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by Charles Darley

Scale speed of model boats

From research that I have done, you do not just take the scale of the model and use that to calculate the scale speed of the model boat.

For instance a 1:36 scale model whose real ship would have the speed of 20 knots would result in a speed of only 0.5 knots (20 divided by 36 = 0.5).

The bow wave at such a speed (0.5) would not be realistic.

To approximate a realistic scale speed we have to enter into the science behind "Tank Testing". Here the scale speed is based upon the SQUARE ROOT of the scale. In the example given above this would give a speed of 3.3 knots. at such a speed the bow wave would be realistic.

For the Scot Ranger with a real speed of 11.5 knots when ballasted and the model at a scale of 1:32 the scale speed should be of the order of 11.5 divide by square root of 32.

Thus 11.5 divided by 5.65 = 2.03 knots. A slow walking pace.

The maximum speed that a boat can reach, before it becomes a planing hull, is specified by its Hull Speed.

The speed in knots equates to the square root of the hull length in feet multiplied by about 1.34. The constant varies with the shape of the craft with 1.34 being the mid of the range.

Thus for the 9'6" foot craft the max speed is 3.08 x 1.34 = 4.1 knots. Thus the scale speed mentioned is therefore well below the maximum. It is apparent from my own testing of the craft that to reach such a maximum speed, the size of the motor would need to be disproportionately large and of course would be well above the scale speed.


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