Sailing boats' Stability, STIX and Old Ratios

[richardsnowstar]

Hi, Does anyone know where I can get official Stability info for a 1985 Southerly 115. I need it to get the yacht MCA coded for charter. Any help would be appreciated. Thanks, Richard.

[Guillermo]

Perhaps it could be useful if you contact the RYA and ask them http://www.rya.org.uk/Pages/Home.aspx

Cheers.

[richardsnowstar]

Thanks for this, I have since found the information on their site at http://www.rya.org.uk/KnowledgeBase/...ilitydata.aspx

Richard.

[TeddyDiver]

Think this Eric Sponbergs post should be duplicated here. Also link to the original thread center of flotation calculation and implications?

Originally Posted by Eric Sponberg

ON A SCALE OF ONE TO TEN—THE “S” NUMBER (S#)

Wouldn’t it be nice to rate the performance of all sailboats on a scale of one to ten?

Here’s the problem—we have different ways to rate a sailboat’s potential performance in the form of design ratios, handicap rules and ratings, and level ratings. In fact, rating systems have been around for centuries, dating back to England and the realm of Queen Elizabeth I—over 400 years. And in all that time, sailors and designers have continuously argued over what makes a boat go fast, and what should be measured and rated in order to allow disparate designs to compete on equal terms. VPP programs and CFD codes have tried to make performance ever more definable, but these tools require sophisticated programs and specialized people to run them. An alternative solution is to race in one-design boats, but, unfortunately, not everyone wants the same boat. On top of that, not everyone wants to race. Still, we want to be able to judge performance—we always want to know about performance.

A similar problem crops up with advertising hype—this or that boat is a racer/cruiser, cruiser/racer, racing machine, or simply just a dog that can’t get out of its own way. Who defines these things, and how is anyone supposed to make sense out of it all?

A rating number from 1 to 10 might simplify things for the average sailor and designer. What can we do with the information we already have without resorting to a consultant—some way that anyone can rate any boat on a scale from 1 to 10? Has anyone done this? Yes, someone has.

Back in the mid-1980s, I designed a “Boat-in-a-Box” sailboat—that is, a boat that could ship inside a standard 40’ shipping container—for a client in Texas, Mr. A. Peter Brooks. At the time, he and I both were also consulting for Cat Ketch Yachts Inc., the builder of the Herreshoff and Sparhawk cat ketches. Brooks was a retired business consultant and author, and he did some writing and marketing for the company. I designed all the carbon fiber free-standing masts for the boats. Brooks invented the idea for what he called the “S” Number (S#)—a single number between 1 and 10 which could rate the performance of all sailboat designs. This idea was published in Telltales, a southern Texas boating magazine, in April, 1988. I have never seen anything like it, before or since.

The concept is rather simple and is based on the Displacement/Length ratio (DLR) and Sail Area/Displacement ratio (SA/D), both of which we have discussed in the last few weeks. We know that DLR relates to drag—heavier displacement for a given length results in more drag, and boats with high DLRs are slower than boats with low DLRs. We also know that SA/D relates to power—more sail area for a given displacement results in more speed, and boats with high SA/Ds are faster than boats with low SA/Ds. We have also plotted SA/D versus DLR in a chart, and have seen how the spread of data points relates to boat performance. We use these same ratios—SA/D and DLR—to calculate S#, so we don’t need any new computer program to achieve our goal—just one new equation.

The equation for S# is an exponential and logarithmic function using DLR and SA/D as the primary variables. We already know how to calculate DLR and SA/D, and I am going to remove the slash “/” from SA/D so that it is less confusing in the S# equation— we’ll use the term “SAD.” Although the S# equation looks complex, it can be easily programmed into a calculator or a spreadsheet. Here it is:

S# = 3.972 x 10^[-DLR/526 + 0.691 x (log(SAD)-1)^0.8]

Brooks developted this equation with the assistance of Dr. Fred Young, at the time Dean of the College of Engineering at Lamar University in Beaumont, Texas. I spoke with Dr. Young by telephone some years ago just to make sure I understood the equation correctly, and he was very helpful.

Brooks collected a list of boat designs and their particulars from various published sources and calculated their S#s. Then he classified the boats according to the following categories:

· Lead Sled: S# = 1 to 2
· Cruiser: S# = 2 to 3
· Racer-Cruiser: S# = 3 to 5
· Racing Machine: S# = 5 to 10

The reasons for the ever-broadening scale of category names is simply a function of the logarithmic scale embodied in S#. This appears to be an asymptotic function. You can never reach the number 10, and you can never reach the number 1, both of which are the asymptotes.

Now we have a way to definitively categorize boats, not a wishy-washy, vague notion; we got a unique number for each and every design! I attach the spreadsheet that I used to calculate the SAD and DLR numbers in that chart I posted last week (S# Chart 01). Included in that spreadsheet is the S# calculation (pink column), and next to that is the category name for each boat. I sorted the data according to descending values of S# so that you can see how the categories play out. Also included in the spreadsheet is S# Chart 01 of SAD vs DLR for these boats. There are other charts there, too, which you can study or modify at your leisure.

The magazine sources for these sailboat designs are listed at the top of the data table and in the left-most column with the date of publication in the second column. They are all published data from advertisements and design reviews that I have collected over the years. As I review the magazines, I continually add data to this table. The original Telltales data that Brooks used and published in 1988 is included. I find it discouraging that in recent years the magazines have been slacking in publishing worthwhile design data on boat designs. Sometimes, it is difficult to get even the most basic of information—some small piece is frequently missing, and you don’t necessarily find it on the manufacturers websites. But we gather what we can.

Now here is where my contribution to S# comes in. Overlaying this chart of SAD vs. DLR, I have calculated and plotted the traces of constant S# so that you can see how they subdivide the boat population. I attach this chart as well (S# Chart 02). In the data table, I highlighted in yellow a few of my designs, and then have labelled them in the chart, just to give you some context.

So what do we see? S# Chart 02 can be interpreted as follows:

A boat that is very lightweight and has lots of sail area will have a low DLR and a high SAD. It has a high power-to-weight ratio, and so it will be very fast. Its S# will be between 5.0 and 10.0. It will be a “Racing Machine.”

On the other hand, a very heavy boat that has a small sail area will have a high DLR and a low SAD. It has a low power-to-weight ratio, and it is not going to be a very good performer. Therefore, its S# will be between 1.0 and 2.0. It will be a “Lead Sled.”

For S# values between 2.0 and 3.0, the boat will have a decent amount of volume to carry people and goods but won’t necessarily be a real hot-shot sailer. We can place these boats in the “Cruiser” range.

For S# values between 3.0 to 5.0, the boat will be in the middle ground between “Cruisers” and “Racing Machines”, so we can call them “Racer-Cruisers” (or “Cruiser-Racers” if you prefer.)

Therefore, the net result of the S# is a clear delineation of sailboat performance using a convenient scale from 1.0 to 10.0, and by this we give definitive meaning to typical descriptive names. In fact, Brooks claimed that the S# is a fairly reliable predictor of PHRF or IMS rating. For two boats of the same length, the one with a higher S# will be faster, will take less time to sail around a course, and therefore will have a lower rating. However, in the same article, this footnote appears: “Both Dr. Young and the author stress that the ‘S’ number is not a handicapping or rating system, but a guide to probable boat performance vs. other boats of comparable size.” I personally agree with that opinion.

Something else that is quite interesting is shown in the next chart (S# Chart 03) also included in the spreadsheet. I had the notion to divide SAD by DLR and plot that against S# and got a surprising result. All of the data forms a unique cluster in a very well-defined curve. These are two independent functions plotted against each other. Rarely in science do we see such a profound correlation of data. I am not absolutely sure of the ramifications of this, and maybe I am reading into it more than I should, but I would have expected a broader spread of data points in this chart. The relationship of the SAD/DLR ratio to S# is extremely solid as indicated by the cluster of points along its trend line. The equation for the trend line shown at the top of the chart is another way to approximate S# in a simpler cubic equation. Throughout the lower categories, S# follows the trend line almost exactly, and it is only in the Racing Machine category where there is some scatter away from the trend line. If we plot S# versus some simple dimension or factor such as LOA or Displacment, we see no discernable relationship to S# at all. But S# vs. SAD/DLR gives us a very unique view of sailing performance.

I am not mathematician enough to explain why this works as we see it. I have, however, on occasion, presented plots like S# Charts 02 and 03 to clients to review the performance they have in their current boats or are trying to achieve in a new design. It seems to give them a clearer understanding of what their current boat does or their new boat is going to do. S# is a way to be a little more scientific in layman’s terms. This gives us a better tool to clearly showcase performance without having to go to the model tank, measure resistance factors, plot results, correlate them to full scale, and then do VPPs on top of all that. A picture says a thousand words, and this seems to do a pretty good job.

You will also see in the data table and in the charts a calculation and plot of Ted Brewer’s Motion Comfort Ratio (MCR) plotted against S#. I will explain that in next week’s topic.

In the meantime, you are all free to use this database and spreadsheet as you please. You may add to it as you do your own designs or review the designs for others. You may change it around and expand it however you want. You may do other analyses and manipulate the data at your will. Send it to your friends, fellow designers and fellow sailors. Pass it around. Discuss it. Use it. The S# is for the public domain, and I hope it adds to our better understanding of sailboat performance. Time will tell.

Eric

[Guillermo]

Thanks for bringing my attention to Eric's excellent post, Teddy. Not enough time now to participate in the forums these days (work traveling), I'll study this interesting S number carefully when back home.

All the best.

[Alfredoc]

Hi all,
someone can help me?
I've tried to calculate the STIX of the Jeanneau SELECTION 37 but whit infruttuosely results!!!
Could you help me

Cheers

Alfredo

[TeddyDiver]

Pretty sure they don't announce all the necessary data you to do that. Have you the spreadsheet for the STIX formula? If not here's a copy. Just throw the numbers in, but be aware, crap in..

[Alfredoc]

Thank's Teddy,
i've tried but i haven't all the numbars, in few week i have read all this iteresting post, I'm not a technician but a simple corintian sailor who wont make a sail race who the committee want boats whis a stix >32 like a A class

ciao

Alfred

[Guillermo]

Alfredoc.
I have found something useful here: http://cruisingresources.com/Jeanneau_Selection_37

But to be able to work out an approximate STIX (roughly) I need a profile plan showing the sail plan and the underbody. If you provide me with such a plan I can do the numbers for you.

Cheers.

[Alfredoc]

Dear Guillermo,
first of all thank you for this trade is really interesting for learnig about boat and her stability.
the only numbers who i have are the one-desing roule: http://selection37.com/francais/aps/JaugeDec05.pdf

personally i need changing mast (more short ) but if my boat have a low stix and avs is unnecessary

here i join a irc certificate of a selection modified (with the short mast)

ciao

Alfredo

[Guillermo]

Those are more accurate numbers, excellent. Now we need a profile drawing.
Cheers.

[Alfredoc]

This are the only two design who i have

a lot of thank's

ciao

Alfred

[Guillermo]

OK.
I'll work on them this weekend.
All the best.
Guillermo.

[Guillermo]

Sorry I have not been able to work on it this week end (busy boating and spearfishing with kids). I'll try to do it some day this week.
Cheers.

[Alfredoc]

Guillermo don't warry !!!!!!!

I've no hurry

When you can, you are giving me a great gift

Thanks a lot

ciao

Alfred