How do you calculate the scale speed of a model boat?

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19 October 2009 at 08:14 #23591
Rick Devonshire
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@rickdevonshire
How do you do it?

Many years ago I got into slot car racing and remember a debate in “Model Cars” about this matter as a number of people were claiming their slot cars were doing speeds of 300 to 800 mph when this was scaled up in line with the scale of the cars.

One or two ‘wise owls’ then came up with a formula which, if my memory serves me correctly, containe the square root of the scale of the model and which gave far more realistic figures when applied to the situation.

Unfortunately I can’t remember the complete formula or reasoning behind it.

Have we any similar ‘wise owls’ in the boating world? May be someone who has worked at the NPL , Vosper Thornecroft or other research institute would be able to help?

Rick.
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19 October 2009 at 08:14 #3437
Rick Devonshire
Participant
@rickdevonshire
I wonder how to calculate the scale speed of a model boat, any ideas?
19 October 2009 at 08:59 #23592
Bob Abell 2
Participant
@bobabell2
Hello Rick

The calculated speed must be correct and the fact that some expert has devised a formula to reduce the figure makes no sense at all……..IMO

Can`t wait to see how my 1/6th scale Slo Mo Shun performs!

A speed of 1.5 feet per second represents 180 mph…….We`ll see!

Bob
19 October 2009 at 12:48 #23593
Colin Bishop
Moderator
@colinbishop34627
There was an article published in the February 1983 issue of Model Boats by Ron Warring on the subject of scaling all sorts of things down, Displacement, Strength, Speed, Winds, Time etc.

The ones we are interested in here relate to Speed and Time and the following are relevant extracts. When you think about it it all makes sense although I doubt if they teach this sort of thing in schools today.

Colin

Scale Speed.

True scale speed is equal to the square root of the scale of the model multiplied by fullsize speed. For examplewith a 30 knot destroyer at 1/100 scale the calculation would be 1/10 x 30 = 3knots which is fast walking speed. In calm water that would look pretty much correct.

Scale Time

Scale time is derived from speed being distance divided by time so things appear to happen faster when scaled down, a boat will roll more quickly for example. This effect used to be used in cine photography to make model ships look more realistic. The scaling down is on a similar basis to the speed above so for our destroyer one minute in scale time would be the square root of the scale which is 1/10 x60 seconds = 6 seconds.
19 October 2009 at 17:38 #23596
Rick Devonshire
Participant
@rickdevonshire
Many thanks Colin, that’s where the sq, root comes into the calculation.

Rick.
19 October 2009 at 18:21 #23599
Dave Milbourn
Participant
@davemilbourn48782
Easy!

You post exactly this question onto a couple of model boat forums, then you wait for about seven days. After that time, add up all the answers (there will be dozens). Divide the total by the length of your model in mm; add the square root of the stall current of the motor…..

and then eat the result…….

If it looks about right then it’ll do. If it don’t then it won’t.

DM
19 October 2009 at 18:22 #23600
Colin Bishop
Moderator
@colinbishop34627
Paul Freshney has also suggested looking at this site:

http://www.nhsouth.com/crafts/workbench/ssc.htm

Colin
19 October 2009 at 18:54 #23603
Bob Abell 2
Participant
@bobabell2
Colin says..

“Scale speed = sq root of scale x full size speed”

Slo Mo………….sq root of 1/6 x 180 mph

1/2.45 x 180 = 73 mph……………What?

Bob says……………180 mph / 6 = 30mph…………..more like it?

(The brushless boats at Bury travel at 40 mph)

Who`s right?………………..Bob
19 October 2009 at 19:11 #23604
ashley needham
Participant
@ashleyneedham69188
whatever looks best on the water!!

and in your case Bob as slow as you can get away with, just get it on the plane will look ok I recon, as its a big beastie.

Ashley. (always overpowers boats for more fun, and to escape the Swans chasing the Canada geese off the pond)
19 October 2009 at 19:40 #23605
Colin Bishop
Moderator
@colinbishop34627
I’m not asserting anything Bob, just repeating the stock formula from R.H. Warring who was a respected engineer in his time I believe.

But why shouldn’t it be 73mph? 30-40 mph may indeed be the speed the models go at but that doesn’t necessarily make it scale.

I think the standard formula quoted above was intended to produce a wave and wake pattern corresponding to the full sized vessel for ships which have a “normal” operating speed of between 10 & 30 knots or so. Applying the formula to a speed of 180mph would perhaps take it out of its comfort zone.

My personal view is the same as Dave, if it looks right then it is about right and that does tie in with the result for a 1/100th scale model travelling at around 30 knots.

Colin
19 October 2009 at 20:16 #23606
Bob Abell 2
Participant
@bobabell2
Just been checking my figures…………..and I`ve made a serious error!….Apologies all round, chaps

Slo Mo speed was 180 mph………..at 1/6th scale………this becomes 30mph

30 mph is 44 feet per second!……………………………………Just possible?

73 mph is 107 feet per second!…………Blimey!……………Impossible!

So 30 mph seems to be the correct scale speed afterall?

All good fun……………….Bob
19 October 2009 at 20:46 #23607
Colin Bishop
Moderator
@colinbishop34627
In the case of Slo Mo Bob, if it’s going round the lake chucking vast quantities of water at all and sundry, making a noise like an amplified banshee and causing everyone to cover their ears and cower on their knees in fear – then I’d say you’ve got the scale speed about right.

Colin
19 October 2009 at 20:59 #23608
Bob Abell 2
Participant
@bobabell2
Well, lets hope it does!

In certainly is a Pandora`s box!

Bob….with fingers crossed!
20 October 2009 at 10:59 #23614
Colin Bishop
Moderator
@colinbishop34627
Just to pick up on Bob’s earlier post – if you applied his formula to the 1:100 scale destroyer I mentioned you could launch it from the pondside and collect it from the opposite bank a week later – Slo Mo Shun indeed!.

Colin

Edited By Colin Bishop on 20/10/2009 10:59:44

Edited By Colin Bishop on 20/10/2009 11:00:19
21 October 2009 at 16:12 #23628
Paul T
Participant
@pault84577
If its faster than your hand to eye co-ordination then its too fast……….walking speed is my limit. except on a nice long straight bit when no one is looking..(but then the bow wave does cause some comment)
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