Brett,
I would agree for IOM?s but I think the graphs do apply for some of the larger classes like 1/12th scale AC class here in the states for example.
Have had a look at Claudio’s graphing showing apparently contradictory curves relating ‘ideal’ Cp to a given fraction of hull speed. The curves on the graph are curves of ‘least resistance’ - that value of Cp which gives lowest resistance when the hull is towed without leeway and without heel in a towing tank at the indicated speed. My guess is that the discrepancies in these curves comes from the different assumptions each curve makes about waves in the tank, and about the hull shape towed.
The first question is, what was the shape of the hull being towed? The problem with Cp is that it doesn’t tell you how the hull volume is distributed fore and aft. And we know that this is the ‘big secret of yacht design’ – the ‘only’ (!) problem of hull design is where to distribute the volume so as to minimise drag for a given set of conditions. (Before you ask, no, no one actually knows the answer, and those who have a good ‘feel’ for what the answer might be are certainly not telling anyone else. When did you ever read anything said by Bruce Farr or Juan Kouyoumdjian that shed any light on this topic?!?) For example, Cp doesn’t tell you if the bows are particularly full. We know that hull resistance is primarily a function of wetted surface area at low speeds, and a function of wave-making drag at higher speeds. Well, fuller bows will probably give you higher wetted surface area. Not good at lower speeds…
The second question is, how flat was the water in the towing tank? Was the hull towed through any waves? In reality, high boat speeds only occur at high wind speeds which means there must be significant surface waves. If you want to go fast through a lot of waves, you may prefer to have more fullness in the bows. So testing your hull for resistance on the flat water of a towing tank doesn’t make too much sense…
Conclusion? IMHO Cp is a rough and crude measure which, once understood, is of little further use in detailed hull design…
Exactly Lester…
Bret & Lester it is very interesting .
So why to boder to put curves that are meaningless if not supported by the measurements set-up conditions ?
I wonder if I have to put a big Cross on the Graph I have presented since all are apparently in the books just to increase the number of pages .(polite version).
Why then I should buy another book ?
It just happen to discover that a very very successfull model (all weather I assume) exibit in fact PC of 0.58/0.60, so why to think about to 0.53/054 ?
Claudio
Claudio,
The graphs are not meaningless, they are roughly fitted curves derived from tow tank testing. They are never going to be 100% correct, what they are is a starting point.
Rant:
Ok all the wana be designers out there listen up, and consider the following.
A racing yacht moves in 6 degrees of freedom. It resides at the boundry layer between two fluids. It gets a driving force from one fluid and a drag force from the other. Throw all that together and you get one big nasty problem.
In short, Yacht design has no silver bullets. Stop looking for the optimum LCB, Cp, Bwl, it does not exist. I have said this before and I will say it again, everything related to yacht design is a compromise.
The graphs in these books are NOT for model yacht design,they are for real boats.
Dans right,there aint no silver bullet
All right.
has been reported that the tests made in tanks were made with scale models of similar size of ours, and from there derived the “rules” for the big ones, as “reported” in the books. I remember having seen around 78’ the Australia model used for tank testing of about 1.5 meters long.
Actually, I suspect, that these data were probably not very satisfactory , since it appears that actually the tests are made on larger scale models sized up to 1/4 or 1/3 instead of 1/10 or even 1/20 as in the past. Having questioned two architects working on America Cup design and developpement, one has already replied suggesting for low wind & low Vr and calm sea a PC of 0.53 and for breeze & high Vr and choppy waters a PC of 0.57. I’m waiting for the second answer, (ex Silver Bullet), booth knows that I’m talking about models.
It would be nice to ears also wich PC have you used, if eventually considered, during the design phases.
Thanks and Ciao
Claudio
Have just come back to this thread and am pleased to see that you really were talking about resistance and not some magic optimal CP.
I can’t add anything to Brett, Dan’s and Lester’s comments, all of whom I agree with.
Claudio - you are correct in your observation that larger tank models are used in an attempt to remove scaling effects. 1/3rd and 1/4 scale models are used in big boat design development if the budget allows.
claudio,
try this link, you might find it interesting. http://www.users.zetnet.co.uk/behemoth/slender/slender.htm
for cp i prefer the .535 - .555 range.
Indeed Dan - actually I will probably chose for the 1st model out of 2 something around 0.57
Thanks
claudio
That was interesting reading.