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Old Jul 11, 2007, 12:54 PM
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Brooks's Avatar
Bozeman, Montana, United States
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Measuring and increasing a tug's bollard pull

I used a rubberband and a foot ruler to measure my vac u tug's bollard pull. I taped the rubberband (a single strand of rubber from a cut rubberband) to the 0 end of the ruler. Using a string between rubber and tug, I measured the stretch of the rubberband while the tug pulled on the string.

I found that sinking the prop slightly deeper than the design waterline increased the pull by 50%. I sank the prop by putting 6oz of lead on the aft deck. While the stern is noticably deeper riding with the lead weight, the appearance of the tug was not altered enough to make it look un-tuggish. In fact, unless you notice that the normal waterline (boundary between red oxide undercoat and black hull) is no longer evident, you'd probably not think anything of it. The extra thrust makes a difference when pulling my barges. It would make a difference if you were trying to haul another boat off a sandbar, I am sure. One disadvantage of sinking the prop is that it reduces the prop-walk, or "paddlewheel" effect somewhat. If you are used to taking advantage of this effect while manuvering your tug, then you might want to practice your usual manuvers before showing off at the regatta :-).

Measuring pull with a rubberband and ruler is not as straightforward as measuing pull with a metal spring balance (fisherman's scale). Whereas the spring constant (cm of spring extention / oz of pull) is relativly stable wrt load, the rubberband constant is not. Eg. if 1 oz. pulls the spring out 1 cm, 2 oz. would pull the spring out about 2 cm. But a rubberband would behave differently, eg. for my rubberband 1 oz. = 1 cm of stretch, 2 oz.= 4 cm, 4 oz. = 16 cm of stretch. As the pull increases, a rubberband stretches more and more. This is because the cross sectional area of a rubberband decreases as weight is added. With less area to take the stress, the band elongates more. A steel spring does the same, but to a much lesser degree (although this is one reason why spring balances are not legal for selling food). All this means in practice is that you need to calibrate your rubberband with several different known weights, hopefully weights in the vicinity of the tug's actual bollard pull. I measured 3 known weights, and used my calculator's built-in linear regression program to figure out the appropriate rubberband K factor for each pondside measurement. The K was a pretty good fit to a straight line, so you could just graph the data, and skip the math, if you wanted.

You could skip the calibration altogether, and just go on rubberband stretch distance, if you were comparing 2 different props, for example. However, you might be mislead as to the magnitude of improvement in pull from a new prop; For the rubberband above, 4x the stretch might only mean 2x the pull.

Some bollard pull results for my vac u tug with standard motor, Phil Pace's 4 blade prop, 6cell NiCd battery :
Without lead weight 3.2-3.8 oz.
With lead weight 4.2-5.4 oz.
Phil's results, for comparison 4.5 oz.

Working solo, it was hard to keep the tug perfectly lined up with the ruler and rubberband, so there was some variation in recorded rubberband stretch.

If your bitts can stand it, a running start generates much more pull, say for jerking a casualty boat off a sandbar. 'With a running start, the rubberband stretched longer than the ruler, so I don't have an oz of pull figure, but it was a significant increase.

In real tows, there is a significant horsepower requirement difference between hauling several separate barges on multiple towlines, and hauling them rafted up into one big barge. My tow textbook says up to 30% of the resistance can be attributed to the drag of the towline, so dragging multiple towlines can slow you down. With the light barges I play with, this effect was evident to some degree:
Pull needed for 4 barges, each tied to the preceeding barge: 1.5-1.9 oz.
Pull needed for 1 big long barge made up from the previous 4 : 1.3 oz.
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Last edited by Brooks; Jul 11, 2007 at 01:21 PM.
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Old Jul 11, 2007, 01:15 PM
USA'd ex Brit
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Quote:
Originally Posted by Brooks
Measuring pull with a rubberband and ruler is not as straightforward as measuing pull with a metal spring balance (fisherman's scale). Whereas the spring constant (cm of spring extention / oz of pull) is relativly stable wrt load, the rubberband constant is not. Eg. if 1 oz. pulls the spring out 1 cm, 2 oz. would pull the spring out 2 cm. But a rubberband would behave differently, eg. for my rubberband 1 oz. = 1 cm of stretch, 2 oz.= 4 cm, 4 oz. = 16 cm of stretch. As the pull increases, a rubberband stretches more and more. This is because the cross sectional area of a rubberband decreases as weight is added. With less area to take the stress, the band elongates more. A steel spring does the same, but to a much lesser degree (although this is one reason why spring balances are not legal for selling food). All this means in practice is that you need to calibrate your rubberband with several different known weights, hopefully weights in the vicinity of the tug's actual bollard pull. I measured 3 known weights, and used my calculator's built-in linear regression program to figure out the appropriate rubberband K factor for each pondside measurement. The K was a pretty good fit to a straight line, so you could just graph the data, and skip the math, if you wanted.

Some bollard pull results for my vac u tug with standard motor, Phil Pace's 4 blade prop, 6cell NiCd battery :
Without lead weight 3.2-3.8 oz.
With lead weight 4.2-5.4 oz.
Phil's results, for comparison 4.5 oz.

If your bitts can stand it, a running start generates much more pull...
Or you could just buy a digital fishermans scale from WalMart..

Tigger (standard Springer) pulls 1lb 8oz, Scoot (straight Kort nozzle) pulls 2lb 12oz, (tapered Kort nozzle) 2lb 14oz... Mustang Sally (proposed Springer) should easily break the 3 - 3 1/2lb barrier..

A 'running start' is a 'no no' as it will probably result in the Bitts being pulled from the deck of your tug.. or barge.. You wont see a real tug doing that!.
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Old Jul 11, 2007, 01:32 PM
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Bozeman, Montana, United States
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With your big pull capacities, you could start a new Springer event: Salvage of sunken ship/barge/treasure. A line would run from the sunken object up to a pully suspended from a large buoy (large enough to support the sunken object if it were hauled off the bottom). The salvage line would have a small buoy attached to the end. A Springer would engage the end of the line, and try to pull the treasure to the surface. The large buoy would probably need an anchor to keep it in place while the Springer tugged away on the salvage line. Or maybe it could be a team event, with a 2nd Springer holding the large buoy in place until the treasure emerged, then helping the 1st Springer to manuver the whole gaggle to shore. A 6 pack of suitable beverages might make a good treasure to rescue :-).

I only mentioned the running start in case the value of the casulty was sufficient to risk damage to the model tug. One of the key mental steps one has to take in salvage is the shift from "retrieve the casulty just like it was" to "get it back anyway you can, broken if necessary". Whether a broken tug is acceptable is a judgement call, of course.
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Last edited by Brooks; Jul 11, 2007 at 01:40 PM.
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Old Jul 11, 2007, 02:17 PM
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Quote:
Originally Posted by toesup
A 'running start' is a 'no no' as it will probably result in the Bitts being pulled from the deck of your tug.. or barge.. You wont see a real tug doing that!.
That will not be a problem with my set up...as the tow line is not actually connected to the bollard, rather only lays on top of it then goes straight down a tube and is looped around the metal servo clevis rod that is in a hard wood block on the bottom of the glass deck. I feel pretty sure I can run at full speed and snap that line all day long with out ill effects to the tug. I built it that strong.

Now a normal tug is another matter all together.

Captain Slick
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Old Jul 11, 2007, 02:25 PM
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(I'm sure this has been done)

An ole fashioned TUG-O-WAR would be fun! Pick teams of tugs to share in the war just like humans do.

Captain Slick
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Old Jul 11, 2007, 02:38 PM
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that would be fun hoggy, or perhaps a sumo style push-out
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Old Jul 11, 2007, 07:27 PM
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Quote:
Originally Posted by Hoghappy
An ole fashioned TUG-O-WAR would be fun! Pick teams of tugs to share in the war just like humans do.
Quote:
Originally Posted by Ghost 2501
that would be fun hoggy, or perhaps a sumo style push-out
Its been done guys...
http://media.putfile.com/Scoot-the-Springer
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Old Jul 12, 2007, 10:30 AM
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Hmm. If you hadn't sealed that rear deck, she'da had you sunk!
- 'Doc
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Old Jul 12, 2007, 10:56 AM
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Cool Video...

...but...that sure was a short tow rope...lol

What I have in mind is a group of boats on one side, pulling off against another group at the other end of the main rope...all at the same time. They could be bow to stern, but I think side by side, all tied to the end of the main tow line that maybe has a loop in the end to tie several tug boat tow lines to, is my vision. A true team sporting event. Choose whatever method to pick the teams, but at random would be cool or club VS club? No real limit to the number of tugs pulling at the same time in this event.

Captain Slick
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Old Jul 12, 2007, 02:46 PM
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Tug 'o War

That sounds like the prelude to a grate how to class... "Now that we have recovered all of our boats and drained the water from them, lets dry them out and oil everything" LOL

Scott
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Old Jul 12, 2007, 11:16 PM
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This is a happy coincidence. I was looking at a digital fish scale at K Mart last Saturday with this very thing in mind. I was thinking about mounting it to a 2x4 or a snow fence post with a pulley near the base. The towline would pull down on the scale and the towline would pass through the pulley to run horizontally out to the tug. Anyone see a flaw in this plan?

I am converting one of my old tugs from a single Graupner/Marx Z drive to twin screw conventional. The one z drive barely moved the boat. I'm hoping the twin props will give it some muscle. I hope the measuring rig will give some reliable data.
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Old Jul 12, 2007, 11:16 PM
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Quote:
Originally Posted by RhumbLine
That sounds like the prelude to a grate how to class... "Now that we have recovered all of our boats and drained the water from them, lets dry them out and oil everything" LOL

Scott
No water ingress was reported in either Templeton OR Scoot during the making of that motion picture...
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Old Jul 12, 2007, 11:22 PM
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Quote:
Originally Posted by Prins Willem
..I was looking at a digital fish scale at K Mart last Saturday with this very thing in mind. I was thinking about mounting it to a 2x4 or a snow fence post with a pulley near the base. The towline would pull down on the scale and the towline would pass through the pulley to run horizontally out to the tug. Anyone see a flaw in this plan?
I just tie my scale off to the nearest immovable object (..ok, a sun lounger ) and pull away...
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Old Jul 12, 2007, 11:52 PM
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I hope you are sitting on the sun lounger, Toes!!! I know Scoot could drag one of those, "unattended", right into the pool!!!
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Old Jul 13, 2007, 08:34 AM
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Quote:
Originally Posted by Prins Willem
This is a happy coincidence. I was looking at a digital fish scale at K Mart last Saturday with this very thing in mind. I was thinking about mounting it to a 2x4 or a snow fence post with a pulley near the base. The towline would pull down on the scale and the towline would pass through the pulley to run horizontally out to the tug. Anyone see a flaw in this plan?
I'm not a rocket scientist, but wouldn't the pully increase the pull power thereby giving you a false reading? I'm thinking a direct hookup would be more accurate.

Captain Slick
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