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Author Topic: Scale speed calculation  (Read 3792 times)

warspite

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Scale speed calculation
« on: June 04, 2009, 09:31:14 PM »

Recently i posted a question regarding scale speed, and someone (my apologies for forgetting your name) stated that the method was correct in how i arrived at the inches per second figure at 1:1, then said that the scale speed was the square root of the scale.

This effective reduced the 10 foot test length time from 8-10 seconds to a staggering 1 and a half seconds, with this in mind does any one have a boat vid link and is at 1/72nd scale doing the equivelant to 45 KNOTS
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FullLeatherJacket

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Re: Scale speed calculation
« Reply #1 on: June 04, 2009, 11:39:47 PM »

Phil

As a designer of model aircraft and boats for the last Thirty-Lord-Knows-How-Many years, my advice to you would be thus;

You're dealing with a very small boat with a crazy power/weight ratio on full-size water in a bathtub, so forget any idea of calculating  realism. If it looks about right to you then it's generally in the right county.

Nuts to anyone who says otherwise.

Works for me.

FLJ
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warspite

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Re: Scale speed calculation
« Reply #2 on: June 05, 2009, 07:52:52 AM »

Was looking on youtube for ideas of scale speed and saw two PT boats, one in sweden, elco type and a chase boat, T something or other, there was a smaller grey boat filming the old elco and when it switched to the elco to film the grey thing the difference in the way it preformed was vast, the grey thing crashed through the water like it was flat fronted and the old elco gracefully sliced effortlessley through the water, what a difference.

will see saturday afternoon
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Bryan Young

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Re: Scale speed calculation
« Reply #3 on: June 05, 2009, 07:21:22 PM »

This seems to be an easy thing to answer...(?)
If the full size ship / boat whatever takes "x" no. of seconds to travel its own length then surely just dividing that time by the scale of the model will give an accurate(ish) scale speed. But of course it doesn't end there. You will need a couple of marker sticks set at the length of the model, and adjust the speed accordingly. You will in all probability find the result to be too slow. Correct perhaps, but visually too slow. I find that a very slow walking speed is visually more appealing. BY.
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Notes from a simple seaman

warspite

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Re: Scale speed calculation
« Reply #4 on: June 05, 2009, 07:48:37 PM »

 :} hopefully if the weather clears up i will be sailing her tomorrow afternoon, she's on charge now, from the bath test she appeared to be doing about a calculated 10 mph

if 45 mph = 95.23"/s and it travels 78" in 3.5 seconds = 22"/s then 95.23/45 = 2.1162"/1 mph therefore 22/2.1162 = 10.4mph - oooouuu!!!! i have a headache - but is it right?

 :o
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malcolmfrary

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Re: Scale speed calculation
« Reply #5 on: June 06, 2009, 07:57:19 PM »

When you have your boat making patterns that look right in the water, figure out how fast it is actually going, guess how fast the real thing would have been going, and you will find that the square root of the scale rule is involved.  Test tank users have been using this for donkeys years.  It also works the other way, knowing the performance of the original and dividing by the square root of the scale will tell you how fast it should be going to look right in advance.
45kt about 60mph = (60*5280)=316800ft/hr, (316800/3600)=88ft/sec,
scaling, sqrt72 near enough 8.5, (88/8.5)=10ft 5in per second.
If I take my 1//72 boat for a walk around "my" lake, its 1 real mile.  The boat has cruised a scale, linear 72 miles.  If I take an hour, it will have looked like it was cruising at about 8.5 mph.  For the crew, they will think that they have been out for 8 and a half hours.  No mysteries, everything is nicely accounted for.
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johno 52-11

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Re: Scale speed calculation
« Reply #6 on: June 07, 2009, 07:24:27 PM »

Having worked the scale speed out for a model Lifeboat a few years back. There is a page here http://www.freewebs.com/lifeboatmodels/severnspeedtesting.htm On how it was achieved.

As has already been stated the square root of 72 is about 8.5. So if we divide the real boat speed of 45kts by this we get a model speed of 5.3 kts.

There are 1852 Meters in a nautical mile so the model would need to cover 9815 Meters in an hour. Divided this by 3600 and you get Meter per second of  2.7

To test your speed find a measured length of water and divided it by the speed you want to give you the time it need to take to cover the distance. e.g. 30 Meters divided by 2.7 is 11.1 seconds.

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johno 52-11

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Re: Scale speed calculation
« Reply #7 on: June 07, 2009, 07:28:26 PM »

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warspite

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Re: Scale speed calculation
« Reply #8 on: June 07, 2009, 08:22:54 PM »

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