# froude's number

Discussion in 'Hydrodynamics and Aerodynamics' started by deanlife, Oct 5, 2012.

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### deanlifeJunior Member

Hello guys
I'm doing some research about froude number. And I'm with a doubt about Froude's number, wich parameter I use when I'm analyzing the froude's number of high seed boar, like launch or jet skis? I mean, I've heard that I've to use the beam of the boat, but I'm not sure about it. Could you help me by giving some sources about it...?

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The Froude number is generally related to length, as an indicator of wave making resistance.

Some use beam, or volume, but these are often only used/referred to in research papers to establish some commonality between difference type of hulls in order to produce a curve of sorts which could be considered "constant", for the investigation.

So, Fn = V (speed in m/s) / sq.rt[g (9.81 m/s^2) x Length (m)]

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### deanlifeJunior Member

I'm familiar with froude's number. I'm just having problem of what parameters (lenght, beam, volume) I have to use in high speed hulls, are there any pappers with this information?

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See my post above. The parameter is length.

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### deanlifeJunior Member

As I said I know that parameter is lenght. But when use the others?

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If you're unsure then the reason is probably beyond your understanding of hydrodynamics.

Since the areas in which you wish to investigate, requires a certain amount of background knowledge. If you're still unsure which parameter to use (even though I gave you the answer above) then you are highlighting a lack of depth in your understanding of when/how and why.

No different to a structural engineering asking...which parameters do I use to calculate the modulus of a section.

Whilst the question may be simple, the answer is related to a somewhat more detailed understanding of the principals and requires more depth and study than the answer provides.

Plugging a "number" into an equation wont help you, if you're unsure where the equation comes from and why. That is beyond a one line reply.

May I suggest you read a book or two like:

Mechanics of Marine Vehicles - Clayton & Bishop
Hydrodynamics of High Speed Marine Vehicles - Faltinsen

Then after this you can read many research papers where each author uses a different parameter. The reason is laid our in the research. Thus one picks and chooses to suits ones area of focus.

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### rxcompositeSenior Member

You are talking of two different things. Froude's number is a relative measure of speed. The "g" is a constant, the "L" is costant (a boat length does not change with speed*), so the only thing that changes is the velocity.

You are talking of "planing conditions" which is lift or more specifically, hydrodynamic lift. So the question should boil down to "At what speed will I start to gain hydrodynamic lift?" "What do I need in order to gain lift?".

To get lift, you need velocity as a measure, surface area, angle of attack, efficiency of the surface/airfoil relative to velocity, to overcome the (frictional and other forms of) resistance and the mass that will be lifted.

Since a planing boat is similar to a kite (an airplane in a perfect stall) where what changes is the velocity (the speed of the wind), It is analogous to saying;

At what wind speed do I need to fly a small kite? a big kite? a really big kite?

* (Before Ad Hoc catch me) The boat length waterline changes a little at different conditions in the planing speed due to rise of bow/trim/angle of attack/lift.

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### Leo LazauskasSenior Member

For planing hulls, the wetted length is the best to use, but it is not always
easy to find this length.

Sometimes the cube root of the displacement volume is used in comparisons
of hulls with widely varying displacements and speeds.

For finite depth effects, water depth is used instead of hull length.

There are many other uses of Froude numbers, for example densimetric
Froude numbers etc.

Good luck!
Leo.

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### rxcompositeSenior Member

Called the "wetted length". See, they catch me early.

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### deanlifeJunior Member

It's what I'm trying to say. Now I know the conditions that I have to measure to calculate my froude's number. Because intuitively I know that the boat will make more waves if the velocity grows, but how measure it? At planing conditions is hard to have a answer for that.

I conclusion, what I'm really like to know is, Is possible to test a model in planing conditions (like froude's number > 1.2)?

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### Leo LazauskasSenior Member

It is not only possible, but you should be able to find many papers that report
experiments with planing hulls.

It is not compulsory to use a Froude number if that is too difficult. It is
acceptable to present results as drag versus speed, as long as you provide the principal dimensions of the hull.

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### rxcompositeSenior Member

You can dowload the excellent spreadsheet written by Dingo (Tweedie) here in this forum. I know there is another one, "Savitsky Planing boat Calculations". It should be in the library. You will be limited to button punching but if you are methodical in changing the parametric inputs, you will get an insight of how it interact with each other.

If you want to know more about planing hulls, order "The Prediction of Performance on Planing Craft" by JB Hadler from SNAME. It contains graphs on the lift coefficients, center of pressure, ect. There is also a link to Savitsky's paper in this forum. It was discussed some years back.

If you need basic understanding of the theory of lift, center of gravity, center of lift, drag, ect, visit the NASA website http:www.grc.nasa.gov/W W W/K-12/airplanes

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