# A simple prismatic coefficient question

Discussion in 'Boat Design' started by BASIL J WALL, Jan 7, 2013.

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### BASIL J WALLdesigner

Hi...Just a simple question ...In the Delft systematic series data, which I have skimmed through...I cannot find which cross sectional area they use to determine the Cp...Is it the midsection area or the largest section area?...Perhaps the distinction is not significant, but I do find that this particular area is not carefully stated when the Cp is give in many designs...
I guess there is one other question...Which area do you use and is the difference significant?
Thanks
Basil

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### daiquiriEngineering and Design

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### jehardimanSenior Member

And remember, the maximum section is from the upright waterline at nominal trim & draft and generally calculated without appendages. So if you are looking a vessel with significant deadwood or keels, they are not included, but the "apparent shape" of the canoe hull is...that any clearer?

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### BASIL J WALLdesigner

Thanks... for the quick responses...but I still need to know which area the Delft series uses...I'm assuming that both of you use the max. area...which I think is reasonable...but does "Delft" do the same?
Basil

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### TeddyDiverGollywobbler

That's in the formula so using anything else is flawed.. But if you want to find the max cross sectional just calculate it from Cp, Disp and Lwl

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### daiquiriEngineering and Design

The mid-ship underwater section is sometimes used for cargo ship hull forms and similar, where mid-ship section usually coincide with the maximum section. In case of sailboats, the maximum underwater section area is used for the calculation of Cp.

However, you can also try to send an e-mail to Dr. Keuning and ask him personally which definition did they use. My guess is he is probably willing to reply to such technical questions regarding his work: http://www.3me.tudelft.nl/en/about-...n/staff/persoonlijke-paginas/keuning/keuning/

Cheers

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### lumberjack_jeffSawdust sweeper

Not necessarily germane to your question, but freeship uses the midship section for the Cp and Cb calculations. For boats and yachts, one can manually specify the "midship" position as the point of greatest underwater area.

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### BASIL J WALLdesigner

Thanks to all especially daquiri...I e-mailed Dr Keuning and he replied instantly...
The Delft systematic series used the max. cross sectional area to determine Cp...
and interesting to note that there is no standardization in the definition...
I would like to see the Max. area used exclusively...or at least the area used should be stated explicitly when quoting the Cp...
Thanks again...
Basil

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### daiquiriEngineering and Design

But that IS how it is defined, as noted before. I have made that remark about cargo or tanker ship boxy hull forms simply because in that case the midship section generally coincides with the maximum section. But even in that case, it is still in line with the accepted definition of Cp.

Lumberjack Jeff's good point about Delfship / Freeship tells us that we also have to use the common sense and double-check every output from the design software. A good example of garbage in / garbage out situtation.

I often have much more troubles with the way some people use the Froude number, for example. I've seen papers with plotss of Resistance vs. Froude number, with no hint about WHICH Froude number it is referred to...

Cheers

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### Part TimeJunior Member

Orca 3D also uses the max midship section.

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### daiquiriEngineering and Design

Be careful there - it is either max or midship section. Only in some particular cases the two coincide.

Cheers

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### Part TimeJunior Member

oops, daiquiri you are correct. I should have said Max section.

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