S. Siva Chidambaram
Naval Architect / Marine Engineer
Comparison between Monohull and Catamaran Design as a research
An Oceanographic Research vessel must be characterized based on the following
Work Deck Area
Ability to set the research equipment
Demi-Hulls have a greater wetted surface area
than a Monohull and therefore catamarans
have a higher Resistance than Monohulls at
lower speeds, Fn ≤ 0.3. At high speeds (Fn ≥
0.5), the narrow demi-hulls have a lower Wave
resistance than an equivalent Monohull.
Monohulls naturally have less wetted surface
area and so the Resistance is Less when
compared to catamaran in case of lower Fn But
when Fn increases the resistance will be high.
A catamaran usually have a propeller and a
rudder in each hull and this gives very good
Maneuverability characteristics is reduced
when compared to Catamaran.
Seakeeping may not be so good and stresses in
the cross – connection (Bridge) between the
demi-hulls in an oblique sea may not be quite
Seakeeping good when compared to
The only factor where monohull challenges
Lower Wave Resistance because of the demi-
hull spacing distance. But increased Frictional
Wave resistance and Frictional resistance are
normal as of a Monohull.
Very High stability because of Larger GZ and the
twin hull benefits.
Stability concerned completely on the hull form.
however, it cant challenge catamaran in case of
Large Deck Area- Accommodation High. Smaller Deck, Working deck area and laboratory
space are the premier commodities on any
The Research Vessel (R/V) Kilo Moana is owned by the U.S. Navy and operated by the
University of Hawaii Marine Center. This 186-foot general oceanographic research vessel is
designed to operate in coastal and blue water areas.
Swath Ships are not specifically for high speeds, the main consideration being good
Seakeeping. The small water plane area twin hull (SWATH) vessel is a displacement ship
having two demi hulls.
Each demi hull is made up of a semi-submerged hull resembling a body of revolution and a
strut which pierces the water surface. The separation between the demi-hulls is bridged by a
box cross-structure. Thus the SWATH ship combines the favourable features of catamarans
and semi submersibles.
This enables the SWATH ship to retain the essential advantage of a large deck area combined
with controlled motion and reduced wave making drag by placing its displacement well
below the water surface.
Complete Details on
for Research Vessel (R/V) Kilo Moana , which is ultimately a SWATH is provided in the below mentioned
One important point needs to be considered in case of choosing a catamaran. It’s Cost. Cost of a
catamaran is high. Reason lying
Larger surface area = hull materials = Cost to build.
Larger Volume = accommodation = fit out = Cost to build
Machinery = 2 engines = Cost to build
Multiple duplicated systems = Cost to build
Design of a swath
Steadiness in a disturbed seaway:
It is well confirmed that a properly designed and built SWATH ship will substantially reduce
motions induced by moderate to high wave conditions.
SWATH ships can be designed to suffer only one-half to one-fifth of the heave, pitch, and roll
motions of a monohull of equal displacement in seas driven by wind speeds over 20 knots.
Furthermore, SWATH ships can be configured such that motions are nearly independent of
wave direction relative to the heading of the ship, both underway and dead in-the-water.
Ability to maintain speed in high sea states:
The amelioration of slamming by high waves allows SWATH ships to steam at speeds not
possible in comparable monohulls.
The submerged hulls running below wave motion, and the main hull elevated by the slender
small waterplane columns (struts), together with some other design tradeoffs can make
moderate size vessels relatively immune to slamming.
Since the chief benefit of SWATH designs depends on having their buoyancy compartments
well below the disturbed sea surface, a deeper draft is required for similar sized monohulls.
This can be lessened by the variable draft design.
High propulsion power:
The greater wetted surface of the submerged hulls causes greater frictional resistance
and total drag at low and moderate speeds. At higher speeds, the lower wave- making drag
of a properly designed SWATH lessens this disadvantage.
Because of the small waterplane area and wide separation of its buoyancy compartments, a
SWATH design will tend to have larger trim and heel excursions than will have a monohull.
The SWATH ship also will experience greater draft changes (about four times greater) than
an equivalent monohull.
Basic Design involves the determination of major characteristics affecting cost & performance.
· Main Dimensions: L, B, T, D
· Hull form: Lines Design.
· Preliminary General Arrangement
· Major structure.
The proper selection of the above should satisfy the following mission requirements
· Good sea keeping performance.
· The desired speed.
· Cargo capacity
· Dead weight
Process of Design
• It includes the Technological feasibility studies to determine the fundamental elements
of the proposed vessel such as Length, Breadth (B), Draught (T), Depth (D), fullness (Cb),
power or alternative sets of characteristics which meet the required speed, cargo cubic &
• It includes preliminary lightship weight estimation.
• The selected concept design forms the basis of obtaining approximate cost
• It refines the major ship characteristics affecting cost & performance.
• Its completion provides a precise definition of the vessel that would meet the mission
requirements (This provides the basis for the development of contract plans &
• A ship is essentially a part of the profitable transportation, industrial or service system.
Hence the viability of its Economic operation is a major factor. It also involves a continuous
interaction with the production processes and procedures in shipyards.
• The design that is created therefore must be producible at a low initial cost.
Later comes the Final Contract Design, where the contract is signed between the two Parties.
• A controllable pitch propeller (CPP) is a type of propeller with blades that can be rotated
around their long axis to change their pitch.
• If the pitch can be set to negative values, the reversible propeller can also create reverse
thrust for braking or going backwards without the need of changing the direction of shaft
• It adds complexity and a higher initial cost, but it does provide the operator almost infinite
speed variations from nearly zero thrust right up to the vessel’s rated speed.
• The reduction gear is simpler, because no reversing gear and clutch is required.
• A controllable pitch propeller system lends itself more readily to dynamic positioning,
provided there is also a thruster and it is also a part of the dynamic positioning system.
• In Many small vessel controllable pitch propeller systems are well- proven, very reliable,
and do not significantly increase operational costs or maintenance requirements.
• A VPP can improve vessel maneuverability by directing a stronger flow of water onto the
• On a large ship the VPP requires a hydraulic system to control the position of the blades
• A Fixed pitch propeller (FPP) is a type of propeller in which the pitch of the blades is fixed.
• This propeller is optimal for most vessels, not only because it’s cheaper, also because it is
more efficient than a CPP for a specific rotational speed and load condition.
• A FPP-equipped vessel needs either a reversing gear or a reversible engine to reverse.
• Compared to a FPP, VPP is more efficient in reverse as the blades’ leading edges remain as
such in reverse also, so that the hydrodynamic cross-sectional shape is optimal for both
forward and reverse.
• The Voith Schneider propeller (VSP), also known as a cycloidal drive (CD) is a specialized
marine propulsion system (MPS).
• It is highly maneuverable, being able to change the direction of its thrust almost
• From a circular plate, rotating around a vertical axis, a circular array of vertical blades (in the
shape of hydrofoils) protrude out of the bottom of the ship.
• Each blade can rotate itself around a vertical axis. The internal gear changes the angle of
attack of the blades in sync with the rotation of the plate, so that each blade can provide
thrust in any direction.
• A Voith-Schneider drive merely requires changing the pattern of orientation of the vertical
• In a marine situation this provides for a drive which can be directed in any direction and thus
does away with the need for a rudder.
• It is highly efficient.
• A choice is made on the basis of perceived performance requirements. Instead of a Kort
nozzle, VSPs are often fitted with a "thrust plate" or "propeller guard" which acts as a nozzle
at low speed, protects the VSP against grounding and provides another blocking location
• The propeller rotates 360 degrees around the vertical axis so that the thruster can perform
both the propulsion and steering duties for a vessel, or positioning for a semi-submersible
rig, drillship or FPSO.
Mechanical transmission: where a motor inside the ship is connected to the outboard unit by
gearing. The motor may be diesel or diesel-electric.
Electrical transmission: are more commonly called pods, where an electric motor is fitted in the
pod itself, connected directly to the propeller without gears. The electricity is produced by an
onboard engine, usually diesel or gas turbine.
Azipull is the newest azimuth
thrusters in the Rolls-Royce range
and is a low drag, highly efficient
pulling thruster. It is designed to
offer efficient propulsion and
maneuvering on higher speed
vessels (Typically 20-25knots).
mountable thrusters : A
robust, heavy-duty L-drive
azimuth thruster specifically
designed for extended and
reliable operation on offshore
rigs and drill ships.