# Fore and Aft Cp -- unresolved questions

Discussion in 'Boat Design' started by Paddlelite, Feb 19, 2013.

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### Leo LazauskasSenior Member

There are an infinite number of solutions to this problem.
For example, you could have one design with slightly high skin-friction but
low wave resistance; another could have lower friction but higher wave
resistance.
The propulsion is pulsatory, so the velocity is not constant.
The "fuel" is limited, so there will be differences between the speed range at
the start and end of the race.
And what is so special about a speed-to-length ratio of 1.3?
(Or is S/L something else?)

I can see what you are trying to do. I puzzle over this sort of problem too
and have my own ideas for kayaks and rowing shells, but I'm not telling you

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### Leo LazauskasSenior Member

Interesting. I haven't heard that interpretation before. I'll look out for it if it bobs up in the literature.

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I think that there is only one best answer given theoretical (call it magical) constant propulsion, because only one paddleboard will reach the other side of the lake first. I chose S/L = 1.3 because I had to chose something to prevent the complaint of "too many variables" and because that's about the speed I'd paddle a 12 ft. board.

Of course I admit that in the real world, there is no constant propulsion, and in fact, in the "glide" phase between paddle strokes, lasting less than a second, paddleboards decelerate about 1 mph at that speed. (That would additionally raise the question of what speed in that range you target in your design parameters -- the middle of the range, or maybe the top?) Neither is there perfect tracking in the real world. So, I would be unable to perfectly evaluate each expert's design and accompanying claims, allowing each to proclaim that their unique design and features produced the best result. That delimma, in turn, leads to the question of what is the best test -- what real world, repeatable standard of evaluluation could you employ, perhaps short of tank testing.

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I guess I'm trying to get the admission, or lack of dispute, that with the current state of hydrodynamics and boat design:

a simple and practical problem can still be posed for which
(a) there clearly must be a best answer, but
(b) that answer cannot be determined, and
(c) if a best answer were claimed, it could not be proven.

That is in no way to diminish all the accomplishments within the field, or those who have acheived them. It's more to say that anyone's educated guess might be as useful as anyone else's educated guess.

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### Leo LazauskasSenior Member

Sounds like post-modernist claptrap to me.
But, hey, I'm happy for post-modernists to choose the colour schemes for
bridges, as long as they leave the engineering and construction to those of a
more scientific persuasion.

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Not at all. I've always had great faith in, and dependence on, math and science, and especially math, for which I have a degree with highest honors. I am simply noting, or maybe proving, how quickly we reach the limits of what is known, or perhaps what can be known. I'd call that philisophical, not post-modernist.

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### philSweetSenior Member

Known, and publicly available, are not the same thing. There are esoterica in religion and recondite knowledge in science. If you created five different challenges for a group, I'd bet there would be a high correlation to the finish order of the participants.

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### Remmlingerengineer

Leo, it's about time to compliment you for your use of the English language. I keep improving my vocabulary by reading your posts. "claptrap" was unknown to me, but it is a word one desperately needs these days.

To the discussion about knowledge in hydrodynamics just one remark:
The knowledge is available, but to apply it to a specific design problem is sometimes tedious. It requires the identification of the main particulars or drivers of the problems, may be a computer simulation and a lot of work. I learned that in engineering. In the end practical experience will tell you whether your design meets the requirements. You will not get the answer from a discussion in the barroom.
Uli

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Oh how true

I'm sure there are many more words we can provide you to better help describe such situations in a more colourful vocabulary

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### Leo LazauskasSenior Member

To be fair to Paddlelite, it's only that one sentence that I found amusing and slightly postmodernist in its sentiments.

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Thanks, Leo. And I will note that the quest for more knowledge and answers never stops. I, for one, never give up.

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