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[ian-mccaffrey]

hi guys . is there a formula or some way of finding out the scale bollard pull of my tug ....

[dreadnought72]

A spring balance and some tow rope would seem to be the most obvious way.

Andy

[F4TCT]

Always wondered myself how to do the conversion.

Will have a mooch on the internet and see what i find

Dan

[Tug Fanatic]

I don't think that the conversion is really possible as scale bollard pull is not the same as scale ability to do towing work.

Lets take a standard modern tug with a bollard pull of about 60 tonnes and a nice big container ship which weighs around 130000 tonnes.

If we use simple percentages this gives us 1 unit of bollard pull per 2167 units of weight moved. This is ridiculous for a model as it means that we should tow a tonne (2200lbs) with a 1 lb bollard pull tug. I am guessing that this is about what most Springers give.

If we work on the tug alone and assume a scale of 1/32 ( which is the tug towing standard I think) the we can take the full sized bollard pull of 60 x 2240 (to give lbs) and divide by the scale which is 1/32 x 1/32 x 1/32 (length x width x height) we get a figure of 4.1 lbs bollard pull which using the ratio above should tow 4.1 x 2167 = 8885 lbs or about 4 tonnes.

The problem is that whilst we can scale the model we cannot scale the real world. Water, wind, time etc are all full sized in our model world & effectively destroy our scale calculations.

If you are going to measure your bollard pull using the spring balance method make sure you use a decent length tow line (2 metres would be nice) that puts your model well away from the bank or you will get a false reading.
My 42" tug has a bollard pull of about 12lbs which I suspect is about what a lot of the "professional" tug towing models work with and they use that to tow loads of around a tonne (which is a guess) & which is about 12x the power/weight ratio indicated above.

If you are going to measure your bollard pull using the spring balance method make sure you use a decent length tow line (2 metres would be nice) that puts your model well away from any bank & in deep water or you will get a false reading.

[malcolmfrary]

http://en.wikipedia.org/wiki/Bollard_pull
tells you why there isn't.  Even the best, and probably hideously expensive, simulation software, still needs to be validated in real life.  So the best way is to use the components that youfeel will do the job, then, if you are really that keen to put a number on it, get out the spring balance and piece of string.

[catengineman]

At a scale the Graupner Parat and the Graupner Tito Neri can move a 4.5 tonne vessel side ways with out too much problem so if scale bollard pull is worked out to be around the 4 tonne mark then that would probably be about right

R

[ian-mccaffrey]

lovely job thanx guys ..... i did the test today ......... 4 meter tow rope .....  test 1 ...15lbs and the knot slipped and i broke free ...... test 2       21lbs and the knot held !!!!          so i got a bolard pull of 12 lbs astern .... could some one work that out for me  using tug fanatics formula ????.................

(     If we work on the tug alone and assume a scale of 1/32 ( which is the tug towing standard I think) the we can take the full sized bollard pull of 60 x 2240 (to give lbs) and divide by the scale which is 1/32 x 1/32 x 1/32 (length x width x height) we get a figure of 4.1 lbs bollard pull which using the ratio above should tow 4.1 x 2167 = 8885 lbs or about 4 tonnes.   )

[andra]

Hi Ian,

Your question is an interesting one as it would allow tug fans everywhere to correctly replicate, in a dynamically correct way, the actual towing behaviour of the full size vessel. Often we are content to just divide the full size engine power by the cube of the scale factor, however as has been correctly stated we need to allow for the greater losses between motor/propshaft and bollard present at model level due to our inability to scale down water.

By far the best way then is to measure by effect (bollard pull, what we are all after) rather than cause (engine power, which is reduced by varying and often practically unmeasurable drivetrain efficency, apparent and real propeller slip, etc) when setting up our model powerplant and deciding what prop. diameter/blade number/pitch, motor voltage/gear ratio etc. we will experiment with. Calculators at the ready for the next part!

Simply stated without all the symbols etc. found in the naval architecture/marine engineering texts,


                                                          scale bollard pull = 1/scale factor^2.5

So if we have, say, a 1/100 scale model of a tug with 90 tonnes bollard pull, the model should have a bollard pull of

                                                   90 tonnes divided by (100 to the power of 2.5)

which works out as

                                                   0.0009 tonnes = 0.9 kg bollard pull from the model.

The formula applies of course to all boat/ship models and you do n't need to have a scientific calculator, just divide your full size bollard pull by the scale factor, then again, then finally by the square root of the scale factor to get your answer...oh and don't forget to convert from tonnes to kg by multiplying by 1000 at the end...or we are gonna see more underpowered models out there!

Hope this helps!

[ian-mccaffrey]

Hi Ian,

Your question is an interesting one as it would allow tug fans everywhere to correctly replicate, in a dynamically correct way, the actual towing behaviour of the full size vessel. Often we are content to just divide the full size engine power by the cube of the scale factor, however as has been correctly stated we need to allow for the greater losses between motor/propshaft and bollard present at model level due to our inability to scale down water.

By far the best way then is to measure by effect (bollard pull, what we are all after) rather than cause (engine power, which is reduced by varying and often practically unmeasurable drivetrain efficency, apparent and real propeller slip, etc) when setting up our model powerplant and deciding what prop. diameter/blade number/pitch, motor voltage/gear ratio etc. we will experiment with. Calculators at the ready for the next part!

Simply stated without all the symbols etc. found in the naval architecture/marine engineering texts,


                                                          scale bollard pull = 1/scale factor^2.5

So if we have, say, a 1/100 scale model of a tug with 90 tonnes bollard pull, the model should have a bollard pull of

                                                   90 tonnes divided by (100 to the power of 2.5)

which works out as

                                                   0.0009 tonnes = 0.9 kg bollard pull from the model.

The formula applies of course to all boat/ship models and you do n't need to have a scientific calculator, just divide your full size bollard pull by the scale factor, then again, then finally by the square root of the scale factor to get your answer...oh and don't forget to convert from tonnes to kg by multiplying by 1000 at the end...or we are gonna see more underpowered models out there!

Hope this helps!

 no it dont help what soever haha .... you have totally fried my brain now !!! ...... if i gave you the amount of lbs my boat can pull ... would you be able to work it out for me ????

[andra]

I almost fried my own little grey cells with that last post...if I could predict your actual bollard pull i would be a rich man LOL!  The formula says what you should aim for, getting it in practice methinks is where the fun starts!

[ray123]

lovely job thanx guys ..... i did the test today ......... 4 meter tow rope .....  test 1 ...15lbs and the knot slipped and i broke free ...... test 2       21lbs and the knot held !!!!          so i got a bolard pull of 12 lbs astern .... could some one work that out for me  using tug fanatics formula ????.................

(     If we work on the tug alone and assume a scale of 1/32 ( which is the tug towing standard I think) the we can take the full sized bollard pull of 60 x 2240 (to give lbs) and divide by the scale which is 1/32 x 1/32 x 1/32 (length x width x height) we get a figure of 4.1 lbs bollard pull which using the ratio above should tow 4.1 x 2167 = 8885 lbs or about 4 tonnes.   )

  wow  thats some pull you have there    whats the tug you have there?   are you running brushless???  regards ray

[ian-mccaffrey]

 wow  thats some pull you have there    whats the tug you have there?   are you running brushless???  regards ray

here is a video of her  on youtube ... and she is an asd tug kit from www.marksmodelbits.com

http://www.youtube.com/watch?v=OEfssIG4UKw

and im using 555 brushed motors and 5 bladed props in kort nozzles

[catengineman]

If you can find a vessel which weighs 4.5 tonne then see if you can move it
http://i85.photobucket.com/albums/k49/adonis_spanner/Tugs%20and%20tankers%20at%20Marchwood/MarchwoodLake013.jpg

I cant remember the spring pull from the tug but I know that the trainer tanker weighed in at 4.5 tonne the VS drive tug pushed it side ways with no problem and all at scale speeds.

A 1:32nd  Portgarth tug with twin ASD's managed to pull 12 Lb on a spring scale before the skipper let the string go...
http://i85.photobucket.com/albums/k49/adonis_spanner/Tugs%20and%20tankers%20at%20Marchwood/MarchwoodLake010.jpg

[ian-mccaffrey]

thanx for the answers guys . but im sorry i asked now lol ... you havd fried my brain

[Oldsmokey]

Hi Ian
Some years ago Brian Ward, (Tugnology) started to introduce the bollard pull test into the tug towing comps.
That only involve the spring balance and line method, it only gave a given models bollard pull.
Not sure if it is still is used by the Northern clubs.
In the Southwest only used if Brian sponsored the comp. Inter club or SWAMBC events no.   

[Tug Fanatic]

Ian

I am really impressed that you can get 10kg bollard pull from a pair of 555 motors.

Your numbers are fine but as I said originally some things don't scale. A 4 tonne load in a flat calm is very different to the same load in a breeze if it has any side area - which of course real ships and good towing models do.

[ian-mccaffrey]

well thats what it says on the scales .... i could take a pic for proof if you like

[Tug Fanatic]

I am not doubting you but I am still surprised. Are you sure that your "scales" are reliable?

You sound to have done everything right - nice long tow rope etc.

Perhaps others will come back & tell me that 5kg pull from a 555 is standard. Are you using the standard MMB power pack with 50mm nozzles? Make of Prop? Can you measure how many amps you are drawing & at what voltage?

This would make a fabulous Springer power source. Toesup has probably the most powerful Springer in the world and his is only 2kg with a near 100mm prop:

http://www.rcgroups.com/forums/showpost.php?p=8046449&postcount=1980