What did I resurrect? A well known designer makes erroneous statements, and no one calls him on it??!!

oh, plenty have. It just opens a can of worms that is hard to close :P

Hi cat_scratch_fever

you are of course free to have your opinions, but it would look a lot better if you posted under your real name.

Maugan,
Instead of discouraging a new person to the forum, maybe you could send him a private message and explain what you are talking about.

Mary,

Agreed, but I'd just prefer to post it here to save us from travelling down the same roads over and over again.

That was my intent.

Cat scratch fever…

The best thing for you might be to do a search of Bill’s past posts on this Topic before taking him to task for his comments. He has explained his reasoning adnauseam. Just when the dust settles someone new comes on board without the benefit of his prior explanations and stirs the pot all over again…Mr. Roberts must have the patients of Job to keep answering the same questions over, and over, and over again…Read with an open mind. If you are really interested in the topic the time spent researching this thru past posts will be quite rewarding.

Bob

Hello CSF,
I have not seen or read about the M20 winning an ISAF Trials. I did read about an ISAF Trials a year or so ago where the M20 was damaged the day before the race and was not able to compete. A Tornado won every heat of that race series. It was written up in Multihulls World magazine.
I have seen the M20 in 4 or 5 Miami to Key Largo Races and it has never won. I don't think it has beaten the first Tornado either.
I have read about the M20 in the Around Texel Race for the past few years and the M20 hasn't beaten the first Tornado yet. It is always a very close race.
My comment about the M20's performance and its light weight is that I do not clearly understand why it is not light years faster than the other boats. The race results that I have seen do not comfirm its super superior performance. It leaves me puzzled.
Bill

Any chance you would write a column explaining how sailboats work? Beyond CE/CLR, I'm lost!

I've just started "Principles of Yacht Design" by Larsson and eliasson, and it is easy to see that this project alone will take years to fully understand. That is, if it can be done at all in a self-study mode.

Pete,
That's a pretty big question -- "how sailboats work"? Can you narrow that down a little? I'm not sure this web site can handle an answer to the "how."

The question is general in scope. Often, the technical jargon is so arcane that I'm unable to follow the conversation. Specifically, I have no idea what "prismatic coefficient" means or why I should care.

The question was asked precisely because it is so broad and complex. So much so, that I doubt I'd ever figure it out on my own.

"Prismatic coefficient"??? Why would you care? Why would any normal sailor care? Pete, you are getting me very worried about the future of sailing if people are thinking about things like that.

Hi Mary,
You might have reason to worry about the forum turning into a technical debate site! I appreciate the others tolerating some occasional nerdishness. Seems like a high percentage of catsailors have some kind of engineering training. Anyway, if the last poster wants to know about yacht design more power to him! One more guy who can help keep the commerical boat marketeers honest. Some good books that even I can read and understand: Skeene's elements of yacht design (Kinney); Sailing Yacht Design (Douglas Phillips Birt); Yacht Design (Mead and Jan Gougeon); The Science of Yachts Wind and Water (H. F. Kay); The Common Sense of Yacht Design (L. Francis Hereoff).

Kevin Cook

Hey Guys,
Mary is right. Prismatic coefficient will not help any of us sail a boat faster. It is already built into our hull shapes and there is nothing we can do about it.
For those who are interested, it is a ratio of two hull volumes below the waterline. It ranges in value from 0.0 to 1.0. The numerator is the actual hull volume below the design waterline. The denominator is an imiginary hull volume equal to the maximum actual hull cross sectional area integrated over the full length of the boat below the same waterline. So, when this number is small, it means the hull max cross sectional area occurrs for a very short length of the hull with almost straight lines connecting the max cross section of the boat to the bow and stern. You would call it a "pointy" hull shape. On the other hand if the prismatic coefficient is large, like near 1.0, then the hull maximum cross sectional area or near maximum cross sectional area is carried well forward and well aft from the point of max cross sectional area and the hull lines appear full with a blunt bow and stern. A barge hull shape has a large prismatic coefficient. A canoe hull shape has a much lower or smaller prismatic coefficient. See, I told you it wouldn't help you make your boat go any faster.
Bill

In addition to Bill's post, consider the two hull shapes shown below (in a rather simplistic form), hull A would have a low prismatic coefficient and hull B would have a high prismatic coefficient if length and displacement were equal (I hope I have that right).





However, because the prismatic coefficient is a function of the length of the hull, it alone is not enough to define or even compare hull shapes (i.e. a short stumpy hull can have the same PC number as a long fine hull). It's kind of like saying "I live on the east side of town". It tells us the general area but it's not enough to get us to your house for dinner. In fact, it is a number that's really used near the end of the design phase of a hull.

Looking very generally at sailboat hulls, first we define a target speed we want to achieve the highest efficiency at. There are then two routes we can take to define the basic displacement type hull shape (not including planing hulls) and we need to decide which one we must use first. This decision depends on the ratio of hull length to width (finess ratio). Somewhere around 11:1 and lower (a 20' boat with a 1.9' or wider hull; monohull territory) you need to consider the speed that the hull generated bow wave travels in order to try and optimise the hull shape. Unless you've figured out how to make it plane, this wider hull is limited to the speed of that bow wave. However, higher finess ratios than 11:1 (i.e. catamarans) no longer contend with the bow wave and play in a different set of rules since it can drive through the bow wave (and doesn't create much of one). Once you have decided what kind of hull you have, you can calculate a few more parameters to define it's behavior nearing your target speeds at the boat's available power limit. Once you've got all this, you can start drawing your hull to conform to the optimum prismatic coefficient (length and finess ratio being the same). The thing that gets complicated is that the finess ratio, the speed to waterline length, the prismatic coefficient, and any other hull design parameter are all intermingled. If you change one you very likely affect the others - yikes! You can either just start building in your garage like the creators of the Tornado did, experiment with drawing different shapes and recalculating these parameters until you arrive where you want to be, or try and remember all that calculus you studied in college so you can arrive at the hull shape you want in one sitting. This is where my bus stops - it's at my recollected knowledge limit - everyone off!

A great little primer regarding hull shape, speed, and power design for finess ratios less than 11:1...i.e. monohulls...can be found here:

http://www.kastenmarine.com/coefficients_of_form_equations.pdf

I just realized I did not phrase the above properly. I did not mean that the people on this web site could not handle the answer to the question -- what I meant was that a complete explanation of how sailboats work would probably be too huge in volume for the website to handle.

Mary:

Sorry about all the fuss, just wasn't thinking straight.

Bill, Jake and Kevin: Thanks, I needed that!