# Froud number limitation in Michell's and Noblesse Theories

Discussion in 'Hydrodynamics and Aerodynamics' started by A.H.Sabahi, Jan 11, 2017.

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### A.H.SabahiNew Member

Hi every one

I am working on optimizing a sport sailing trimaran. I want to use a theory for calculating my model's wave-resistance. I have searched in the internet and I have found two theories; Michell's theory and Noblesse theory. the former one has been more used as I have found. But I don't know the differences between these two theories completely. Now my question is:

Is there any limitations of Froud Number for using any of these theories? Because as I have found in researches, They have used these theories for vessels with Fn below 0.36 or in some cases below 0.5, but in my case I have a trimaran with Froud number about 0.7 up to 1.3. Can I use them? and which one may be better?

I will be thankful if any one could help me

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### Remmlingerengineer

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### daiquiriEngineering and Design

Bear in mind, however, that neither Michlet nor Flotilla take into account any eventual hydrodynamic lift force or (especially) trimming moment generated by the hull.
I am saying this because if your hull is designed to generate any amount of hydrodynamic lift, it's effects will not be visible in the calculations' results. While hydrodynamic lift may not play a big role in terms of direct contribution to the sum of vertical forces acting on the hull, the associated trimming moment may be relevant for some hull shapes (likely not for yours, though) - and the trim does alter resistance results.
Cheers

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### A.H.SabahiNew Member

Thanks Remmlinger. I will go on finding out more about Flotilla software.

Also thanks Daiquiri for your good point. It should be one of the parameters for optimizing.

But for wave making resistance do any of you know which theory ( Michell or Noblesse ) could be better?

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### Leo LazauskasSenior Member

Flotilla calculates the trim and sinkage. It also calculates trimming moments and sinkage forces, but I'm not sure if I actually wrote those quantities to an output file. (Michlet definitely does NOT account for sinkage and trim.)
FIRM took into account squat in its calculations.
REMEMBER: Not only does the hull sink and trim, but so does the water around the hull.

Happy New Year, Slavi!

Leo.

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### daiquiriEngineering and Design

Hi Leo,
I was referring to hydrodynamic forces, i.e. dihedral or flat-bottom hydrodynamic lift. I have understood that Flotilla doesn't include those into calculatons either, but if you say I'm wrong then that's how it is.

Happy New Year to you, Leo. Hope things are going well for you.

S.

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