Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Hydrodynamics in Advanced Sailing Design
Hydrodynamics in Advanced Sailing Design
Pages 635-660
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From page 635... ...
Existing procedures in areas including wave resistance, lift and induced drag of appendages, and resistance due to sea waves are reviewed, and their roles in the design process are evaluated. New procedures and results in the areas of viscous flow over appendages, thrust due to unsteady motions and sail aerodynamics with influence of the hull beneath the sails are given.
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From page 636... ...
The second category is the applied hydrodynamic research on sailing vessels stimulated by competition in the America's Cup yacht races. These events, typically held once every 3 to 5 years, have attracted extraordinary and unique international interest and intense competition.
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From page 637... ...
angle. For prescribed values of the wind speed, Vie, and the sailing angle AT, all six terms in equations 2 depend on the boat speed, the heel angle and the leeway angle.
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From page 638... ...
Although it is the most common information source, such data suffers from being in a form which makes it impossible to separate hull hydrodynamics, sail aerodynamics and sailing ability of the crew. At its best, initial data on the relation between hull geometry and the hydrodynamic aspects of performance come from towing tank model tests, supplemented by numerical results on the added resistance in sea waves as it is very difficult to determine this experimentally with sufficient accuracy.
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From page 639... ...
At very low wind speeds the appendage friction is largest component after hull friction. The boat speed is low enough in these conditions for the residuary resistance to be considerably smaller than the appendage friction, and the sea waves are small enough for the added resistance to be small as well.
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From page 640... ...
nation of boat speed, sail side force, and sea waves makes the residuary resistance, resistance due to heel and side force, and added resistance similar and large enough for the fraction due to appendage friction to be somewhat less. This exercise is one demonstration of the requirement for using velocity prediction programs.
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From page 641... ...
5 Towing Tank Testing A complete consideration of the towing tank testing of model scale sailing vessels will not be presented here as it is well covered in the literature (c.f. Van Oosannen, 1993 and Milgram, 1993~.
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The aforementioned statements about determining the wavemaking drag from "wake cut height measurements" and from using numerical computation apply here too. Without either a robust numerical tool or an experimental "data base" for friction drag, towing tank 642
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From page 643... ...
Development of numerical methods for calculating hull friction is currently being pursued as an area of research. Some RANS codes show promise and extension to free surface flows of the integral boundary layer equation method described subsequently for appendages is an exciting possibility.
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The objective is to find a solution for M and the other boundary layer parameters that satisfy the integral boundary layer equations: AS + (2 + H)
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From page 645... ...
Hence, as the boundary layer is altered toward correctness, iteration by iteration, the outer flow which generates the pressure distribution that drives the boundary layer is simultaneously altered toward correctness. Figure 7 shows drag coefficient vs.
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From page 646... ...
For example, with a one-third scale model of a 20 meter long vessel, if the frictional resistance is in error by 4070 due to an error in estimated form factor, the full scale predicted resistance is in error by less than 1% at typical sailing speeds. 6.3.1 Numerical Methods The most attractive possibility for the use of numerical hydrodynamics in minimizing residuary resistance is computation of the wavemaking resistance for differing hull geometries.
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From page 647... ...
Both the basis double body flow and the perturbation wave flow are determined by a source or source and dipole based panel method using the Rankine source Green function. Two notable extensions and applications of Dawson's method to sailing vessel hulls are those of Rosen et al.
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,~ 10 BOAT SPEED (KNOTS) Figure 14: Total Resistance Drag Areas for Two IACC Yachts with a Heel Angle of 20 Degrees as Predicted from Towing Tank Model Test Data.
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However, there does not appear to be any major roadblock that would stop extension of the time domain panel methods for wave resistance to the nonlinear case. Section 6.2 describes the ongoing work related to coupling inviscid panel methods with the integral boundary layer equations for determining forces on three-dimensional objects in the absence of a free surface.
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From page 650... ...
However, the ratio of the induced drag coefficient to the square of the lift coefficient is the same for both methods and that is the quantity which mea sures the induced drag efficiency of a lift-producing three-dimensional object. Now, two examples of the use of the vortex lattice code in support of design are given.
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From page 651... ...
Making the keel small at the bottom, to the extent permitted by structural considerations, minimizes the friction drag due to reduced wetted surface, but increases the induced drag unless some other means is used 651
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From page 652... ...
Figure 17 shows the added resistance computed by the panel methods and equation 30 for two IACC yachts. One is a "base boat" and the other has the waterline beams increased by loo and the canoe body depth decreased by 10% so as to maintain displacement.
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From page 653... ...
6.6 Mean Forward Thrust Due to Unsteady Motions An intriguing subject is the potential for dynamic thrust, which is a mean forward thrust due to the flow on appendages associated with unsteady vessel motions and sea waves. On traditional vessels this thrust is very small, and therefore it has not received much attention in the history of sailing vessel hydrodynamics.
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From page 654... ...
S(~e ~ He (41) o Equation 38 is the fundamental equation for dynamic thrust and can be used when the viscous and parasitic drag of the thrust-producing wing are accounted for in the appendage friction drag.
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From page 655... ...
Figure 20 shows the dynamic thrust less the friction and interference drags in fully developed seas for a wind speed of 9 knots, both upwind and downwind, as functions of aspect ratio. All other quantities are the same as in the preceding example except the taper ratios are 2/3 and the wave angle for the downwind case is taken as 30 degrees from astern.
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From page 656... ...
_ IFORM AND ~ FRICTION DRAG A, DRAG UFT ~ ~ FORCE RESOLVER FORWARD FORCE HFF' FORCE HEEI JNG MOMENT 1 . - - - - ~I~A~ r Or _ ~—~ _ MODEL TEST HULL VPP _ BOAT SPEED AND APPENDAGE DATA Figure 21: Method of Evaluation of Sail Shape on Performance.
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From page 657... ...
Figure 7 shows the hull, sails and panelization for which we have done computations. The sailing conditions are for a 4.52 m/s wind speed at a height of 10 meters, a boat speed of 4.51 m/s, a heel angle of 20 degrees, and a true wind angle of 40 degrees from ahead.
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From page 658... ...
Rankine panel methods for transient free surface flows. In Proceedings, Sixth International
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From page 659... ...
Seakeeping and added resistance of lace yachts by a threedimensional panel method. In Proceedings of Eleventh Chesapeake Sailing Symposium.
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From page 660... ...
~ ~ 7 8 9 10 11 BOAT SPEED (KNOTS) Figure 12: Error in the Wave Making Resistance of an lACC Yacht Computed by Sclavounos.
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