and a safety margin. The factors are calculated on the areas of ship exposed to wind or water - but not the displacement of the ship. (Generally a bigger ship will displace more, but for example a loaded oil tanker will be mostly underwater with little windage relatively speaking, whereas a container ship of the same sort of size stacked high may be quite the opposite.)  So if it's a calm day at neap tides one tug might do the job; if it's blowing hard at springs you might need several.

sean Half-pint works

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Re: Bollard pull question
« Reply #5 on: February 23, 2013, 10:09:25 AM »

Dererk,

I will have a look at those later on... Thanks!

Oic,

Thats an interesting answer, so basically for modeling, use common sense and look at what you are towing (for scale tows at any rate) Basically they calculate against the estimated resistance of the tow, as opposed it its displacement/ overal weight?

Sean
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lighterman

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Re: Bollard pull question
« Reply #6 on: February 23, 2013, 10:35:06 AM »

In these days of ISO and SMS there is no place for "she'll do it" they have to have a set of figures to add to the accident report should things start going wrong. In days gone by I have towed 1000ton mud hoppers behind 150hp toshers! The insurers would pull their hair out now. Also moving deep loaded craft in the dock from round a ship to say the lock would be done by hand, or a rope over your shoulder. My experience of this was always when the wind was blowing the wrong way. When you asked the foreman if you could cadge a tow down the dock which would cost the firm a couple of quid the reply was always NO 'why tow when you can blow/row"
in these pages and other towing website you may come across mention of "FUNNEL POWER" this was a reference to steam tugs and some early diesel boats in that the bigger/taller the funnel the more perceived horse power she had a sort of conjurors trick and it was the skill of the skipper and to some degree the engineer to get the best from the boat and sometimes save face for the pilot or master of the ship when a planed move started to go wrong. the finest example i can give of this i was working as mate along side a skipper that never got flustered and we went to pull a ship and swing it off a berth and head down with a 500hp tug. the swing was going ok then the pilot thought we were not swinging fast enough so he put on more revs this made the tug really start to lean I was stationed by the hook ready to slip and as we started to go through the water sideways and the smoke from the exhaust grew blacker and blacker the tow rope was singing and it was drawing us closer and closer to the ship we were towing we finally got the ship head down and got the order to slip. which i did with haste!. on going to the wheelhouse the skipper said "F&%$ me that was close!" and then lit his pipe.. cool as a cucumber! and with only 500hp or (6 tons BP) under him...
if you understand railways then the power grading of loco's was not always fair take the southern Q1's could pull like a good un... but getting a fully loaded un-fitted train to stop was another matter.. ooops another rambling post...  :embarrassed:
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sean Half-pint works

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Re: Bollard pull question
« Reply #7 on: February 25, 2013, 09:22:29 AM »

Lighterman, I enjoyed that post, and don't worry about rambling, it is amusing!
 
Sean
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dodes

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Re: Bollard pull question
« Reply #8 on: February 25, 2013, 09:28:18 PM »

If it is of any interest, in the RN manual for navigation, every 10 kts of wind on the beam equates to 2 kts of tide, due to thier high windage above water to thier hull in comparison to thier draft.
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dodes

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Re: Bollard pull question
« Reply #9 on: February 25, 2013, 09:29:12 PM »

I forgot to mention my last comment was in regard ti type 42 and 22's.
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Umi_Ryuzuki

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Re: Bollard pull question
« Reply #10 on: February 26, 2013, 01:23:50 AM »

Common sense for the lead, and the brake tug.
I have seen lead tugs overturned and sunk because the trailing tug
was not controlling the forward momentum.

Also, "Who needs a bollard pull rating? When ya gots lots o' tugs"
http://www.youtube.com/watch?v=SVyZvItgspE

 8)

geoff p

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Re: Bollard pull question
« Reply #11 on: February 26, 2013, 06:10:36 AM »

Brilliant, Umi.  Thanks for that link.

Some years ago I was asked to help dispose of a dead-and-decomposing whale, using my 42-foot semi-tug.  It was washed up on rocks; the jaw and the tail had been cut-off by souvenir hunters, we could only get a hawser around its middle.

With a few jerks, the thing came off the rocks and floated, broadside-on to the rope.  At near to full power I guess we made less than a 1/2 knot.  (At full power, the prop was cavitating and the effective thrust dropped noticeably.)

After a couple of hours it was deemed we were far-enough away from land for the smell to less of a nuisance, so we shackled some old anchors to some old steel-hawsers around the carcass, and left it for the crabs to enjoy.

Geoff
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deadwood

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Re: Bollard pull question
« Reply #12 on: April 10, 2013, 11:21:52 PM »

How does Bollard pull relate to the loads a tug can pull?  :P

Maybe not quite the answer to your question but at least an interesting paper nonetheless that gives some hints how to arrive at props that deliver maximum bollard pull.



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deadwood

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Re: Bollard pull question
« Reply #13 on: April 11, 2013, 12:38:49 AM »

...but with ships (incl. barges, rigs, etc) wind and currents are a large factor alongside simple inertia of course.
...
 The factors are calculated on the areas of ship exposed to wind or water - but not the displacement of the ship. (Generally a bigger ship will displace more, but for example a loaded oil tanker will be mostly underwater with little windage relatively speaking

I don't want to underestimate the added forces due to wind, waves and current.
But as far as the inertia of accelerating (or rather decelerating) large floating objects (viz. the notorious ULCC) is concerned the involved masses literally do play  a massive role indeed.
Especially the added mass component known as hydrodynamic mass can contribute considerably to the required stopping force.

The factors are calculated on the areas of ship exposed to wind or water - but not the displacement of the ship.

Well, even if it was only for the frictional resistance to overcome.
Doesn't along with the displacement volume the wetted surface increase too?

But then we also have inviscid resistance components which towed and towing objects must overcome, such as potential (displacement) and wave making resistance.
Of course the the latter predominates rather at higher Froude Numbers which maybe aren't so common for towing.

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Tug Fanatic

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Re: Bollard pull question
« Reply #14 on: April 11, 2013, 08:15:17 AM »

The way that the tug delivers the power is also significant. A traditional propeller has a very limited range of directions of thrust through the 360 degrees around the tug. Modern drive systems allow pretty well equal thrust in any direction. For ship handling this make an enormous difference but obviously doesn't for say barge towing.
Thus both what you are moving and how you put the power down matter + wind + current + ...............................................
 
I get the impression that insurers & pilots dictate what a tug can tow & that they demand ever increasing tug power for the same job.
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dodes

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Re: Bollard pull question
« Reply #15 on: April 11, 2013, 02:18:23 PM »

Voith Snieder propulsion systems have max power ahead or astern only, as you apply side thrust so power comes of the towing thrust, the MoDn tuts would do 12 kts ahead or astern but only 3 kts sideways. So it was important with them to keep the tow wire dead astern with very little side thrust used.
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Umi_Ryuzuki

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Re: Bollard pull question
« Reply #16 on: April 11, 2013, 06:30:11 PM »

Over all the VS drives have less power at the propeller than direct or schottel drives.
But the thrust does not diminish if it  is vectored in any direction.

The reason that a VS drives so slow sideways, is not because they don't have power, but
because the thrust has to be vectored when the boat is flanking or moving sideways.
When flanking, the thrust is diverted at an angle to the bow and stern from either drive.
It is not a direct thrust that allows the boat to "crab".

It is the same with other drives, but I imagine that one of the bigger factors is the
issue of drag against the surface area of moving a boat sideways.  ok2

dodes

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Re: Bollard pull question
« Reply #17 on: April 12, 2013, 10:21:14 PM »

All I know is that as soon as you put side thrust to control the movement , you lose power from the towing wire , more so than a screw and rudder.
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deadwood

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Re: Bollard pull question
« Reply #18 on: April 12, 2013, 10:42:28 PM »

That sounds plausible, even to the uninitiated like me, since the surging movement of the towed seems to then result from the cosine component of the towing force
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Umi_Ryuzuki

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Re: Bollard pull question
« Reply #19 on: April 13, 2013, 08:20:00 PM »

All I know is that as soon as you put side thrust to control the movement , you lose power from the towing wire , more so than a screw and rudder.

I can see how that would occur, due to the fact that if you steer the VS drive it vectors
all your thrust away from the "tow" and into the turn. Where as a screw and rudder vectors
half the thrust from behind the propeller to initiate a turn.

Interesting dynamic.

However as a brake tug, many of the VS driven tugs have been designed to provide additional
bollard pull by using the drag of the boat hull rather than the power of the engines and "propeller".

So when towing with a VS drive, do you make your corrections at a slower rate to maintain tension on the line?

dodes

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Re: Bollard pull question
« Reply #20 on: April 24, 2013, 01:48:52 PM »

You try not to make large corrections yes, but it depends alot like all tugs on the speed of the vessel being assisted, I know years ago the tut Careful with its full amount of 5" wire paid out on Invinceable, nearly knocked out going around Devils point because the carrier was moving too fast. As to stern drag, this tug often assists on turns with moving vessels, an does this usually with an indirect tow, which means thrusting the opposite way to which the tows stern is wanted to go. In doing so, the water acts on the tugs skeg which gives a very strong pull.
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TMIS

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Re: Bollard pull question
« Reply #21 on: November 05, 2013, 09:39:44 PM »

Bollard pull (BP) can also be a good 'sales statistic' for getting a towing vessel on to a charter. Quoted figures can sometimes be a bit optimistic - based upon when the vessel is new, engines are tip-top condition, no deformation of propeller/s and the bollard pull test conditions are most favourable (ie in a fjord or other sheltered water). Bollard pull can also be quoted as 'static' or 'continuous' - the latter being more useful for commercial applications. As a vessel ages it can be reasonably expected her bollard pull can drop off to something a bit less than new - a bit like the horsepower of your car. As previously mentioned, underwriters of specific marine adventures will specify what the total minimum BP is for the work to be covered and in some cases trials will need to be done in advance of that work commencing to prove the vessel/s meet the minimum spec. Where the vessel is towing on a winch then its specification also needs to be considered. Modern anchor handlers have winches that will pull say 300te and hold say 600te on the brake but teh ship will only have a BP of say 250te. Commercial applications need to have these factors considered before the vessel is chartered. BP may be reduced when a diesel/electric powered towing vessel is operating on the high seas and using transverse thrusters against wind/waves/current or other heavy consumers that cause pitch reduction or power reduction of the main propulsion when thrusting against external forces (Bourbon Dolphin). So, BP is a bit of a spurious number and should only be taken with a 'pinch of salt' and always weighed up specifically for the work the vessel is expected to complete (safely). Well, at least in the real world that is. Rambling now - sorry......
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