Yacht Design

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Yacht Design Technology

• Hydrodynamics

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Lecture Contents

• How is resistance determined?
• Components of resistance
• How can resistance be minimised?

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Resistance
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Determining the Resistance of a Design
1) Model testing in a towing tank
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Determining the Resistance of a Design
2) Calculation by Computational Fluid Dynamics
(CFD)
Using numerical techniques to solve equations
defining fluid flow Equations solved are
numerical approximations, hence inherent level of
approximation in solution
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Determining the Resistance of a Design

• 3) Systematic Series
• Calculation by empirical formulae - determined by
  regressional analysis of Systematic Series
• Series of towing tank test models all derived
  from one particular parent
• Change one parameter at a time and keep others
  constant
• Empirical formulation for determination of
  resistance of arbitrary shape

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Components of Calm Water Resistance
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Components of Calm Water Resistance
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Influence of Speed on Resistance
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Components of Calm Water Resistance
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Frictional Resistance

• Dependent on
• area of hull/keel/rudder in contact with water
• forward speed
• frictional coefficient

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Frictional Resistance
Total frictional resistance of yacht is
Subscripts c canoe body k keel r
rudder
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Frictional Resistance
Cf determined from experiments with flat plates,
now a standard equation, ITTC-57, is used
Note that Van Oossanen gives
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Reynolds Number

• Dependent on
• length
• forward speed
• kinematic viscosity of fluid

Remember that flow changes from laminar to
turbulent flow at around Rn 4.5x105
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Reynolds Number Canoe Body
For ships L is taken as waterline length. For
yachts this is not a realistic representation.
Therefore typically a value of L is taken between
70 90 of the waterline length. This
obviously leaves space for interpretation.
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Reynolds Number - Foils

• Average chord length used to determine Rn
• If taper ratio (difference between chord length
  at tip and root) greater than 0.6, then appendage
  divided into strips and total skin friction found
  by summing skin friction of all the strips.

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Components of Calm Water Resistance
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Viscous Resistance - Form Drag

• The actual frictional resistance of the yacht
  will differ from plat plate frictional resistance
  due to shape of hull or form, i.e. flow is 3D
  rather than 2D.

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Viscous Resistance - Form Drag

• Form Factor, k, is determined from tank tests
  using Prohaska Plot. Obtain k from CT/Cf versus
  Fn4/Cf plot
• k may also be calculated from Holtrop 1977.
• For sailing yacht k0.1
• Additional increase to viscous resistance caused
  by effects of hull surface, since ITTC-57
  accounts for smooth surface only.

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Viscous Resistance - Form Drag
Keel and rudder Form Factor may be determined
from data available in literature e.g. Hoerner
Fluid Dynamics Drag Fluid Dynamics Lift
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Components of Calm Water Resistance
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Viscous Resistance - Transom
Pressure drag caused by immersed transom is a
component of the viscous resistance
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OK, we now know what Viscous Drag is - how do we
minimise it?
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Minimise Viscous Drag

• Reduce wetted surface area
• Maintain laminar flow as far back as possible
  (use straight lines in forebody)
• Minimise form factor by ensuring that flow lines
  along hull are as straight as possible
• Straight flow obtained by adopting
• slender waterlines in low BWL/Tc
• slender/straight buttock lines in high BWL/Tc
• avoid pronounced bilges in diagonal flow

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Components of Calm Water Resistance
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Wave-Making Resistance
Wave-making resistance is associated with the
energy involved with generating the pattern of
waves seen when a vessel travels along the
surface.
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Wave-Making Resistance

• Flow along hull reduced (in relation to yacht
  speed) at bow and stern while increased at
  amidships.
• This is responsible for
• increase pressure in bow region
• decrease in pressure amidships
• increase pressure at stern

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Wave-Making Resistance
The length of the wave is a function of the wave
speed
This means that as the speed of the yacht
changes the interference between the waves
generated by significant parts of yacht hull e.g.
bow, shoulder, stern changes. hull speed is
when l LWL
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Wave-Making Resistance
Influence of yacht speed on wave length
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Wave-Making Resistance
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Wave-Making Appendage Resistance
The volume of the keel produces wave-making
resistance. To avoid abrupt changes in
lengthwise distribution of volume, keel volume
may be faired into Curve of Cross Sectional
Areas. Work by Keuning Binkhorst (Chesapeake
1997) measured forces on keel and rudder
separately from hull forces. Results clearly
showed residuary drag on the keel in upright
condition (2 5 of overall resistance).
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OK, we now know what Wave-Making Drag is - how do
we minimise it?
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Minimise Wave-Making Drag

• Design hull to have long effective waterline
  length
• Carefully distribute displacement volume along
  length
• More volume towards bow and stern, decrease XSA
  of maximum section of hull - this increases
  prismatic coefficient, Cp

• Effective wave-making length of the hull is
  increased (distance between wave peak at bow
  wave peak at stern increased)

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Components of Calm Water Resistance
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Induced Resistance

• Going to windward hull, keel and rudder develop
  side force.
• To generate side force flow requires angle of
  attack with respect to hull centreline.
• Induced resistance is directly related to side
  force generated by hull and appendages. It is
  dependent on
• wing geometry
• flow around wing tip
• aspect ratio of wing
• presence of the free surface

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Induced Resistance

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Induced Resistance Wing Geometry

• Induced resistance minimised when wing has
  elliptical load distribution over span
• Elliptical plan form is not strictly necessary
  for elliptical loading - taper ratio ct/cr0.6 is
  effective (ct tip chord cr root chord)

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Induced Resistance Wing Tip

• Induced resistance strongly related to strength
  and shape of tip vortex changes to shape of
  wing tip may influence induced resistance.
• Flow around tip, from high-pressure side to
  low-pressure side, must be restricted to minimise
  RI.
• End plate may be used to minimise tip losses.
  Hull is one end plate. Wing tips or bulbs may be
  used at other end.
• End plates bulbs however have additional
  resistance e.g. large wetted area form drag.

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Induced Resistance Aspect Ratio
Aspect ratio is ratio between wing span and the
wing area. A long slender wing has a high aspect
ratio. For high AR wing, effect of wing tip on
overall performance of wing is small. Lift/RI
increases with increasing AR
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Induced Resistance Free Surface Effect
Induced resistance effect due to pressure field
around keel being close to free surface as yacht
heels. This pressure field generates waves which
manifests itself as resistance.
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Induced Resistance Free Surface Effect
When sweep angle increased pressure field is
spread out over longer portion of free surface,
hence reducing wave generation. Has led to
development of inverse taper keels and
winglets. Interaction between pressure field
around keel and free surface may not be neglected
during keel design.
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Heeled Resistance
Forces on sails produce heeling and trimming
moments in addition to drive force for
yacht. Running trim will lead to a bow down
attitude, unless counteracted by crew
movement. This will change both the viscous and
residuary resistance
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Change of Viscous Res. due to Heel
When yacht heels underwater part of hull will
become asymmetrical and there will most likely be
a change in the wetted surface area. New wetted
area may be found from hydrostatic
calculations.
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Change of Residuary Res. due to Heel

• This is more significant than change in viscous
  resistance due to heel.
• When yacht heels there will be a change in the
  distribution of of the cross sectional areas over
  the length of the yacht.
• Depending on hull geometry this will lead to
  change in hull shape parameters
• waterline length
• waterline beam
• canoe body depth
• LCB may lead to change in trim (bow down as
  LCB moves aft)

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Change of Residuary Res. due to Heel

• Most influential are B/T ratio and LCB.
• Hullform with increased B/T ratio tends to have
  greater increase in residuary resistance when
  heeled.
• Trimming effect can significantly increase
  resistance by 10-15 at high speeds.

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Hydrodynamic Side Force
Ability of yacht to sail close to wind and
achieve good VMG is mainly dependent on ability
of hull, keel and rudder to develop substantial
side force without significant resistance. Side
force produced when hull has yaw or leeway angle
relative to track of yacht through water. Yachts
with good windward performance can generate high
side force at small leeway angle, whereby leeway
is reduced.
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Hydrodynamic Side Force

• Keel rudder must be symmetrical this limits
  lift to drag ratio to the order of 10.
• (Non-symmetrical cambered wing sections can have
  lift to drag ratios of 30 for small angles of
  attack).
• Flaps may be used on trailing edge to increase
  lift, though drag penalty also present.
• Canoe body is inefficient producer of side force
  with max L/D ratio about 5 6 at low speed and 2
  3 at high speed.
• Large Keel
• high sideforce, large wetted area, VB low, bTW
  small
• Sails high and slow
• Small Keel
• low sideforce, small wetted area, VB high, bTW
  large
• Sails low and fast

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Side ForceAspect RatioLift increases with
angle of attack until flow separates from foil
and it stalls.High AR wing more effective at
producing lift.High AR wing generate high lift
at small angles of attack stall very soon.
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Side Force Aspect Ratio

• High Aspect Ratio
• High lift production
• Small leeway angles
• Minimal induced drag
• Reduction in WSA lowers frictional resistance
• Drawbacks
• After tack large angle of attack and wing may
  stall
• Water depth
• Structural implications
• In waves angle of attack varies considerably due
  to motions, also lower speed, hence wing may
  stall.

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Side Force Section Profile

• Thickness Ratio (max. section thickness/chord)
• Greater thickness increase max lift.
• Slightly higher resistance.
• Thicker foils less sensitive to stall than
  thinner foils.
• Longitudinal position along chord length of max.
  thickness
• Determines extent of laminar flow on foil.
• Move position aft laminar flow may be
  promoted.
• Too far aft and boundary layer will separate at
  low lift coefficients.
• Good reference Theory of Wing Sections Abbott
  von Doenhoff

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Hull Form Lines Development

• It is recommended that every Naval Architect
  draws a lines plan by hand at some stage in
  their career.
• Slow work but gives excellent appreciation of the
  process of simultaneously drawing 3 fair
  orthogonal views.
• Possible Technique
• Work with parameters length, displacement, beam
  waterline, Cp and LCB.
• Draw profile
• Draw maximum section shape
• Examine Sectional Area curve
• Adjust for displacement using selected Cp

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Hull Form Influences

• Class Exercise - Try and define a possible
  influence of the
• following conditions or hull form parameters
• Heel
• Bow type
• Flared topsides
• Displacement
• Cp
• LCB

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Hull Form Design for Heel
Boats spend large sailing time at heel. Tend to
trim bow down as they heel - aft shift in LCB
sail force trimming moment. Need to consider
heeled lines as much as upright lines.
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Hull Form Bow Type

• Cruising yachts styling, flare forward, shape of
  deck edge in plan view, sea conditions.
• Racing yachts Rating rule
• IMS system gives fine forward waterlines
  vertical stem profile.
• IACC rule measurement at waterline overhanging
  bow encouraged (Meter bows).

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Hull Form Flared Topsides
Greater asymmetry results in greater drag at
heel. Flared topsides (high B to Bwl ratio)
create asymmetry. Deck beam important for
crew-righting moment water ballast.
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Hull Form Displacement
Determines general character of boat. High
L/disp tends to give increased beam-draft ratio
since will derive stability from form rather than
ballast.
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Hull Form Prismatic Coefficient
Typically Cp optimised for Fn 0.33-0.35 (upwind
sailing for racing yacht in medium winds) Cp
typically vary from 0.52 to 0.56
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Hull Form LCB
LCB typically range between 3 6 aft of
amidships. LCB towards gives fine bow less
added resistance may be appropriate for planing
at higher speeds. However may nose dive in large
waves trim bow down with heel.
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Delft Systematic Yacht Hull Series
Series established in 1974 by Gerritsma et al. at
Delft university of Technology in order to derive
empirical expressions for hydrodynamic forces on
sailing yachts. Over 50 systematically varied
models tested upright, heeled and yawed at
various speeds. Parent hullforms have evolved as
yacht design has developed
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Delft Systematic Yacht Hull Series
The following hull parameters were chosen
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Delft Systematic Yacht Hull Series
The total resistance is calculated from the
addition of the residuary resistance and
frictional resistance (ITTC-57) The residuary
resistance is determined analytically from the
individual contributions made by the hull and the
keel. Using the polynomial equation with hull
form geometry coefficients as variables the hull
residuary resistance may be determined from
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Delft Systematic Yacht Hull Series

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Delft Systematic Yacht Hull Series

• A similar procedure is utilised for determining
• Appendage resistance
• Induced resistance
• Hydrodynamic side force

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Delft Systematic Yacht Hull Series
Good Reference for Delft Series Keuning, J.A.
Sonnenberg, U.B. Approximation of the Calm
Water Resistance on a Sailing Yacht Based on the
Delft Systematic Yacht Hull Series 14th
Chesapeake Sailing Yacht Symposium, January
1999.
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Recap/reflect

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