# Help : Emperical Formula of Prismatic Coefficient for Catamarans

Discussion in 'Boat Design' started by Laranjo123, Aug 26, 2012.

1. ### Laranjo123Previous Member

I would like to ask is there any emperical formula of CP for Camatarans.
I need some formula to calculate the CP of a Catamaran.

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### TANSLSenior Member

empirical,-ca adj. That is based on experience and observation of facts.
Is that, really, what you need?

3. ### Laranjo123Previous Member

anything that would get me out of the CP Calculation.
exp:
CP = ( 1.15 - 0.6 ( V/ L^0.5) where L = LBP in feet

i used this formula but the answer gave me a negative result.

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### DCockeySenior Member

Do you know the definition of Prismatic Coefficient?

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### TANSLSenior Member

Or Laranjo123 ignorance is huge or he is looking for one thing, he calls CP, but I do not know what is.
I can not guess what he called CP and would be happy to help him to solve his problem, if he can explain what he needs. Perhaps my ignorance is also huge.
In any case, that with all due respect, we may be doing a time consuming we could use in some more interesting things.

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Tansl, we will not be wasting our time if we can help someone who is sincerely trying to learn.

Loranjos equation appears to be some combination of S/L ratio with a coefficient of difference. He might be trying to establish a recommended Cp for a particular boat velocity or some such. Like where V is used as a variable to identify optimum Cp. I do not see how he got a negative number from that equation, nor can I guess where the (1.15-0.6) came from.

Maybe I will learn something here too.

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### Red DwarfSenior Member

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### TANSLSenior Member

messabout, I understand and agree with your words. I also try to help when there is a question about a topic, that I know something about, and therefore I can contribute with something positive. And I'm learnig a lot from these forums.
That said, I must also say that we should demand a certain rigor in our conversations and at least learn the technical language, before you start asking for help. Not least because, if we pose a question that is meaningless, it is unlikely that the response has it.
I do not understand how laranjo123 is asking for an "empirical" formula to define parameters of a ship. I do not understand what does he mean by "CP". You think he can treat the prismatic coefficient, but why is not the center of pressure?. I hope he does not believe that the prismatic coefficient can be calculated taking into account the speed. I'm perplexed ¡¡¡¡
So I say, or expressed better or we will lose time.
Best Regards for everibody, laranjo123 included

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I suggest that Laranjo123 search the multihull forum to get the answers that he seeks.

My antennae went up when I observed that his avatar says that he is a NA and engineer. But he asked a question whose answer should be easy for an actual NA. His personal profile says that he likes math but the equation and conclusion in his post is slightly flawed. He has also posted similar questions in the sailboat section.

Charitably, we may have a language barrier that seperates our understanding. On the other hand, this may be the work of a mere troll. The personal profile also said that he liked games. Sufficient evidence?

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### DCockeySenior Member

Last September Laranjo123 posted "I'm a BS.NAME student 5th year i'm about to graduate next year. ...." which means he probably has graduated by now.
http://www.boatdesign.net/forums/boat-design/how-become-effective-yacht-designer-39820.html
He asked another question about design ratios of catamarans. http://www.boatdesign.net/forums/boat-design/question-design-ratios-catamarans-44443.html It looks to me like he is trying to figure out how to start designing a catamaran.
Based on his posts here there might be some questions about what he learned while studying naval architecture. But I've seen nothing to indicate he's a "troll".

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Agreed. He's just very inexperienced and knows very little, despite what he is being taught or told by his "university"!!

12. ### Laranjo123Previous Member

Ok here we go again. First of all i don't know who the heck changed my Subname. I was shocked. When i logged in here last monday i think. And saw that my name was changed into Naval Architect /Eng'r FG i don't really think i was the one who did that (Noob). In spite of all the things i've been working. I've been studying 3ds max since the day i've graduated. And really it's hard to go with the flow i have. Realizing that i've took the course that i did not chose it was my parents recommend to me. But i said to them i can take any course but in my mind i was like saying i really like Computer Science not this NA but programming. Second the formula came from INA by comstock pp.160 and i can't find the book. That's hard!!!. Third i'm just asking if you might know some emperical Formulas for preliminary design of a ship ( catamaran ). And it's a formula for prismatic coefficent .I'm asking what if you have only been given the LBP and the Vs of the ship how can you calculate your Cp. Of course you will need Emperical Formulas. Now which part do you think you can't understand from me. Btw Ad Hoc you are right i came from university. But what if you climbed in a tree and found it very annoying what would you do? You took the course not because you like it but because your parents told you so.

I understand you guys. You are seniors. Juniors, Engineers. Associates. I respect all of you. When i ask questions you reply back. And some would ask wht? some would say his troll. But ask me about computers. Damn.... Ask me about games. I'm a human bro's. And every human has its own weakness at the very beginning of his time. Like a baby needs a mother's care. I'm an NA graduate. I'm not that noob i think but since you been asking me.Now i do. And i saw some folks from greece to Nepal etc. Some Naval has the same questions that they've been asking like me...??? Just a formula guys that is all i want to know from you if you might have preserved some may i know it please?????.....

To Messabout i think you already figured how i ask things out. All i need is just another Formula for calculating the Cp of a Catamaran. I already gave mine sorry the formula is not right
rewrite: ( Cp = 1.15 - {0.6*(Vs/[L^0.5])} ) where L = 33m Vs=28 knots that's a negative....
I still have some formula here from "Ship Design, Resistance and Screw Propulsion, p.74 by G.S Baker" i don't own the book

Cp = 1.02 - ( Vs / {10 x [Displacement ^ (1/6)]}) This Cp was used in Yachts still a negative result. where Vs =28 knots Displ=240 tons

And saw some ranges of catamaran Cp = 0.63 - 0.65 i don't want to use these ranges unless i have no choice......

Thanks and best regards
with respect

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### Boat Design Net ModeratorModerator

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14. ### Laranjo123Previous Member

Wow the mod got here. Thx.

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### TANSLSenior Member

Mr. Boat Design Net Moderator, I would like you to answer me, in all honesty, if possible someone can edit my profile to change my data, if someone can edit my responses to change what I've said, if someone can change the pics in my gallery, if anyone can send messages on my behalf, all without my authorization.
If this were so, and I am convinced that it is not possible, it would be a very serious security problem, which poses boatdesign.net forum. Probably it would be the breach of the laws that help protect the privacy of our data.
Any chance that whatever laranjo123 says has happened with his private data?.
Thank you, Sir.
Ignacio López

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