Help : Emperical Formula of Prismatic Coefficient for Catamarans

If you login on a public computer that other people have access to and don't log out, it's very possible someone else using the computer could. If you login on a public computer, please remember to click the log out button and also uncheck the "remember me" when you login so it doesn't keep your account logged in.
It's also always a good idea to please use fairly secure passwords, not something that is too easy to brute force. Other recommendations would be to periodically scan computers to make sure they are free of any malware, use different passwords for different sites, and update passwords periodically for good measure. If you ever suspect your password has been compromised, please update your password immediately and send a PM with as much relevant information as possible so logs can be reviewed for anything suspect.

 

What I do not understand, not due to my lack of knowledge, but because is an absurd question, is how you pretend to calculate the prismatic coefficiente if you only know the LBP?. If you knew what the prismatic coefficient is, it is possible that you would not formulate your question in those terms.
And I repeat, no boat coefficients are derived (by definition) from empirical formulas.
By the way, you can use the speed for many calculations, but never to calculate the prismatic coefficient. Do not dwell on this topic.

 

I think he needs recommendations/formulas to choose CP(Fn) based on empirical data. The graphs posted below are exactly for cats, from my paper.

 

You're probably right, Alik, but who knows what he wants, if he does not tell us?

 

I have not posted in a while (because of the growing adversarial tone of many posts) but I feel compelled to come to the defense of the OP.

In fact, he did tell us what he wanted.

In his second post (third overall, I believe), he stated he was looking for a relationship between CP and speed-length-ratio (his "V/L^0.5"). This is a very common early-stage design check, and one that has been developed for many different hull types. Alik and Laranjo123 posted examples. It is a perfectly reasonable possibility that he is competent using this "design lane" for monohulls, but needed appropriate data for cats.

It is hard to say what his circumstances are. But that's the point - it is hard to say what his circumstances are, so why presume?

Don MacPherson
HydroComp

 

If I am guilty of asking rigor in the expressions we use, I publicly apologize.
If my tone has been "sour" in any case, I also apologize.
B.R.
Ignacio López

 

Mr.Tansl. as you can see i've changed my subname more confident way. <removed>

BTW would you like to share me some formulas or not. If not then probably i'll use the CP ranges that i found. But it will be great if you folks could share your formulas.

Any Formula guys any formula is good to share. Just formula of CP's either graphs i already gave my example mr.Tansl all i need is another formula empericals experimental formulas. Either its a s/l related or Displ/L related any just share it if you want.

^__^

BTW mr.Alik Thanks for sharing.

Actually because of studying 3ds max for months on my own. I already begun to forget things. This is damn way too flawed.

My example Design using not 3ds max but autocad 2012 this is a yacht not a cat
https://www.facebook.com/media/set/?set=a.462421550440302.122237.100000173844983&type=3

 

Just formulas that's it Like a student asking for a formula of a circle A=3.1416r^2
any CP formula. Still can't understand me.

Again which part do you think you can't understand from me???? Actually my classmates after seeing me with insults. They don't want to register here because you get insult first before you get a good reply....Now i'm not saying something here, what i'm trying to say is this is reality....

 

Laranjo123, Dear friend, the only way I know to calculate Cp, and I gladly share with you, is: Cp = Displacement/ (Area Main Section x Lwl).
I don`t know any other and I'm happy to put at your disposal.
If what you want is to know, in the first stage of the project, an approximate value for the Cp should have your boat , I think the graphics (empirical) that Alik attached in post # 18 will be very helpful.
Sorry but my knowledge do not give for more explanations.
If what happens is that I can not understand you, I beg you to explain what you need so that even someone like me, be able to understand.
Do not let me be the interpreter wthat you need, say you clearly what you need.
Another issue: I care very little the subname to appear in your profile, or any other debater. What interests me is what you say and how you say it.
I used an unkind tone with you but I do not think I've insulted you at any time. To say that your ignorance is enormous, is not an insult, is simply to say what is in my opimnión, in your level of training. or knowledge. I would like you to realize that you have much more to lose than me, if you do not use this forum properly.
Best regards

 

I empathise with your defence of the OP, since many are here to learn or understand.

But, this is not the first type of thread by the OP and as you have already pointed out, the answer has been given by Alik above, yet for some reason keeps asking the same question. Perhaps you can now understand a measure of frustration by the other posters when their replies, answering directly, appear to be ignored.

To paraphrase Armstrong...there comes a time when enough is enough!

You still do not get it, do you? Despite endless protestations to the contrary and the patience of many other posters.

The Cp is a simple formula, which you can obtain from any, yes ANY book, even Wiki has it here.

Thus if you're able to log on to this, wiki is a day at the beach! The Cp is simple a measure of the longitudinal distribution of the displacement of a vessel.

It is Cp = Volume/(Sectional area at midships x waterline length)

That is it, no magic, nor smoke and mirrors. Even those text books you say you have read, has this!

So, you need 3 parameters: Volume, Cross Sectional Area of Midships (or max section) and the Waterline Length.

As already noted above by TANSL, you can't calculate Cp with just LBP!

In case you missed it, see here:

If you are unable to understand any of the above, then you need to recognise your limitations and you should get someone to do the work for you. Reading an anatomy book doesn't make me a brain surgeon no matter how many times I say I am!

Exactly..

 

I know before posting that I'm going to regret it, but this will be my final comment on this topic, so here goes. I do not believe that the OP was claiming to want CP as a function of LBP. Look at this post, and you will see that he wanted CP as some function of V/L^0.5 (the speed-length ratio). His statement about L=LBP was clarification as to type of length used (i.e., LBP, LWL, LOS). However, I get the gist of the frustration here, and the posting by Alik should have given him what he needed. Sorry, but as someone who is competent to write only in English, I tend to be sympathetic to those writing in English where it is not their native language - and I try to read between the lines to get the intent of the posting, and not worry about the actual prose. Of course, there are times where proper terms are necessary for successful communication, but I'm not sure this topic is one of those times.

Don MacPherson
HydroComp

 

I know the formulation of CP that is Cp = Vol.Displ / L x AM

that is the common formula. Every naval architect knows that . And a thumbs up for alik for sharing his sheets. Well Folks we all know that we cannot Solve Cp by having only Lbp or something. All i wanted is another formula that is like of what i've shown you guys. Like
Cb = Vol.Displ / LxBxd <---this is the common way of calculating the cb
But another way of solving Cb is this

Cb = 1.2 - {0.39 * [V/ (L^0.5)]} <-----know this?....just one of my formulas been using this ever since i try to determine particulars of a ship.....

 

I appreciate your generosity in sharing with us your formula: Cb = 1.2 - 0.39 * {[V / (L ^ 0.5)]}. I did not know and recognize that it is very useful.
Surely there will be qualified for the type of boats is valid and what margins between lengths and speeds can be applied.
Full of excitement I have applied to a simple case: a tanker 285,000 m3 displacement, L = 200 m, B = 50 m, T = 30 m, with a cruising speed of 12 knots (6.18 m / s) . My big surprise was to verify that the boat Cb (which according to the traditional procedure, is 0.95) according to the formula was 1.02.
I need explanations, please.
After this I am convinced that the formula is not correct for large tankers and that should be explained to what type of boat, and why, it is applicable.
Thanks in advance.

 

The answer is 0.869

Cb = 1.2 - {0.39 * [ 12 / (200^05)]}

Cb= 0.869

This formula is applicable to all type of ships.

 

Gooday 'messa' - with all due respect - we will be wasting time with this young person.

1/ in his previous posts he states that a book of some 350 pages or so - is to big to be bothered reading - or some such comment of equal value.

He has a very qualified professional person less than 3/4 of an hours drive from him - that is xxxx - no - was willing to help him - but that seems to be to far away for him to make the effort. I'd wonder if he's old enough to have a drivers license yet ??? Suspect not ! !

2/ my first post to him was - that if he was real & not just wasting our time - we could help him - I am still waiting for an answer. Wont hold my breath ! !

3/ his actions - have been - to change subject - change post healings - not address reality & waste very valuable time & our patience & resources. At least mine - if not other knice people in here.

Good luck with your continuing to assist him - - London to a brick - you're wasting your time. ciao, james