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Sailboat Design Heavy Seas
A number of measurements and formulas
are used to describe the sailing characteristics and stability of
sailboats. Some of the most commonly used descriptors are discussed
below:
Sailboat Design & Characteristics
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 | LOA
Overall length.
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| LWL
The length of the hull at the water line (load waterline length).
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| Bm Width at the widest part of the boat
(Beam).
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| Disp
Displacement or weight of the boat.
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| Ballast
Weight of the bilge or keel.
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Maximum Hull
Speed Formula (Ref 1): This formula
estimates the theoretical maximum speed of a hull displacement boat. (1.34 times
the square root of the length of the hull at the water line of a boat
loaded with cargo.)
Go To
Speed Calculator
SA/D Ratio is
the sail area/displacement ratio (Ref 1). This ratio indicates how
fast the boat is in light wind. The higher the number the faster
the boat.
Go To
SA/D Ratio Calculator
* Cruising Boats have ratios between 10 and 15.
* Cruiser-Racers have ratios between 16-20.
* Racers have ratios above 20.
* High-Performance Racers have ratios above 24.
D/L Ratio is
the displacement to length ratio (Ref 1). This indicates if the
boat is a heavy cruiser (results greater than 325) or a light
displacement racing boat (results less than 200).
Go To D/L Ratio Calculator
Ballast
Displacement Ratio (Ref 1): This ratio is calculated by dividing
the sailboat's ballast by the boat's displacement and converting the
result to a percentage. This ratio indicates the resistance
to heeing or a sailboat's stiffness. An average ratio is
approximately 35%. A higher ratio indicates greater stiffness.
Motion
Comfort (Ref 2): This ratio predicts the predicts the overall comfort
of a boat when it is underway. The formula predicts the speed of
the upward and downward motion of the boat as it encounters waves and
swells. The faster the motion the more uncomfortable the
passengers. The higher the number, the more resistant a boat is
to movement. This ratio was created by boat designer, Ted Brewer.
It is useful in comparing different boats. A higher value predicts
a more comfortable ride. Asailboat with a LOA of
42 would be expected to have a Motion Comfort Ratio in the low 30's.
Go To Motion Comfort
Calculator
Reference 1:
Reference: Rousmaniere, J, The Annapolis Book of Seamanship
Simon & Schuster, New York, New York, Chapter 1: The boat
p26-35, 1999.
Reference 2:
Ted Brewer Yacht Design
http://www.tedbrewer.com/yachtdesign.html
Heavy Seas and Boat Stability
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Where are you going to sail the boat ?
Heavy weather in the open ocean, with a long fetch and high
waves, is much more dangerous than that encountered in a sound or lake.
High winds alone can easily capsize a small boat, but a larger sailboats
may only heel to a near 90 degree angle. However when combined with
high waves even large boats can easily be capsized.
Thus, if one is sailing in an inland waterway, it is less likely that
the boat will be capsized. A boat with a larger beam, which heels less
and has more living space may be desirable in this setting.
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A longer boat is desirable in heavy weather and high waves. No
matter how big a sinusoidal wave is, it should not capsize a boat.
A breaking wave is a different story, these waves have a slope of 70
degrees and if higher than a boat is long, the boat cannot go over them.
An extreme breaking wave hitting the boat's stern or bow may cause it to
tumble over end on end (pitchpole or pitchpoling). If a
breaking wave hits the boat on the beam, it can more easily broach or
capsize the boat. Thus, one of the most important factors in
surviving high waves is the length of the boat.
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A high stability is most important when
hit beam-on with a breaking wave and may only give a relatively small advantage.
Boat "stability" or resistance to being capsized is most important when
a boat is hit beam-on with a breaking wave. Boats with a wider
beam are less stable that those with a narrow beam. However, boats
with a wider beam are stiffer (heel less) and have more living room
below.
According to Andrew
Claughton in Heavy Weather Sailing 30th ed. p 21 "This (the
test data presented in the chapter) suggests that alterations in form
(of a sailboat) that improves capsize resistance may be rendered
ineffective by a relatively small increase in breaking wave height."
If a boat is positioned into a breaking wave, most boats (wide and
narrow beamed) can survive a 55% LOA (overall boat length) breaking
wave. However, a 35% LOA breaking wave hitting a wide-beamed boat
beam-on can easily capsize the boat. All yachts tested rolled to
130 degrees. No yacht, no matter how stable, could consistently
resist capsizing when hit, beam-on, with a 55% LOA breaking wave.
(K. Adlard
Coles' and Peter Bruce's (editors) Adlard Coles' Heavy Weather Sailing
(30th edition) Stability of Yachts in large breaking waves.
Chapter 2 pp11-23 International marine,
Camden, Maine)
Putting this in perspective in a 40 foot (LOA) sailboat: In a highly stable boat wave survivability
would increase by 8 feet, if hit beam-on by a breaking wave. A
40 foot sailboat no matter how stable will not consistently survive a 22
foot breaking wave. Thus, in a strong gale with 22 foot seas and
breaking waves, a 40
foot sailboat is at risk of capsizing no matter how stable.
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Most important factor is an experienced
crew: Of all the factors, it is far more beneficial to have an experienced crew
that can either avoid or position the boat into large breaking waves.
The 1998 Sydney - Hobart race was one of
the worst sailing disasters in recent maritime history. And from
it many lessons were learned regarding the functioning of boats and
crews in heavy weather. 115 boats left Sydney and were hit by an
unexpected typhoon. Seven boats were abandon and five were lost.
The 1998 Sydney to Hobart Race Review Committee report, summarized by
Peter Bush, committee chair, reported the following
as one of the significant findings: "There is no evidence that any
particular style or design of boat fared better or worse in the
conditions. The age of yacht, age of design, construction
method, construction material, high or low stability, heavy or light
displacement, or rig type were not determining factors.
Whether or not a yacht was hit by an extreme wave was a matter of
chance." (Ref:
Rob Mundle in Fatal Storm, Publisher's Afterward p 249.
International Marine/McGraw-Hill Camden, Maine.)
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Sailboat Stability: Two formulas are
designed to give an estimation of the stability of a sailboat.
The reliability and value of these
screening formulas
are
disputed. Although reliable in the lab, they may not predict
stability in severe heavy weather conditions.
1.
Capsize
Screening Formula. This formula estimates the
boat's resistance to capsizing. (Ref. 1) This formula does not take into
account the vertical position of the center of gravity (VCG). The
VCG can be lowered by a longer keel or by having more ballast (weight of the keel)
at the end of the keel. However, according to Adlard Coles' "Heavy Weather
Sailing" (Ref. 3) thirtieth anniversary edition, "The effects of
large movements of the VCG on the propensity to capsize was surprising
small". Nevertheless, a low VCG will greatly help the boat in righting
itself once it has capsized. For example, the J-105 is a lightweight
high-performance racer with a D/L of 135 and a SA/D of 24. Stability is not predicted by this
formula but the J-105 has a 3400 lb 6.5' lead keel to stabilize the boat.
Go To Capsize
Screening
Calculator
1) Rousmaniere, John The Annapolis Book of Seamanship Boat
Selection.
Chapter 1 p35 Simon & Schuster, New York, New
York.
2. Ultimate (Latent) Stability.
This is the resistance to capsize and heel. One of the best
predictors of ultimate stability is the "angle of vanishing stability"
or the angle to which the boat can heel and still right itself. A
dingy will have a stability range of about 80 degrees, an inland water
boat should have a stability range of 100 degrees, and an offshore boat
of at least 120 degrees. Boats which have a stability angle of
less than 140 degrees may be left floating upside down once capsized.
Boats with a higher angle will usually right themselves (Ref. 3).
The formula on the website used to estimate
the Angle of Vanishing Stability does not fully take into account the
position of the vertical center of gravity. Thus, boats with a
long lead keel or a lead bulb at the end of the keel may have a higher
angle of vanishing stability than that predicted by the formula.
Go To Calculator of Angle of Vanishing Stability.
3) K. Adlard
Coles' and Peter Bruce's (editors) Adlard Coles' Heavy Weather Sailing
(30th edition) Stability of Yachts in large breaking waves.
Chapter 2 pp11-23 International marine,
Camden, Maine
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