question about PC

[Ad Hoc]

For those of you fixated with Cp of hulls, you can all soak up this graph:



Those wishing to design, you need to define your SOR first.

[DCockey]

What's the source of the graph?

[Alik]

I would just design the shape first, and then CHECK if CP is close to recommendations. There is not much sense playing with those numbers if there is not shape yet, ranges are quite wide and optimum CP say, for sailboat hulls and commercial ships are quite different. I am always opposed to giving single curve as optimum CP as it would be valid only for certain parent hull; it is more fair to give a range of CP.

[ancient kayaker]

A few hundred years ago ships and boats were usually designed with narrower sterns than bows, presumably on the theory that fish were like that. With a more scientific approach things changed, but it was a long time before designers began to explore really extreme designs and we boat folk are still learning. That's why it's so difficult to give or get a definitive answer to questions such as this one.

I am not a qualified boat designer or any kind of theoretician on matters related to boating, but I offer the following theory for consideration.

First, let’s note that canoes and kayaks have the least amount of driving power available of any class of boat. Also they are products of long periods of continual development within their respective cultures. Those cultures were close to nature with less margin for error than our modern mechanically-based civilization, so failure to "get it right" had darwinian consequences.

Putting all that together, it seems logical to expect that the boats they depended on were optimized to make the maximum use of the limited power available, which was not sail, certainly not power, but the strength and duration of the human arm.

If that is the case, traditional kayak and canoe designs should exhibit unusually high speed in relation to the available technology. And they do. So-called hull-speed is routinely exceeded by designs based on traditional concepts.

Traditional craft are by-and-large long, skinny and more-or-less symmetrical fore-and aft, except for highly specialized boats like retrieval kayaks. The kayaks you see in big box stores tend to have higher Cp aft of midships, and are asymmetrical. They are also shorter and beamier than native designs.

So perhaps while greater Cp aft is useful on a beamy boat, it is less so on a long and skinny one. I suspect the primary function of a broader stern is to prevent stern squatting at speed so you do not find yourself sailing uphill. A long and skinny boat doesn’t do that to any significant extent.

Whether I am right or wrong, Ad Hoc is correct, the SOW rules. A canoe is a fairly extreme type of craft, and not the place to start if you are actually contemplating a sailboat, power boat, or yacht tender. Canoes and kayaks were not what the designers who created the hydrostatic formulas had in mind.

[Ad Hoc]

Originally Posted by DCockey

What's the source of the graph?

Does it matter?...here are 2 more:



All basically saying the same thing.

Cp only affects residuary resistance. Thus the "optimum" Cp increases with increase Fn. But to look any deeper just misses the point. One does not design a boat/hull using the CP as the reference or starting position.

You need to know what the SOR of YOUR design is, and the Cp ends up, what it is, that satisfies the SOR. If if falls outside of these Cp curves yet satisfies the SOR, what are you going to do....scrap the hull?

I have never used Cp as a reference in more than 20years of design.

But if others wish to "hang their design" on one arbitrary value.....that is their prerogative. As they also probably do not design boats for a living and thus find coeff's of "value" as a point of measure, rather than the finished design that satisfies the original SOR.

[Alik]

Originally Posted by Ad Hoc

But if others wish to "hang their design" on one arbitrary value.....that is their prerogative. As they also probably do not design boats for a living and thus find coeff's of "value" as a point of measure, rather than the finished design that satisfies the original SOR.

That's true, they always stuck to values and Internet publications, and claim why CP is only 0.65 in our design made for them, it should be 0.671 from some graph of formula.