# Froude Number

Discussion in 'Boat Design' started by ShahriarTanveer, Apr 20, 2010.

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### ShahriarTanveerJunior Member

How can I find froude number of 50 m cargo ship? Is it same if beam changes?

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### zeronameNaval Architect

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### AlikSenior Member

Amazing is that a person with 'naval architect' in his signature is asking this question...

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### daiquiriEngineering and Design

I agree with Alik.

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### Leo LazauskasSenior Member

It's a bit like signing yourself Leo Lazauskas, B.A. Oxford, (failed).

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### ShahriarTanveerJunior Member

Thank you all. But still I have some confusion.
Look The formula is : Fn=V/Sqr(g.L)

In Ship, L = Length of Ship
In Channel, L= Channel Depth
In a pipe, L = Diameter of pipe.

Can you explain why ?

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### zeronameNaval Architect

in ship , L is not equal length of the ship .
L = is the length of the ship at the water line level, or LwL .

it is simple phenomena to understand "why"
Take your textbook , you will get da answer in details there.

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### ShahriarTanveerJunior Member

I want to say in a channel why L = Depth of channel, why not width?

Actually I want to know what is the relationship with the flow of water.
For a ship we are taking the length parallel to the water flow but in a channel
we are taking the length perpendicular to the water flow. Why?

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### conceptiaNaval Architect

go back to ur fluid mechanics textbook.. it is der....

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### daiquiriEngineering and Design

Mr. Tanveer, sory for being direct - are you really a Naval Architect & Marine Engineer? I can understand an amateur builder asking them, but it seems incredible to me that someone could get a specialistic degree without understanding such basic concepts.

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### ENG StudentJunior Member

dont hate....help

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### DMacPhersonSenior Member

I really didn't want to get involved with this thread - so much work at the office - but what can you do...

Assume for the moment that the poster is indeed someone who claims the mantle of "naval architect" without completing a degreed program. Wouldn't be the first time. Still, his question is interesting at face value.

Let's start with where the coefficient got its name - William Froude. His landmark work was all about "corresponding speed". He codified a relationship where wave systems were visually the same between ships of different size and speed. The value in the relationship was in its "comparative" or qualitative nature. The Froude number relationship itself is (for the most part) meaningless for anything other than comparing the behavior of systems at different scale. Voila' - "Froude scaling". (Sure, Froude number is used as a speed coefficient in resistance prediction algorithms, but that's really just an indirect statistical game.)

Froude's original relationship dealt with comparing surface wave systems using vessel speed and length. For different analyses, we are not limited to using waterline length, but we are free to scale and compare any system by using a meaningful "length" dimension. For planing craft, we use chine beam or the cube root of volume. For propeller scaling, we can use diameter. And for shallow water effects, the reasonable comparisons come from using a depth-based Froude number.

(Water depth is by far the critical "length" dimension for analysis of shallow water effects, but in a restricted channel you could make a case for the square root of the cross sectional area, or perhaps something akin to the "hydraulic radius".)

Regards,

Don MacPherson
HydroComp

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