Model speed

[Max CormickParticipant @maxcormick]

hi,

I'm working on a new model: Bismarck 1/250

I calculate his scale speed 30 kt  in about 1,00 mt/sec

Is there a tool or a way to measure the speed of a model in the pool?

 

Edited By Max Cormick on 24/03/2018 14:40:20

[Max CormickParticipant @maxcormick]

[Charles OatesParticipant @charlesoates31738]

Scale speed is the square root of the scale multiplied by full size speed. A formula that's been around for a long time. I think Ron Waring used it back in the 1960s, but it's probably much older.

The speed of the model on the water will have to be measured as time over a known distance. That's a bit of a faf to set out, but doable.

Many of us, probably most of us, are happy with the model looking right on the water, with some in reserve for emergencies.

Chas

[Max CormickParticipant @maxcormick]

Hi,

I used

v = V/√S where v= speed in scale S = scale V = real speed so v=30/ square( 250)=1,90kt –>1,00mt/sec

[Dave MilbournParticipant @davemilbourn48782]

Given that Chas' formula produces a scale speed of 474 knots I think that Max's version is more likely. Malcolm Frary allegedly has the correct formula tattooed on the inside of his eyelids so I'm sure he'll confirm it one way or the other.

DM

[Charles OatesParticipant @charlesoates31738]

Dave, I think you took the root of 250, not 1/250. Which is .004, or are my senior moments getting too much for me.

Chas

[Max CormickParticipant @maxcormick]

hi

V	scale	v = V/√S	km/h	mt/sec
30	250	1,897367	3,604997	1,001388

 

Edited By Max Cormick on 24/03/2018 17:52:45

 

 

Edited By Max Cormick on 24/03/2018 17:53:14

Edited By Max Cormick on 24/03/2018 17:55:57

[Colin BishopModerator @colinbishop34627]

As Charles says, it is fairly easy to judge by the way that the model looks on the water and a bit of power in hand is always very useful.

No need to get too tied up with the statistics!

Colin

Edited By Colin Bishop on 24/03/2018 18:35:21

[Max CormickParticipant @maxcormick]

Hi

I see and I agree

[Paul TParticipant @pault84577]

You all need to revise your calculations as a truly accurate measurement of scale speed should also include water salinity, viscosity and temperature.

[Dave MilbournParticipant @davemilbourn48782]

Chas
You're right, you know. I can have senior moments, too – especially after the trauma of moving house!

Paul
I've seen far too many models driven at grossly unrealistic speeds and with tight turns which, to scale, would be well beyond the capability of the full-size. That being the case I doubt if your additional parameters would make any significant difference – except to the academic.

DM

[Paul TParticipant @pault84577]

Dave

If calculations are worth doing then they must include all parameters and variables.

But I have to admit to leaving out the overall weight of the ship including any used fuel, air pressure, prevailing wind and/or weather conditions, the phase of the moon and the dragging effect of deep water.

[Dave MilbournParticipant @davemilbourn48782]

And presumably whether or not there's a letter "R" in the month?

[Charles OatesParticipant @charlesoates31738]

And how much beer has been consumed?

I'm for practical modeling, I admit to never calculating the correct speed for a model, or anything except using a suitable motor and making sure it sails right way up.

As long as we each enjoy whatever we do, not much else matters.

Chas

[Dave MilbournParticipant @davemilbourn48782]

And so say all of us!

[Malcolm FraryParticipant @malcolmfrary95515]

When it looks right, it will be right. When it is measured looking right, it will be found that the square root formula has been agreed with.

What you can not scale is the reaction of the water in forming those nice creamy wakes. That depends on the surface tension of the water, and that remains constant, so whatever bubbles form are always the same size.

One thing that is useful is using a cube root formula to guess at the power needed. Important factors there are trusting the quoted figures for the ship in question (naval authorities are known to sometimes be economical with the truth when state secrets are involved), then knowing that there is a difference between power output (full size) and power input (to a model motor) and the efficiency difference between getting the power into the water. Since they need sea trials to find out if the real thing lives up to expectations, there should be no surprise when it happens that there is a measure of guesswork in the model world. TLAR gets used a lot, as does TSAR.

[Kev.WParticipant @kev-w]

If it looks too fast for scale ……………………

………….. ease back on the throttle stick until it does look right, you don't have to run 'flat out'.

[Dave MilbournParticipant @davemilbourn48782]

Ah-HA! In spite of all the evidence to the contrary I held firm to the belief that there was at least one other person out there who understood the significance of the words "speed controller" as opposed to "motor switch" – and it's Kip!
Hooray!

DM

[CookieParticipant @cookie15923]

L.O.L well said Kip

[Malcolm FraryParticipant @malcolmfrary95515]

As Kip said, when it looks right, it will be right, even if the throttle stick is not at max. It is nice to have a bit of extra go in hand just in case, but it is not mandatory to use it all the time. Unless its that kind of boat.

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