Discussion in 'Boat Design' started by river runner, Nov 13, 2011.

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river runnerbaker

Suppose you were designing a canoe for the Boundary Waters Canoe Area Wilderness. Because it is a performance flatwater tripping canoe, you put the LCB a little aft of center (like most BWCAW boats). You are figuring a speed/length ratio of 1.1 which would give you a prefered PC of .54.
My question is, with the LCB off-center, should the bow and stern prismatic be the same, or would they be different and only average out to .54?
Even if the LCB was in the center, would the bow and stern prismatic be different?

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And where do these figures come from?

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river runnerbaker

I'm not designing a canoe for the Boundary Waters (at least not right now), I'm just using that as an example, so the actual numbers don't realy matter, but a speed length ration of 1.1 would be reasonable for such a canoe and I got the .54 from a table in Skene's Elements of Yacht Design. It says the optimum PC for a S/L ratio of 1.1 is .54. Is there a problem with that?
The PC is important in boat design. I need to know how to apply it correctly.

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Leo LazauskasSenior Member

Yes. Skene's book is old, yachts are not canoes, and he doesn't, IIRC, give calculations supporting his claim that those CPs are "optimal".
Optimal for what?

The Prismatic coefficient is a fairly crude design parameter. Wouldn't it be best to "design" the boat first, and then check that the CP seems reasonable when compared to other canoes used for the same purpose?

Good luck!
Leo.

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river runnerbaker

If what you say is true, then my question is how do I find out what the optimal prismatic would be for a canoe with a given speed-lenghth ratio. Canoe companies don't list the prismatic in their catalogs and I would think that they'd consider it a trade secret. There is no way to find out what other canoes have for their prismatic. I'm actually more interested in designing another river dory right now. I know of no company that makes river dories that posts their boats PC either.
As stated above, I'm trying to learn about how to use the PC. I don't have a design that I want to apply it to right now. And, yes, my normal design process would be to draw up a design, and then check the numbers. But I don't have a design to check right now. I'm just trying to learn all I can before I start a new design.
When I first joined this forum, one of my first posts was to ask if Skene's Elements of Yacht Design was relevant. The resounding response was yes.

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Try not to get hung up on technicalities like Cp. Canoes and kayaks are not fast boats. Your proposed S/L ratio of 1.1 is optimistic except for short sprints. A boat of this sort will profit more by some effort to minimize wetted surface. The paddler will be operating predominantly in the speed regime that is most influenced by wet surface. In the end, the surface quality of the wetted part will influence your required power input. That is a craftmanship consideration not a design criteria.

Design your boat to have a smooth, reasonably balanced, and conventional curve of areas and dont worry too much about prismatic.

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flo-moJunior Member

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No.

There is no "trade secret" as such. It is just a single parameter. A boat consists of many parameters all conflicting with each other. To establish which way to go, you write an SOR...a statement of requirements, i.e. what you want your design to do. Go 10knots, 30knots??..carry 3 people, 20 people??, have one main sail, or a keel, sleeping quarters, not get wet when paddling, self righting...carry a RIB..or a portable BBQ set...or have A/C...or just a simple head etc etc....all this, forms your SOR. The SOR defines what YOUR design shall satisfy.

Design your boat and then work out what the coeff is for YOUR boat, then compare it against others. You are putting the cart before the horse. One does not design with a specific coeff in mind with all else being ignored.

What is your SOR...it is as simple as that. If you have no SOR, throwing numbers about from text books etc is just meaningless. Your SOR is unique to you...thus the design is, what it is.

If you think there is some holy grail and an "optimum" hull, a one size that fits all, you have been standing too near to the open pot of glue!

There is no such thing as the perfect or optimal hull. The design is to suit your SOR, I'll repeat YOUR SOR. If the coeff's of you boat end up being totally different from every other boat on the market, but does what you want in your SOR...so what?? If the design satisfies the SOR, job done!

If you wish to find some holy grail/magic pill of coeff's, good luck with your quest....tell your wife you'll be gone a while!

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philSweetSenior Member

The essence of recreational small craft design is to achieve sufficient versatility. That is why it is a handicraft. The tighter you optimise for one circumstance, the less versatile the vessel becomes. The canoe is a wonderful generalist.

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river runnerbaker

I'm not designing a canoe. I was just using that as an example. I just want some information that I can use in the future. I don't plan on designing the whole boat around a specific PC. I just want to know if the bow and stern prismatic are usually kept the same, or if they should be different, and if so, for what reasons. If I ever design another boat (I didn't even check the PC on the last one) knowing this information will give me another tool in my arsenal. Not the only tool, just another one. I just want to know: if I design a canoe, dory, sailboat, or whatever, with the LCB aft of center, would you normally keep the bow and stern PC the same?

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At the risk of sounding terribly repetitive,...it depends upon your SOR.

It seems that unless you are actually designing and building one, for real, you shall not grasp this simple concept of what design to an SOR really means.

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timothy22Junior Member

OK, I'll give it a go. Having studied thousands of boat plans in the last 50-odd years, and having built and worked on more than a few, I am comfortable suggesting in very general terms that unless there is a specific reason not to, small boats are either balanced or slightly fuller aft, but this thread told me way more than I thought I wanted to know on the subject.
http://www.boatdesign.net/forums/boat-design/cp-prismatic-coefficient-fore-aft-8654.html

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DCockeySenior Member

What part of an SOR would affect the answer to his question?

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DCockeySenior Member

An aft LCB locationa will generally result in the Aft Prismatic Coefficient higher than Foreward PC for smooth and fair hull shapes without an immersed transom.

I'd be more concerned about the sectional area distribution, also refered to as the longitundinal volume distribution, rather than trying to split PC into two components.

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