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Slogging to Windward
by Chuck Merrell
sloggingtowindward@hotmail.com 

January  2001

DESIGN ANALYSIS RATIOS
Part 1 - "What’s Required"
(on to Part 2)

The year behind, the year ahead:

When I started this column, I decided that the best way to try and keep it interesting would be to vary the theme from month to month. The first column in September took a cut at sacred cows (more of that to come). October told the story of two boatbuilders, one famous and one not who unlike most home boatbuilders, actually completed their projects and then did something significant with the completed vessels. November posed the question: Why do the most ignorant people want to make the biggest and most uninformed changes when working on or building a boat? December offered free plans for Apple Pie, a simple but elegant yacht tender, and to date about 800 copies of the plans have been downloaded. Hopefully some of you will build this little boat and send some pictures for the web page-please, please, suck, suck! This month and next the subject is: Judging a design by the numbers. In coming columns the spotlight will fall on themes such as "The Magic Carpet Syndrome"; "Choosing a Designer"; "The real story on Shallow Draft and Water Ballast"; "Shipwrecked on purpose" and "Whose design is it anyway?" So, here goes . . .

Probably one of the most difficult things to define is physical sensation by degree, but by various means, verbal and mathematical we continue to try. Comedian Carol Burnett was once asked by an audience member to describe the pain of childbirth. She said, "No problem. Just grab your lower lip and pull it up and over your head. That’s how it feels!"

The reason why we want to know what something "feels like" fundamentally is because human beings (masochists excepted) are in general, comfort seeking animals. It’s nice to have an idea of how some new situation (riding in a different boat, car, buckboard, whatever) might compare to a previously experienced similar scenario, especially if we’re thinking of buying the program in some way. Boat designers are often asked: "How will this boat feel and perform in operation? Will it be comfortable, manageable and safe? Where is a boat with these specifications best used? In other words, what’s the idea behind this new item?

Since it’s almost impossible for the individual to personally test every boat, over the years there have been various approaches concocted to help predict a boat’s hypothetical performance and "feel" by utilizing some type of mathematical formula. For example, Bob Perry, as a layman’s guide to performance, popularized the Displacement/Length Ratio over twenty-five years ago. Since then, many boat writers include this and other numbers as part of the text as suggested answers to the above, and other questions.

All you need to calculate most design analysis ratios is the information usually given when a design is published, or en brochure. On the other side of the coin though, this information isn’t only for the edification of those interested in a particular boat. Designers themselves pay close attention to these numbers as they evolve in the course of creating a new hull. In fact, the resultant preliminary calculations often (usually?) drive changes and modifications to a design in progress.

The information usually given for a boat by the designer (as a list of specifications) is: LOA (Length Overall); LOD (Length On Deck); LWL/DWL: (Length of Waterline, or Length of Waterline as Designed); Beam (Width of boat on Deck); Displacement: (How much the boat weighs floating on the DWL); Draft OA (How deep the boat sits in the water, including any appendages-keels or boards); Draft Body (The depth of the hull in the water LESS the salient protrusions); Sail Area (If it’s a sailboat, total amount of canvas in the working sail plan).

In addition to the information listed above, you can see from the hull drawing/information sheet of TestBench* that I also provide clients and plan buyers with additional other information about the hull design and I usually try to convey it on a single sheet for clarity. Makes it easier to look at, think about and relate.

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*TestBench resembles a hull that Phil Bolger might draw, and in fact looks a bit like Micro. However, TestBench is constructed of 1/8" steel from the horizontal line (about a foot above the LWL) down to and including the keel, and made of " plywood for the topsides, deck and lighter wood for the interior. The final design will look almost nothing like a Micro or Micro derivative. The purpose of this design started out to provide a good platform with which to economically test the Hasler/McLeod Modern Junk Rig. Now, with a client in the picture, the idea has evolved into trailerable and a very robust but comfortable, well equipped, mini-offshore cruiser in which a lone sailor (or maybe a young couple) can economically and safely explore remote places like the Bahamas, Caribbean or "ship it there", the islands of the South Pacific etc. I also think TestBench would also be a great boat to use in more or less duplicating a trip to Alaska as written about in Johathan Raban’s best seller (last year) "Passage to Juneau". Click here: http://www.boatdesign.com/bookrevu/revu4.htm to read Bill Samson’s book review:

Now let’s go back to the numbers. I usually also provide the following data: Prismatic Coefficient (It’s not possible to accurately figure the Prismatic from the usual specifications given when a design is published, but more and more lately, designers are showing this number). The same also applies to the CB (Center of Buoyancy), Water-plane Area and Water-plane Loading (an important but obscure index).

So, in order to get a good idea of the potential of a hull design, you need to have all this data at hand, AND the knowledge to interpret it.

In order to keep this column from running entirely too long now--NEXT MONTH, I’ll pick it from here and give you the formulas and instructions for arriving at this information. More important, I’ll also provide some graphs and charts and other information which will help you interpret and integrate this newfound data, based (where possible) on your past experience in boating.

Until then, have a Happy New Year and let’s all hope that the Stock Market makes a big comeback!

Chuck

Go on to part two

 

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