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[Southern Sailor]

Can anyone tell me how to calculate scale speed?  Can this be converted to power required, and hence size of motor, prop size and speed? Thanks and happy sailing.  Brian

[cos918]

have a read of this
http://www.deansmarine.co.uk/index.php?topic=6.0

John

[Bryan Young]

If you run a model at scale speed you'll be appalled how slow it is!
A real ship doing say, 15 knots may well take nearly a minute to cover its own length. So a "scale speed" should take the same time.. might be OK in real life, but looks a bit silly for a model.
Having said that....seeing a 10knot "LibertyShip" being hoofed around at a scale speed of 70 knots is just as daft.
Work out how long it takes for the real ship to cver its own length and try it. BY.

[wartsilaone]

 Scale speed is fine in theory but depending on the model and the scale it can become hard to control at really slow speeds.


Ali

[chris gillespie]

i was told by a wise man
 
" Scale speed " .....whit fur ? well you know to be realistic...........ah is that no whit the throttel lever on the remotes fir...you want slow scale speed dont put the throttle up full, yeh want to have a play or get oot o trouble then put the throttle full...
 
 
 
There is good logic there, as boaters at our pond do get into trouble with weed ( pond variety ) sharp edged concrete...mad boaters.. and mad mating swans.....a bit of unrealistic power comes in handy if and when needed
 
 

[Southern Sailor]

Well now!  Never doubt that you will be helped on this forum.  Wise words guys and sensible.  Thanks.  Actually I was waiting for my better half at the Sun Valley mall in Fish Hoek Cape Town yesterday and started looking at modelling the SWATH boat Silver Cloud and doing some rough calcs on size of model and other design sketches.  Naturally the type of motor and size of prop came up and hence the question of scale speed.  Looking at their website I was surprised to see that the top speed is quite modest at about 15 knots.  It should prove to be quite an interesting project and they have enough info on their site to enable the necessary plans to be drawn up for a model.  (By the way, don't try to do the stability calcs if you want a peaceful day!).  But oh dear, first the completion of Portgarth!  Stay focussed Sailor!  Cheers and happy sailing.  Brian

[malcolmfrary]

The question of scale speed crops up regularly, and usually attracts the reply that it is boring due to a lack of understanding regarding what happens to scale when considering area and volume. 
Empirically, when it looks right, it is right, but when it looks right it is invariably found that the model speed is equal to the real speed divided by the square root of the scale, not the scale itself. 
Consider a 10kt ship at 1:100 scale, rather than doing 10/100=0.1kt, when it looks right, it will be found to be doing 10/sqrt100=1kt.  (Nice round numbers picked to keep explaining it simple)
Power relates to moving a mass, which is proportional to volume, so use the cube of the scale, just as you would to work out the displacement.  With about 750 watts per Hp, if you know the Hp of the original you can convert to watts.  Divide this by the cube of the scale, you have the motor output required.  To allow for small motor inefficiency and whatever the shaft bearings and model prop are going to do at you, multply that by 1.5 to get the right motor input wattage and you can work out the current required at any voltage.  The only black magic concerns the efficiency of the model prop and the motor manufacturers specifications.

[Southern Sailor]

Thanks Malcome.  So at a scale of 1:32, 15 knots real is 15/5.66 or 2.65 kt scale or 4.91 km/hr.  That is 1.36m/sec which seems reasonable.  Brian

[chris gillespie]

you also have to consider,
 
at scale, water is denser for the model to propel through.

[Netleyned]

Wind and water cannot be scaled

Ned

[chris gillespie]

Wind and water cannot be scaled

Ned

of course it can, it would be just a fixed set of parameters, not condusive to other sets of parameters.
 
a scale boat going through water has more friction to go through...think of it as the equivalent of engine oil or soup

[johno 52-11]

We did a lot of work on this with the Severn a few years back there is a page on it here http://www.lifeboatmodels.co.uk/severnspeedtesting.htm

There is a calculation I have come across for working out the power required for a model which is the power of the real boats power divided by the qube of the scale and then divided by the square of the scale and then divided by the effiiency of the electric motor

So for a 1/12 Seven class lifeboat each engine is 1250 hp converted to watts thats 1250x750 = 937500
divided by 12 three times is 542.5 watts divided by 3.4641 square root of the sacle = 156.6 watts
now to take into accuont the effiency of the  motor we divide that by .7= 224.3 watts round up to 225

That would mean that we are looking for a 12V motor that draws 18.75 amps or an 18 volt motor 12.5 amps

the motors in the seven are running on 18 V and draw 14 amps at full power so our motors are not quite 70% but the calculation gives you an idea of what power you need and as long as you go over that a bit you should be fine.

[malcolmfrary]

Wind and water cannot be scaled

Ned

Yes they really can.  You just need to know the rules regarding the differences between length, area and volume.
There are some aspects of water that will not scale, the surface tension which determines the size of bubbles you can get which is why you can't get a proper creamy wake, and the phosphorescent plankton, which add to that creaminess.  The only aspect of air that doesn't scale is the speed of sound through it.  This effects the flyboys, because their models of supersonic planes will never run into actual supersonic problems.  It affects us with sound - a scale model steam whistle is pitched much higher for us than the real thing.  Pretty much all of our sound effects are pitched to please us, not to be true to scale.

a scale boat going through water has more friction to go through...think of it as the equivalent of engine oil or soup

Another mistake - the friction is identical - its just that the real thing, being bigger and thus having a bigger area to act on, gets more friction, but its in proportion.  The power required to move the model, and the distance required for it to drift to a stop do scale exactly from the original, given that the prop probably has unavoidable differences and there might well be structural differences in the size and shape of the hull and the bits n pieces attached to it.
No magic, just physics.

[chris gillespie]

Another mistake - the friction is identical - its just that the real thing, being bigger and thus having a bigger area to act on, gets more friction, but its in proportion.  The power required to move the model, and the distance required for it to drift to a stop do scale exactly from the original, given that the prop probably has unavoidable differences and there might well be structural differences in the size and shape of the hull and the bits n pieces attached to it.
No magic, just physics.

interesting.....
 
one naturally assumes given the particles in water stay the same and reducing the size of vessel to scale, means the model has a tougher substance to penetrate through...
if the terminology of friction is wrong to describe the process of a scale mode going through denser substance than the original.....what would you call it ?
 
 

[dreadnought72]

The density of water does not change.

Density is mass per unit volume.  Both mass and volume are terms that depend on the cube of the scale factor: x³. If you divide one by the other you will get the original density, no matter what x is.

That is, the density of water in a lake, bathtub, or small glass is the same.

Andy

[malcolmfrary]

interesting.....
 
one naturally assumes given the particles in water stay the same and reducing the size of vessel to scale, means the model has a tougher substance to penetrate through...
if the terminology of friction is wrong to describe the process of a scale mode going through denser substance than the original.....what would you call it ?

Thats the original incorrect assumption.  The substance being gone through is not denser, its exactly the same.  The coefficient of friction between water and full-size vs scale model is identical, the amount of friction between the two examples varies according to the scale.  The model has all the reality of the full size item, its just smaller.
A good start here - http://en.wikipedia.org/wiki/Square-cube_law
The thing is, its what you see on the water that looks right or not, when it looks right and you "do the numbers", you wind up with the square root considerations being right.  TLAR rules.

[dreadnought72]

a scale boat going through water has more friction to go through...think of it as the equivalent of engine oil or soup

This is correct. The density of the water hasn't changed, but there's a scale difference between the friction of the full-size and model vessels. Think of it this way: assume for the moment that the friction (drag) on a vessel is a surface effect caused by the hull moving through water.

This surface effect is dependent on the scale factor squared: x²

The displacement is dependent on the scale factor cubed: x³

Newton's Second Law states that a force = mass * acceleration.

Our force is friction, our mass is displacement, and - being negative - the acceleration is better called a deceleration. Rearranging the formula:

deceleration = friction / mass = x² / x³= 1/x

So a model finds this skin friction proportional to the scale factor. Drag on a supertanker is large, but the drag on a model of that tanker is relatively larger.

Andy

[chris gillespie]

excellant stuff dreadnaught 72
 
were talking same hym sheet.
 
water IS denser from the point of view of the scale model.........was my point and where i was coming from.
 
ill stick with my assumptions 

[dreadnought72]

Water is not denser. It's more viscous.

(Poor spermatazoa do the equivalent of wade through molasses at their scale.)

Andy

[chris gillespie]

viscosity relates to flow which is fair enough
 
at the end of the day its about resistance which is interconnected with density and viscosity and all from the point of view of the model.
 
* Back on topic *
 
there needs to be an allowance for the resistance a scale model will go through.

[NFMike]

You'd think speed was about the easiest thing to scale; it's basically one dimensional after all. But even in full size viewing a movement from a distance can make fast look quite pedestrian, so since models are always seen from 'afar' it's tricky. Scale speeds on model railways can make life a bit tedious, but by the time you chuck in the wake (bubble) problem as mentioned by mf above no model boat is going to really convince on the water. So you may as well just bung in enough power plus a bit spare and run it till it looks fair 

[Bob K]

To me scale speed is what looks and feels right, with your model on the lake generating just the right bow wave and wake to look like the original ship in a photograph.  In reality that tends to be quite a bit faster that calculated 'scale speed'.
 
Beauty is in the eye of the beholder.

[chris gillespie]

nicely put Bob K