Nautical and Marine
by Chuck Merrell
DESIGN ANALYSIS RATIOS
Part 1 - "Whats 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. Thats
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.
Its 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 were 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, whats the idea behind this new item?
Since its almost impossible for the individual to personally test every boat,
over the years there have been various approaches concocted to help predict a boats
hypothetical performance and "feel" by utilizing some type of mathematical
formula. For example, Bob Perry, as a laymans 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
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 isnt 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 its 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.