# What's the relation of Fr to S/L?

Discussion in 'Boat Design' started by Laranjo123, Sep 21, 2011.

1. ### Laranjo123Previous Member

What's the relation of froude's number to S/L? (noob here)

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### daiquiriEngineering and Design

Laranjo,

Back in times when I was a student and I wanted to find out a relationship between two variables, no way I would have asked a question like "give me a ready-to-use formula for this". No way. I would have put down the two formulas (for S/L ratio and Froude number in this case), and then after few substitutions and unit conversions I would get the relationship between the two. It's all just an elementary math, and since you are a 5th year student of Naval Architecture you HAVE to be able to do it yourself.
If you just copy a formula from somewhere around, you won't be able to understand what is that formula actually telling you. For that reason you need to work it out yourself.

I would have understood if that question was asked by a backyard builder who just needs to quickly build his plywood dory, but you are a student and it is your job and duty to study and understand the physics and math governing the world around you.

And to prevent a bashing from other forum members for not giving the answer to your question - I HAVE given the answer in the above reply, just not a ready-to-use one.

3. ### Laranjo123Previous Member

Sir i was studying to the point that i am not going to school. just to get the answer. Like If our prof gave us one problem and ask us to solve that. Then i will, because i love to solve problems because i love math. Not to be mention i am a naval student. And yes i did what you've said sir. I was analyzing this Fr and S/L. Thinking what's special to them what's their relationship to each other and i came up to the state that i was blackout. So i went here to ask some people about this manner. And i really do hope that a person like you would answer my question. And I'm at the state to where i am about to fall sir. ( not grades but my self).Btw thx sir. it did help me although I'm doing my stuff here. Paying attention of this formulae.

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### JRDSenior Member

Laranjo, There is an excellent article written by designer and naval architect, Mr Eric Sponberg. It has been published on this site a number of times, and this relationship is defined and transposed for all of us amateur students of yacht design here. Perhaps you should track it down and read it carefully.
Daiquiri makes a good point, this is basic hydro engineering, Are you a student of naval architecture at a university, or a school student who is taking an interest in the subject.

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### Eric SponbergSenior Member

See the link below for that discussion thread. Look at post #305 to download the latest version of the article, The Design Ratios, which is a complete compilation of all the "lectures" in that thread. Speed-length ratio is covered in chapter 4, page 12.

http://www.boatdesign.net/forums/boat-design/center-flotation-calculation-implications-30857-21.html

Eric

6. ### Laranjo123Previous Member

@ JRD yes i am a naval student from a university. O am making my thesis and mr. rxcomposite told me about the fr n S/L so i studied that and was thinking that what was the main purpose of the formulae...anyways thx guys...

7. ### Laranjo123Previous Member

Questions guys ahm is it ok if my parentship's CB are 0.33 -0.40 seems it's not that allowable ? example LOA 50m LWL 46.78 B 9 d 2.85 Displacement 390 tons

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### Tim BSenior Member

Regular reading of a web-based forum, however accurate, is not a suitable substitute for studying to a respected standard.

As someone who's doing a thesis, you should be telling US what YOU think the relationship is, and asking US to comment on the validity of the conclusions you draw from that.

Tim B.

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### rxcompositeSenior Member

#### Attached Files:

• ###### Froude number.jpg
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169.6 KB
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### DMacPhersonSenior Member

Definition of S/L??

Not to further confuse things, but there may be a misunderstanding with respect to what is meant by S/L.

All of the replies to the original post were from the understanding that the author was talking about the traditional Speed-Length Ratio (which I suspect he was) versus Froude number. (They are related, of course, by a simple multiplier as was noted in the previous post - although 0.297 is closer to the correct multiplier when one uses contemporary values for knots and gravity.)

However, the term "S/L" is a standard design ratio for catamarans, as it is used for catamaran demi-hull spacing (S) divided by length on waterline (L). It is not uncommon to discuss interference drag in terms of spacing and Froude number.

My point, I guess, is that there is a valid relationship between an "S/L" term and FN that has nothing to do with speed-length ratio, so probably best that we not be so quick to judge and criticize...

Don MacPherson

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### rxcompositeSenior Member

Cats span to length is usually written as s/L. A small "s" over a capital L

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### DMacPhersonSenior Member

Generally, but not always. Even the luminaries in catamaran research (e.g., Molland, Insel, Sahoo, to name just a few) swap between upper and lower case "s" for demi-hull spacing - sometimes even within the same paper. So, it's not hard to forgive a student for possibly doing the same thing.