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Most anyone can draw a boat; designing one takes a
lot more effort and ability.

Of primary importance in designing a small boat is
knowledge gained by experience. At GLEN-L we design boats, build testing
prototypes, and then test the in-water performance. Most designers don’t
have the luxury of being able to test their final product. We do and the
experience gained is invaluable for designing future craft.

A new design is started by establishing the
parameters. The length, beam, depth, and type of boat is decided on; power or
sail and the material the boat is to be built from must be selected. Every boat
is a compromise and practical experience dicatates the inevitable give and take
features that are best for the particular craft being designed.

ThinkDesigners use coefficients that provide comparative
factors to determine the general shape of the midsection and other factors. The
range of coefficients for a given type of boat have been gathered over the
years and are given in books on boat design. But a designer’s experience of the
performance of previous boats is used to temper the figures, as little
information has been published on smaller boats.

The basic lines of the boat are developed showing
three views; plan, profile, and sections. A designer must rough out a set of
lines that hopefully match the desired coefficients. The term hopefully is used
because accurate coefficients cannot be determined until the lines are

The displacement of the boat must be estimated;
this is the total weight of the hull, motor, passengers, and everything on
board when the boat is on the water. Then an estimated waterline is sketched on
the lines.

A boat will displace a weight of water equal to its
displacement. The underwater volume can be calculated by determining areas of
equally spaced sections (“s”) and entering them into Simpsons 1/3
Rule formula; boat design computer programs do it quicker. The calculated
displacement should be very close to the one estimated. If not, back to the
drawing board to raise or lower the waterline or adjust the lines and
recalculate displacement.

A boat has a balance point called center of
buoyancy (CB) that is comparable to the pivot point on a teeter totter. The CB
can be determined by using figures obtained from Simpsons Rule; the volume on
either side of the CB must be equal. But the CB must be located at the ideal
position to carry the loadings. The approximate desireable location of the CB
is obtained from published figures or percentages of the waterline length
gained by experience. The CB is virtually always aft of the mid-points of the
waterline length. If the calculated CB is not within the design parameter it’s
back to square one and modification of the lines.

Glen L. Witt, Naval Architect at work

Glen L. Witt, Naval Architect at work

But the designer isn’t finished. Weights have
been estimated and must be finalized. The weight of every component of a boat
must be calculated and it’s distance from the CB noted. Consider what is
involved. A wooden boat may have a 1¼” x 3½” keel 18′ long.
The volume of the member times the weight of the wood for a given volume is
calculated and the center of that weight in relationship to the CB determined.
And this is done for everything that will be in the boat. If the total weight
isn’t equal or close to the displacement or the weights are not equally
distributed about the CB, modifications will be necessary. An experienced
designer gets an “eye” or “feel” for a set of lines and can
usually finalize a set of lines with minimal revisions. A neophyte designer may
spend days or weeks bringing all of the factors into balance.

A sailboat will need to be balanced so it will sail
properly without lee or weather helm. Rudder area must be calculated to provide
good steering and the keel or centerboard determined to provide directional
stability If a boat is towed sideways through the water so the longitudinal
centerline of the hull is perpendicular to the towing line that point on the
waterline is called the center of lateral resistance or CLR. The combined area
of the sails must be positioned about the CLR properly (called
“lead”) to assure balance. And of course the sail area forces must be
balanced with adequate ballast so the boat will not capsize.

Powerboats will require calculations on the most
desireable horsepower based on the hull characteristics. If the hull is a semi
displacement type, with limited speed potential, it doesn’t make sense to
overpower the boat. Conversely underpowering a planing hull will make a boat a
real dog.

Tuffy RunaboutSome of the information necessary to make the many
calculations are given in design textbooks. Other factors are obtained through
technical writings and papers collected by a designer over the years. And if
you test boats as we do at GLEN-L, parameters that work can be applied to
future designs.

Most creditable designers have a background of
engineering or schooling on boat design through universities and specialized
boat design courses. Schooling covering the design of smaller craft however is
almost non-existant and the experience gained through testing prototypes is
invaluable on this size craft.

The foregoing outlines the general procedure that
must be gone through to design as opposed to drawing a boat. Computers can do
many of the calculations. However, a computer can’t design a boat. Someone
must put in the figures to form the boat and use the knowlege to select the
proper parameters. It’s been stated that designing a good looking, well
performing boat, is a combination of engineering and art and the best way to
develop this combination is through experience. And with more than 60 years of
designing boats GLEN-L has experience plus.

  • BOOKS:
    • YACHT DESIGNING AND PLANNING, by Howard Chapelle ($35.00)
    • Westlawn Institute of Marine Technology733 Summer StStamford, CT 06901203-359-0500
  • Computer programs:
    • PLYBOATS 2.01Ray Clark Enterprises7071 Warner Ave., #FHuntington Beach, CA 92647email:

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