Scale speed ?

[Nevs]

Have a 51" zodiac like boat I am trying to build with an MHZ outboarder..A 400" 1:1 zodiac could do maybe 60 mph..But a 51" would probably not look fast at (60 x 51) : 400 = 7,65 mph...Is there any formula ?

[Nevs]

Hmm. Found this on another forum:
"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.""
1/4 would be : 0,25 square = 0,5 x 60 = 30 mph
1/5 would be : 0,20 square = 0,44 x 60 = 27 mph
1/10................................= 0,32 x 60 = 19 mph.

[Fluid]

That is the correct formula. When movies used real models instead of CG, the movie makers would slow down the ships' film speed to make them look more realistic using that equation..


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[NativePaul]

To save the maths, for the OP's example of a 51" model of a 400" boat the scale is 7.84 and the scale speed is 1:2.8 which for 60 mph gives a scale speed of 21.4mph

Incidentally squaring of the scale factor scales both distance and time, so to slow down the film speed you have to modify that equation. Slowing the film speed does look great though, if you slow the footage down by the square of the scale it scales gravity so the boat reacts slower to external stimuli as if it had scale weight. I have not seen it written but I believe if you want to shoot as realistic video as possible the equation for the scale speed would be the cube of the scale factor. Taking the OP's situation for example he would run at a 1.95:1 scale speed which is 31 mph and run a camera at 67fps, which when played back at 24fps should look great.