FIGURE 1-1: Powerboat Side View
Draft
Hull
Portuguese Bridge
Freeboard
Bow
Pilothouse or
Wheelhouse
Flybridge
Hull
Length at Waterline (LWL)
Length Overall (LOA)
Chine
Gunwale
Rudder
Propeller
Shoe
Keel
Transom
Cleat
Cockpit
Stern
Salon or Cabin
Mast
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Boater’s Pocket Reference: Chapter 1
Basic Terminology
26
Boater’s Pocket Reference: Chapter 1
For planing
in very rough
water, the
deep vee was
developed
Deadrise Angle
with a deadrise often
greater than
20 degrees
to allow the Chine
boat to cut through the waves. The deep vee requires more
power to get on plane than flatter hull designs. It tracks very
well in all types of seas, so much so that it can be difficult to
turn.
Another development in vee shape hulls is the modified vee,
which has a deadrise decreasing from forward to aft. This
gives it a sharp entry for cutting through waves and a flatter
stern section to aid in planing. When only one deadrise angle
is specified for a modified vee, it is normally given as the
deadrise at the stern.
Other Hull Forms
Cathedral Hull
The cathedral hull is a planing hull that
eliminates some of the slamming action
associated with flat hulls while retaining
much of the efficiency of the flat hull. This design also tracks
well and provides high initial stability.
Sailboat Hull
The sailing monohull is characterized by
a deep keel that reduces slipping sideways
(leeway) from to the force of wind. It can
be rounded or vee shape with a hard or soft
chine.
Trimaran
The trimaran is usually
associated with sailboats and consists of
a main hull with two outriggers, one on
each side, that provide high initial stability. The main hull
and outriggers can be any of the three primary types of hull
designs, and can be designed for displacement or planing use.
The outriggers allow the use of long slim hulls, which are
faster than hulls that have a wide beam.
– ⇐ Righting Arm ⇒ +
Boat Design and Construction
33
Catamaran
Flat Bottom
Cruising Sailboat
t
Powerboat
30
60
90
120
150
180
Capsize Zone
Degrees Heel
FIGURE 1-7: Stability Curves
Capsize Ratio or Capsize Screening Value
The capsize screening value (CSV) was developed after
the 1979 “fastnet storm” in the Irish Sea where several boats
capsized and several lives were lost.
Generally a boat is considered suitable for offshore use if the
CSV is less than 2.0. The higher the CSV is above 2.0 the
more likely a boat is to capsize in adverse conditions. Notice
that this means the hull should not be too wide for a given
displacement. The curve in figure 1-8 represents the CSV at
2.0. The safe area is below the curve. Boats that fall above
the curve are considered unsafe for offshore use.
Capsize Screening Value
25.0
Beam (ft)
20.0
Not Good
CSV
= 2.0
15.0
Good
10.0
5.0
0.0
0 10
20 30 40 50 60 70 80 90 100
Displacement ( x 1000 Lbs )
FIGURE 1-8: Capsize Screening Value = 2.0
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Boater’s Pocket Reference: Chapter 1
Prismatic Coefficient (CP)
The CP is a ratio of the volume of water displaced by the boat
to the volume of a prism like shape with a length = LWL and
constant cross-section equal to the maximum cross-section
of the boat (below the waterline).
Solid line shows hull
volume displaced
Waterline
Dotted line shows prismatic volume
= maximum cross-sectional area
times length at water line.
Area of maximum cross-section
FIGURE 1-9: Prismatic Coefficient
To better explain this, figure 1-9 shows the two volumes used
in calculating the prismatic coefficient. The numerator is represented by the solid line and indicates the volume actually
displaced by the vessel. The denominator is represented by
the dotted line and shows the volume of the maximum boat
cross-section times the length of the boat at waterline.
Equation 1-5 is used to calculate the prismatic coefficient.
CP = D (Volume)
MS × LWL
D (Kg) / 1025
D (Lb) / 64
=
=
MS (SqFt) × LWL (Ft) MS (SqM) × LWL (M)
Where: CP = Prismatic Coefficient
D = Displacement Volume (Seawater)
MS = Midship Section = Underwater Area of
the Maximum Hull Section
LWL = Length at Waterline
EQUATION 1-5: Prismatic Coefficient.
Both the numerator and denominator are volumes and may
be expressed as either imperial or metric units as long as both
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Boater’s Pocket Reference: Chapter 1
Relativity of Boat Sizes and
Measurements
As boat size changes there are certain relationships between
different boat measurements that generally remain constant.
These have been called by various names, such as Froude’s
Law of Comparison or the Law of Mechanical Similitude.
Some of these are:
• Linear dimensions (such as beam and draft) vary proportionally to length (L).
• Areas will vary as the length squared (L2). This includes
wetted surface area and sail area. Resistance varies with
wetted surface area.
• Speed varies as the square root of length (√L).
• Weight, displacement, and volume vary as length cubed
(L3). Corollary: Cost varies with the displacement and
therefore as the length cubed (L3).
A boat that is twice as long as another boat will:
• Have twice the beam and twice the draft.
• Have four times the sail and wetted surface area.
• Travel √2 or 1.4 times faster (displacement hull speed)
• Displace eight times as much.
• Cost eight times as much!
Boat Construction
Hull Materials
General
Today most recreational boats are made from fiberglass,
which in the last 50 years has all but replaced wood as the
material of choice for hull construction. In addition, some
recreational boats are constructed from either steel or aluminum, or a combination of both.
Wood
The structural properties of wood make it an ideal material
for use in building boats. It is easy to work with and provides
excellent strength for its weight without being too brittle (it
will deform to some degree before shattering or failing).
Wood construction is skilled labor intensive, so it does not
easily lend itself to assembly line construction techniques. In
addition, wood is susceptible to damage by water, particularly rot. These two factors have led to the demise of wood as
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Boater’s Pocket Reference: Chapter 1
Another similar in concept technique is the Vacuum Infusion Process (VIP). This was developed as a sufficiently
different alternative to SCRIMP so users would hopefully not
have to pay fees to TPI. The cloth is laid and covered with
plastic, but with channels left open in foam core material so
a vacuum applied at one end will draw epoxy from a drum at
the other end through the channels. Over a period of hours
the lay-up becomes completely saturated with the resin.
The latest and most sophisticated system to be developed is
the Virtual Engineered Composite (VEC™) system developed by Genmar (www.genmar.com). This system uses an inside mold as well as the usual outside mold. The VEC system
allows placement of stringers and boat floor so the entire
structure is constructed as one piece in one step. The inner
mold is lowered and resin is then injected under pressure
rather than by vacuum. Computer control of all aspects of the
process throughout the injection and curing steps, with more
than 500 variables being monitored, ensures high quality and
consistency from one hull to the next. The method is being
used to produce boats up to 24 feet (7.3m) in length.
Structural Properties of Fiberglass
Let’s add an additional complication to fiberglass boat construction. Fiberglass is actually quite a heavy material, with
stiffness much less than aluminum on a pound for pound
basis. As a result, building boat hulls from solid fiberglass,
especially in larger boat sizes, yields a heavier boat, which is
not conducive to a planing or semi-displacement design. In
order to solve this problem and add strength to the fiberglass
hull form, a lightweight core is added in between two layers
of fiberglass. So now we place a layer of gelcoat, a layer of
fiberglass, a relatively thick layer of lightweight material, and
a layer of fiberglass.
Note the deflection in the two different cases shown in figure
1-13. The sandwich construction bends much less, showing
greater structural rigidity and strength.
FIGURE 1-13: Comparative Flexure of Solid and Cored
Fiberglass
By adding a lightweight core, we can considerably reduce
the amount of fiberglass used and end up with the same
structural strength albeit with one major caveat. The thinner
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Boater’s Pocket Reference: Chapter 1
boats with outboard or sterndrive (also called inboard/outboard or outdrive) propulsion. There are many variations of
these with special designs for such diverse activities as water
skiing, river running, racing, fishing, etc. Some examples of
runabouts follow.
Bowrider
Photo of an LXi 208 (Courtesy Larson Boats)
The bowrider is an all-purpose runabout with an open bow
that allows seating forward in the bow. The boat in the
picture is a sterndrive model. Outboards models are available
as well, and jet drives have also become more popular in
recent years. This type of boat can be used for skiing and
fishing and is almost always made of fiberglass. This boat
type with a sterndrive is the most popular model of runabout
for general purpose use.
Cuddy
Photo of a Cabrio 220 (Courtesy Larson Boats)
The cuddy style of runabout has a forward enclosed cuddy
cabin. In smaller boats this would contain vee berths and in
larger models can include a toilet with shower as well as a
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Boater’s Pocket Reference: Chapter 1
Pilothouse
Photo of a Krogen 44 (Courtesy: Kadey-Krogen Yachts)
The pilothouse design is characterized by a raised pilothouse
separated from the salon. Typically there will be a cabin in
the bow as well as a guest cabin under the pilothouse. There
may or may not be a flying bridge as shown in the above
drawing. The Krogen 44 shown in the picture is a full displacement hull design and is suitable for ocean passages.
Sedan
Photo of a 390 Trawler (Courtesy Mainship Trawlers)
The sedan has the salon and cockpit on one level with the
helm being located in the forward part of the salon. A second
helm located on an open bridge above the main bridge,
Boat Design and Construction
71
Ketch
Like the yawl, the ketch is two masted and has a mizzenmast
shorter than the main. The mizzenmast is taller than the mizzenmast of a yawl, is located forward of the rudderpost, and
has a sail with up to half the area of the mainsail. Some have
gaff rigged main and/or mizzen sails and often have more
than one foresail (like a cutter).
Schooner
A schooner has two and sometimes more masts with the aft
mast being the taller mast. The schooner usually has two or
more headsails.