How do you calculate

[KC6OLD]

How much your scale tug Should pull?
if the real one pulls 60 tonnes (2204 lbs per tonne)
and the scale is 1:50 Square root : 0.141
You can't build one that pulls that hard! (over 1800 pounds!!!)
what am I doin' wrong?
Dave

[Tony Mattson]

Scale bollard pull? Seen a few threads on bollard pulls but I don't
think that I've ever seen a thread on true-to-scale pulls in any
discussions on modelling tugs, paddle or otherwise. How interesting.

For screw tugs bollard pull is a function of propeller shaft power,
which is typically measured to an accuracy of ± 3% of the maximum
continuously rated engine output. Either an electric load cell or load
link is usually applied for the bollard pull measurements. Some load
links have the advantage that they can be connected to a recorder or
laptop to give a continuous record of the pull. The pull is taken on the
supply tug's towing hook, or towing cable and the certified load link is
inserted in the cable either adjacent to the towing hook or where it is
anchored to the bollard ashore. Screw size, number of blades and pitch,
presence of kort nozzles, etc., also impact screw tug bollard pull, and
I suppose that hull displacement does as well.

I assume that the same approach should be taken with paddle tugs - that
paddle shaft power is a function of maximum engine output. So, you first
need to have motive power that provides a true to scale shaft power
output. I think that this is a highly unlikely and impractical approach
to modelling - after all we modellers do like to use whatever motive
power is at hand and we generally factor in non-scale speed safety
margins to get our models out of 'trouble'.

In terms of hull displacement - the power of the tug is used to move the
hull through the water as well as apply a load to the tow. Displacement
should also be true to scale - not only in hull dimensions but also in
loading to true water line. Following this thought for a moment,
displacement's a cubic measure, so instead of square rooting there'd be
a cube rooting factor in there somewhere!

Paddle blades might be true to scale, but environmental factors such as
the breeze and fluid dynamics around your favourite sailing waters would
be unlikely to be scale. A puff of wind, a ripple or an eddy won't
necessarily get scaled down by 1:48 squared (forgive me the unintended
pun, oh mighty moderator!)

... at the end of the day, why would you bother worrying about true to
scale bollard pull? Don't you just need to build your model so that it
1. goes, and 2. is strong enough and powerful enough to handle your
club's largest tow barge, if indeed you want to tow things?


regards
Tony
Auckland NZ

At the end of the day I don't rate as a marine engineer's exhaust pipe,
so if there are more knowledgable folk out there then please feel
welcome to correct my ramblings and post the real oil on scale pulls!

[Tony Mattson]

John, you rate yourself below me in marine engineering, then casually
drop Reynolds Numbers into the conversation. Phew, too modest by far!

I think you're onto something: both mu (viscocity) and rho (density)
values are constant for water (OK, a bit of variation for salt vs
fresh water) but both emasures are unaffected by scale and impact the
final number.

I suppose you could apply Reynolds numbers to this but don't they
tend to relate to laminar/turbulent flows in piping where boundary
effects play a role? Unlikely to get boundary effect in a boat pond,
so we could try the more complex Navier-Stokes equations...

...but hey what the heck! Who really needs crusty-speak and scale
bollard pull on a model paddle tug?

Gimme a model that floats, goes splishy splash on a warm still summer
evening, and looks (at least vaguely) like a real'un, and I'll be as
happy as a pig in clover!

[John Roberts]

Hi, Tony,
As I understand Reynolds Numbers, or equivalent, They might have an affect on the resistance (drag) of the boat's motion through the water, varying as to the boat's dimensions.

<<<<...but hey what the heck! Who really needs crusty-speak and scale
bollard pull on a model paddle tug?

Gimme a model that floats, goes splishy splash on a warm still summer
evening, and looks (at least vaguely) like a real'un, and I'll be as
happy as a pig in clover!>>>>>

With this last statement, though, You really hit the nail on the head!!
Regards,
Modeldust (John, in Alabama)

[Paulrjordan]

Great Post, Tony....and I'm in awe of some of the terms you guys use.
Wow!! Reynolds Numbers!

I do think, though, bollard pulls (or ways of measuring a model tug's
tractive effort) are useful for competition classification. Tug
towing events are becoming increasingly popular now as the old
standard "steering course" is giving way to more prototypical tug
operation. Last year our club introduced barge pulling, pushing, log
coralling and a couple of other skillfully challenging tug events to
our regattas. We noticed quite a big difference in tug performance
(twins v singles v korts etc) and it would certainly be useful to
have some way of classifying competitors based on their tractive
effort or some way of simulating a bollard pull rating.

Maybe just stringing up the stern of the tug to a simple series of
weights on a "gallows" and cracking the throttle to see how much
weight it can "lift" might be a solution? Who knows...but if any of
you guys have ideas on this, then I'm paying close attention. As an
aside (but more to stay on topic!) we could perform our own test of
screw v paddle..but I rather think I already know the outcome!

I'm hopeful that one day we'll see serious model paddle wheeler
racing which emulates the strong competitive tradition that has
always existed in these vessels. When that happens, some way of
classifying vessels by tractive effort will be essential.

And, yes, Tony...a model boats which keep out more water than they
let in are GOOD model boats by any standards!

Cheers

PJ

[Tony Mattson]

Paul wrote recently
> I do think, though, bollard pulls (or ways of measuring a model
tug's tractive effort) are useful for competition classification. ...

Maybe just stringing up the stern of the tug to a simple series of
weights on a "gallows" and cracking the throttle to see how much
weight it can "lift" might be a solution? Who knows...but if any of
you guys have ideas on this, then I'm paying close attention.



Hi Paul,

I like the concept for a new Club competition... but weight lifting
is anathema to me (the original hollw-chest 96lb sand-in-yer-face
weakling!).

I wonder if a spring scale might be the trick. If you were to anchor
a suitable sized spring scale by its suspension ring to the poolside
and connect a tow line to the load hook, then you could couple the
tug to the tow line and measure the bollard pull that way. The same
set up might be able to test a wide variety of tugs without the need
for a variety of weights. Other ideas?

Having one of those geekie sort of minds for numbers I'd love to find
out whether club-mate Bob's 1:48 scale TID Tug powered by a trusty
Decaperm on 6V is relatively more powerful than John's 1:32 scale
Flying Phantom with its massive kort nozzle and Robbe 1000 12V drive.
But that word "relativity" is a cow! How would you classify:
- scale ratio
- electric vs steam
- model displacement
- motor size
- battery voltage
- kilowatt output
- number of screws, screw blades, diameter and pitch (for those screw
driven thingies)
- number of paddles, paddle float area (for real boats)
- Kort versus Schottel versus Azimuthing versus Voith-Schneider
- ?

If we could ever agree on one set of standard classifications that
account for all these options then I know at least one Auckland club
that would be keen to have a crack at a bollard pull competition.

As to paddle tug racing... yeah!!!!!! I can just see our boats -
retrofitted with planing hulls, stripped of superstructures, racing
paddles fitted (kinda like those superwide slicks you see on F1 cars!)

anyway, back to reality...
Tony
Auckland, NZ

[Paulrjordan]

--- In Paddleducks@yahoogroups.com, "Tony Mattson" <tmattson@i...>
wrote:> Hi Paul,
>
> I like the concept for a new Club competition... but weight lifting
> is anathema to me (the original hollw-chest 96lb sand-in-yer-face
> weakling!).

Yeh, Tony, but the 96 pounder always ended up with the most
lucrative job, biggest house and best looking wife!

>
> I wonder if a spring scale might be the trick. If you were to
>anchor a suitable sized spring scale by its suspension ring to the
>poolside and connect a tow line to the load hook, then you could
>couple the tug to the tow line and measure the bollard pull that
>way. The same set up might be able to test a wide variety of tugs
>without the need for a variety of weights.

Much better idea, and more efficient than the weights.

> But that word "relativity" is a cow! How would you classify:
> - scale ratio
> - electric vs steam
> - model displacement
> - motor size
> - battery voltage
> - kilowatt output
> - number of screws, screw blades, diameter and pitch (for those
screw
> driven thingies)
> - number of paddles, paddle float area (for real boats)
> - Kort versus Schottel versus Azimuthing versus Voith-Schneider
> - ?

That's the rub..I think the simplest answer (at least in the
beginning) is to rate a tug for towing competitions by simply THRUST
alone on a fully charged battery. This kinda gets rid of lots of
calculating and bickering over different types of props and nozzles
etc.

For speed events, I think the tug has to be rated over a measured
distance and either allocated a handicap number for racing or put
into one of three classes for log coralling etc. Paddlers could be
rated the same way. My past experience in establishing competitive
event guidelines (cars) is to always start simply so everyone has a
bit of fun and gets a generally level playing field and then see how
it develops...or who whines MOST!

Keep the ideas flowing...maybe we'll eventually organize the first
Paddleducks World Paddler and Tugboat competitions? On the other
hand maybe I'd better borrow YOUR words "back to reality!"

Regards

PJ
Victoria, BC Canada