Hello, if I have a calculated Hull speed of 9 knots (45 ft boat) and a Froude number of 0,23 what that means? Is it possible? In the Froude number and the hull speed calculation the only variable is the water line. I use the water line to calculate the hull speed and use that speed and the water line to calculate the Froude number. Is this ok?
They are two different types of measurement. You use the waterline in both but one is non dimensional speed used in calculations, the other is a specific speed point (guess). The hull speed is empirical guess based on experience with bow waves so 1.34 x SQRT( Load Water Line). It gives a good approximation of when the waves are going to start. This was a limiting factor for engineless sailboats, now you can power through them. Froude number allows you to describe any ship of any size with another. You are non-dimensionalising speed with respect to length so you can compare apples with apples. It also tells you the wave pattern and fun stuff like dry transom (Fn=0.33-ish) and so on. Hull speed equates to about Fn = 0.35-0.4 when wave-making resistance takes over from frictional and form resistance increasing the power required, so it is an important part of the model. For your case LWL = 13.71m (45ft), V = 4.63 m/s (9 kn), g = 9.81 m/s^2 (32.2 ft/s^2). So Fn = V / SQRT(g/L) gives Fn = 0.38 and close to hull speed.
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Hello, if I have a calculated Hull speed of 9 knots (45 ft boat) and a Froude number of 0,23 what that means? Is it possible? In the Froude number and the hull speed calculation the only variable is the water line. I use the water line to calculate the hull speed and use that speed and the water line to calculate the Froude number. Is this ok?
They are two different types of measurement. You use the waterline in both but one is non dimensional speed used in calculations, the other is a specific speed point (guess).
The hull speed is empirical guess based on experience with bow waves so 1.34 x SQRT( Load Water Line). It gives a good approximation of when the waves are going to start. This was a limiting factor for engineless sailboats, now you can power through them. Froude number allows you to describe any ship of any size with another. You are non-dimensionalising speed with respect to length so you can compare apples with apples. It also tells you the wave pattern and fun stuff like dry transom (Fn=0.33-ish) and so on. Hull speed equates to about Fn = 0.35-0.4 when wave-making resistance takes over from frictional and form resistance increasing the power required, so it is an important part of the model. For your case LWL = 13.71m (45ft), V = 4.63 m/s (9 kn), g = 9.81 m/s^2 (32.2 ft/s^2). So Fn = V / SQRT(g/L) gives Fn = 0.38 and close to hull speed.