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Deadrise
Measuring a
Vessel's Hull
By Paul Bruno
Deadrise
is measured in two ways, by a linear measurement like inches or centimeters and
by expressing it as an angle.
Let’s
look at the angular measurement first.
Looking
at the cross-section of a hull, draw a vertical line through the center of the
vessel to the bottom of the keel.
The
top of this vertical line should be even with the chine, which is where the
hull meets the topsides.
Now
draw a horizontal line that intersects both sides of the chine and the top of
the vertical line you drew before.
You
should now have a 90-degree angle formed by the vertical and horizontal lines.
Draw
one more line from the point where your horizontal line meets the chine to the
bottom of your vertical line at the bottom center of the keel.
The
triangle you formed is made up of three angles. Deadrise expressed as an angle
is the measurement in degrees of the bottom of the triangle.
To
Calculate in Linear Terms
To
calculate deadrise in linear terms you will use the same triangle as above but
now you will be using a ratio to express the deadrise.
Much
like the roof of a building, deadrise in linear terms is written as inches per
foot.
First,
determine the number of inches from the 90-degree angle of the triangle along
the horizontal leg to the chine.
Next,
determine the measurement in feet from the bottom of the keel to the 90-degree
angle of the triangle. Take the results and write then as inches/foot.
A
Measurement at a Single Point on a Vessel’s Hull
Deadrise
is only a measurement at a single point on a vessel’s hull. Construction plans
will note the deadrise at regular intervals along the length of the hull.
Since
deadrise is a measurement based on the position of the chine it is possible to
have complex expressions of deadrise because of multi-chine and planning hulls.
If
you are asked to measure deadrise you should be given a point to make your
measurement. For example; deadrise at 20 feet from the bow, or deadrise at rear
bulkhead.
Alternate
Spellings
Dead
Rise
Common
Misspellings
Dead
Rise
The
Transition From Chine to Keel
One
way to make a quick assessment of the purpose and ride quality of a vessel is
to view the stern from the rear so you can see the transition from chine to
keel.
If
it's a sharp V shape below the water means the ride will be smooth but the ship
may wallow back and forth ferries and riverboats have this design so they can
operate in both directions without turning around.
If
the deadrise is shallow or flat at the stern the vessel will not have much roll
or wallow but it will slap into the surface with each wave.
A V
shape allows a smooth transition while a shallower deadrise causes a sudden
impact with each wave.
The
flat design has less drag and therefore is found on cargo ships and other low
drag vessels. Cushion effect can be a problem for some heavily loaded cargo
ships in shallow waters like canals.
A
rolled, or soft, chine means the vessel is meant to lean and roll smoothly. This
is true of most sail-powered vessels where there is a counterweight in a deep
keel.
Take
a look at all kinds of common hull shapes to understand more about their uses.
The definition of draft will also be useful when learning about naval
architecture.
Paul Bruno
Maritime Expert
Education
USCG Master's License
B.A., Creative Nonfiction and
Technical Writing, University of Wisconsin
Introduction
United States Coast Guard
licensed Ship Master with Passenger Certification.
A marine contractor familiar
with all ship systems who has worked in the industry for more than 20 years.
Has a USCG Master's
License and a degree in Creative Nonfiction and Technical Writing, from
the University of Wisconsin.
Experience
Paul Bruno is a former writer
for ThoughtCo who contributed work for over five years on the maritime
industry. He has worked with a variety of vessels throughout his maritime
career, including various commercial ships as well as the Great Lakes schooner
reproduction "Dennis Sullivan." As a marine contractor, Paul is
familiar with all ship systems, and his lifelong passion for electronics has
led to a focus on marine hardware and software development. When he is not
working, Paul's favorite hobby is knot work, keeping busy with the nearly 4,000
documented knots, splices, and eyes.
Education
USCG Master's License, 1993
Creative Nonfiction and
Technical Writing, University of Wisconsin, Milwaukee and University of
Waikato, Hamilton NZ
A Message from Paul Bruno
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