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[cos918]

Hello all
There seam to be a trend to get parts on boats working eg on ferry bow door . cranes etc .
I was in Rosslare heading back to Fishguard on the Stena Europa. On the next dock there was the Isle Inishmore. I watch her bow doors close and our stern ramps close. It takes several miniuts say 4 to 5 min . Cranes do the lift in miniutes not seconds and if it is a heavy lift 500+ tones it takes hours.
The question is correct to have a bow door close in say 30 sec on a model or should it take 4 to 5 min.  Realisem is ok but to dock a boat open the bow door then close the bow door 8 to 10 min seams a bit long .Your thoughts would be aprichated.

John

[Colin Bishop]

Does the extract from last year's MB Commemorative Special help?

Colin

Scale Time
There is, however, one important difference between scale on fullsize speeds and that is scale time. Yes, time can be scaled just like anything else. Things happen at a different time rate when scaled down by the same factor governing scale speeds. In fact, this derives from the fact that speed is distance divided by time, or time = distance divided by speed. Distance is a linear dimension (factor S), and scale speed has a factor square root of S. Thus scale time has a factor Scale/square root of scale = square root of Scale (or the same factor as scale speed).

What this means is that things appear to happen faster in scaled down sizes - a model pitches or rolls much more rapidly than a fullsize craft. In a 1/16th scale model, for example, time has also been scaled down by a factor of square root of /1/16 or 1/4. Thus the scale minute has become 15 seconds - or the model is displaced four times more rapidly than its fullsize counterpart.

 This effect is used to 'correct' tine photographs of a model's behaviour (e.g. in a test tank). Projection of the picture is slowed down by the time scale factor square root of 1/Scale when movement of the model appears exactly as if it were a fullsize craft.

Still find scale time difficult to accept? Then think of a grandfather clock with a 1 second pendulum, and a 1/4 size model of that clock. The period for a pendulum is proportional to the square root of its length, so the scaled down clock would have a 'ticking' rate of square root of 1/4 x 1 second = 1/4 second. In other words, by scaling down the clock, 'time' has also been scaled down in the ratio square root of Scale. (It would need different gearing to display correct 'fullsize' time via the hands).

[Sundaysailor]

Hello  Cos  , I worked in ferries for 28 years , in the actual first Bow Door craft that sailed out of Dover "The Free Enterprise 2 " . Was on the bridge as Quartermaster on its maiden voyage . When we went on "stations"for arrival off Calais I was relieved from the wheel , the QM who took over Hit the Peir ! Put a bloody great dent in the bow door , took hours to get it open and the ship unloaded . We had that dent for the rest of the 1965 season till she went for her winter refit !   I was in most of the FE s with bow doors that followed for the next 20yrs .
The speed of her door from closed to wide and locked was about 3 mins . What took up most of the time from hitting the "Pads" to getting the first vehical off was opening the inner water tight door that is behind the bow door . In all maybe 8 mins .   I think you will find that most modern ferries might have both movements combined on one set of highdraulics .
Colin , sorry, but I disagree with your theory of visual size/time  . If you stood looking at the second hand of your wristwatch held up so as you can see the face of a giant clock and its second hand  (maybe 200 yards away) I think you will find that they both pass the same numeral on their respective dials at the same time ! So the timing is the same on both !  But the distance travelled by the tip of each second hand 100 x different .
Get out of that one Einstien !

[Colin Bishop]

Colin , sorry, but I disagree with your theory of visual size/time  . If you stood looking at the second hand of your wristwatch held up so as you can see the face of a giant clock and its second hand  (maybe 200 yards away) I think you will find that they both pass the same numeral on their respective dials at the same time ! So the timing is the same on both !  But the distance travelled by the tip of each second hand 100 x different .
Get out of that one Einstien !

Not my theory but that of Ron Warring who was pretty much an expert on this sort of thing and I'm happy to go along with his arguments.

Anyway, who is Einstien?

Colin

[Umi_Ryuzuki]

Well,... I think, and it is my opinion, that if you closed the bow door over the count of 8-10 seconds, it would look good.

 

[ZZ56]

Colin , sorry, but I disagree with your theory of visual size/time  . If you stood looking at the second hand of your wristwatch held up so as you can see the face of a giant clock and its second hand  (maybe 200 yards away) I think you will find that they both pass the same numeral on their respective dials at the same time ! So the timing is the same on both !  But the distance travelled by the tip of each second hand 100 x different .
Get out of that one Einstien !

I believe Colin is referring to the pendulum that powers the clock, and how it is impossible to get a small pendulum to swing freely at the same rate as a larger one

Never mind scale time, five whole minutes is just boooooring.  Getting it to close in one smooth motion is a better goal, and maybe five to ten seconds should be enough to give people the impression that it's a big heavy door.

[Colin Bishop]

I think it is pretty obvious really. In the moveies, such as 'Sink the Bismark' the model action has to be slowed down to make it more realistic.Model boats tend to bob up and down too rapidly. If you have a model battleship and want to traverse the gun turrets it will look much more realistic on the model at a fraction of the speed the real turret would take to turn.

It is usually fairly obvious on a model whether something like opening a bow door looks realistic or not and I reckon that what seems to be realistic will be very much in line with Ron Warring's formula.

Colin

[deadwood]

Scale Time
...
Distance is a linear dimension (factor S), and scale speed has a factor square root of S. Thus scale time has a factor Scale/square root of scale = square root of Scale (or the same factor as scale speed).

I would rather argue it depends on what form of similarity is viable (see below).

Not my theory but that of Ron Warring who was pretty much an expert on this sort of thing and I'm happy to go along with his arguments.

I'm afraid, I don't know who Ron Warring was.
(just have googled this name and got a link to a tribute/obituary page about a guy who worked for the Aeromodeller magazine; is it him you refer to?)
But I'm pretty sure, as far as (scientific) ship modelling is concerned, that the kudos is due to William Froude who was the first to discover a viable law of ship-model similitude
and from there to devise an ingenious method to predict the power required to propel a ship at a given speed from model tests alone.

By the Froude method roughly the ship's total resistance is devided into a frictional and a wavemaking component.
By towing fully submerged plates Froude could derive a frictional correlation line from which he could extrapolate the frictional resistance of the full scale ship.
The remaining part of the total resistance (which mainly consists of the potential or displacement resistance) was called residual resistance and measured by towing the ship model at Froude similarity,
i.e. at a speed resulting from identical Froude Nos. for model and ship.

If you equate both Froude Nos. for model and ship and cancel out the gravitational acceleration you indeed arrive at the mentioned square root of scale relation,
i.e.

Code: [Select]

Vm = Vs / sqrt(scale)
But this is only half of the story because we also have to account for effects caused by viscosity.
And here comes a second dimesionless parameter/number into play, i.e. the Reynolds No. named after Osborne Reynolds.

Froude and Reynolds Nos. can also both be derived from dimensional analysis of the Navier-Stokes equations which are the mathematical model describing fluid flow.
I would say they can be thought of as a formulation of Newton's Second Law applied to fluids.

Because the NSE must hold for model and ship as well, to achieve full similitude one would have to adhere to identical Froude and Reynolds Nos. for the model at the same time,
which is impossible to do.
For once the model would have to swim in e.g. mercury rather than water.
But what is even more prohibitive or impractical is the resulting model speed.
If you equate the Reynolds Nos. for model and ship and cancel out the kinematic viscosity you arrive at this scaling relation.

Code: [Select]

Vm = Vs * scale
I'm not sure if Froude knew about the works of Reynolds, but he devised a very clever and practical method to the shipbuilders' rescue from this dilemma.

In fact his method is still used today by ship model basins/towing tanks around the world to predict the required power to propel a ship,
as can be seen here and here.
The latter is the by the proposed standard ITTC'78 procedure.

Thus, I would argue that the scaling of time depends on what similarity you want to achieve.

[philk]

i think basically  if it looks right it is right and bug+er the theories

phil

[malcolmfrary]

If it looks right while moving, it will be right, but if you bother to time and measure things, you will wind up with the square root of the scale as the factor for scale time.  As simple and as complicated as that.
The area where scale can't work is sound effects.  If you make a scale model of a steam whistle, the chances are that you won't hear it, but the dogs being walked in the park.  iSame with motor sounds, but full scale sounds are so much more acceptable.

[derekwarner]

  ...good having you on board ...deadwood....."I would rather argue it depends on what form of similarity is viable"

It is always good for the forum to have an additional enquiring/mathematical mind .......looking over over shoulders

I hope you enjoy the model boat forum & look forward to your continued input & comments ...........regards Derek

[wartsilaone]

If you watch my Youtube videos you may guess that I have slowed it down to make it look more realistic. Because of the wind and water conditions neither of which are to scale it is difficult to stay in complete control at such slow speeds.                                               Slowing down the movement of the boat works well but I wouldn't do the same when it comes to doors and ramps. I've seen the real things in action and all I want to do is speed time up to get it over and done with.

Ali.

[Colin Bishop]

I'm afraid, I don't know who Ron Warring was.
(just have googled this name and got a link to a tribute/obituary page about a guy who worked for the Aeromodeller magazine; is it him you refer to?)

Almost certainly, yes. Ron's purpose in the old Model Boats article was to give readers a practical grounding in the subject of scaling down various things (including structures in terms of strength) rather than to delve into the more abstruse areas of theory.

As you say, Froude's genius was to devise formulae whereby shipbuilders could use models to predict the performance of full size craft. There is a lot in the saying, 'if it looks right, it is right' and using Froude's formula for scaling down speed you do get a realistic wave pattern in still water and a model speed which generally looks OK if sometimes maybe a little slow. The only problem of course, as you quite rightly state, is that the viscosity of the floating medium does not alter so instead of getting impressive sheets of spray when pushing into the waves, you get dollops of water instead but there's not much you can do about that.

Us recreational model makers are not too much concerned with the benefit of using these formulae to measure hull resistance, a bigger motor or another couple of volts remedies most deficiencies on an experimental basis!

With regard to this particular query, there is another factor at play too - smoothness. I have seen models with lowering ramps (landing craft etc.) and also models which raise and lower the anchor and the real detriment to realism is not so much how fast they do it but how smooth the motion is. Many are quite jerky in operation and this does affect the realistic appearance.

As a footnote i have also recently seen the saying: If it looks wrong, it probably is but if it looks right then it might be!

Colin

[wartsilaone]

I'm with Colin on that one. When watching model trains there's nothing worse than a loco that keeps hesitating at slow speed.             Well perhaps one thing, the old hand from the sky moment.

Ali.

[Arrow5]

I believe that salt is used in film studios to represent windblown spray.  I wonder if Epsom Salts in the wake of a fast moving ship model would give that lovely green, blue frothing effect.

[malcolmfrary]

I believe that salt is used in film studios to represent windblown spray.  I wonder if Epsom Salts in the wake of a fast moving ship model would give that lovely green, blue frothing effect.

Dunno about green and blue.  Not too sure about the froth, either.

[iron99]

Hi Cos,
If you look at model train clubs, and their operating sessions, you will find they often use a "fast clock" or scale time, so that a session of several hours often represents a full day. Using scale time on your bow doors will probably look real enough. After all, modelling is "representing" the real thing!
Regards

[unbuiltnautilus]

Fit an ACTION Noisy Thing and search the net for some heavy machinery noise to complete the illusion.

[wartsilaone]

Nobody's worked out how to make black smoke yet though.

[Umi_Ryuzuki]

Nobody's worked out how to make black smoke yet though.

I've done that, make black smoke, unfortunately it involves burning the ESC, and some of the wiring insulation.

 

[cos918]

Thank you for the replys. Some intresting thoughts there. I am thinking of about 30 sec to 60 sec to close or open. I do agree 100% on that it has to be smooth.

John

[Arrow5]

Dunno about green and blue.  Not too sure about the froth, either.

  Perhaps some colouring agent, food dye or similar?   Froth was the wrong word , I think effervescent is more accurate for small bubbles in the wake of a ship.

[Umi_Ryuzuki]

  Perhaps some colouring agent, food dye or similar?   Froth was the wrong word , I think effervescent is more accurate for small bubbles in the wake of a ship.

Froth?... Alka-Seltzer glued to the bow, or hidden in one of the bow thrusters...

 

[wartsilaone]

plunk plunk fizz fizz.