# Prismatic coefficient for deep vee planing hull

Discussion in 'Boat Design' started by Mamaboo, Aug 8, 2008.

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### MamabooJunior Member

I am an amateur who fiddles with boat design for fun. So far, I've learned enough to be dangerous, but I'm having fun and am hungry for more knowledge. Lately, I am working on a 37' deep-vee Express-style sportfish to be powered by triple four stroke outboards on an integrated bracket. The hull looks pretty good to my eye, and the displacement is right where I thought it would be at about 19,000-20,000 lbs loaded, but prismatic coefficient is in the .84 range when using midship section to calculate it. From what I've read, this seems a tad high, but I'm not sure if that is the case or even how important it is in a planning hull that should top out around 45 knots. Also, I'm confused on what section area I should be using to calculate the PC. Should I use the midship section even if it is not the largest section (which happens to be the aftermost section)? I'd love for someone to educate me on this subject. Attaching lines drawing in case anyone wants to critique it.

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### ruysgJunior Member

You shuold use your largest area section to calculate your Cp, wich isn't necessary at midship. You should get a more reasonable Cp then.

Hope it helps. Good luck with your boat
Ruy

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I can't help you with your question but I have a few questions for you. What is the beam of the boat? Is it 37' LOA? Where did you come up with the weight estimate? What is the deadrise?

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### MamabooJunior Member

Beam is 12'6". I need to clarify LOA, which is actually 39' 8" once I decided to add the integrated bracket. I actually started the design with the idea that it would use a bolt on bracket, in which case the molded hull was 37'. Deadrise at transom is around 22 degrees. At the dwl shown in the drawings, displacement is calculated at 18,900 lbs as calculated by Freeship. I am aiming for a fully loaded displacement with gear, crew, fuel, etc. of no more than 22,000 pounds, which from my research is pretty much in line with real weight of similar express boats on the market such as those from Pursuit, Contender, or Grady.

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