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Keyword : seakeeping
Results 1 - 4 of 4
The Un-restrained Sailing Yacht Model Tests – A New Approach and Technology Appropriate to Modern Sailing Yacht Seakeeping
Over the recent years the Wolfson Unit has seen a greater impetus from yacht designers and their clients to quantify and compare the seakeeping qualities of their sailing yacht design choices. Modern high performance yachts, fitted with a wide range of appendages generating lift and creating large moments, provide a number of complex challenges for designers. Assessing seakeeping behaviour and performance in a seaway is, indeed, important during the design process since the motions cause unsteady effects on the yacht hydrodynamic characteristics, for instance on the lift generating capabilities of the appendages. Hence it may not be justifiable to assume during the optimisation process that the yacht outperforming other design candidates in calm water would also perform well in waves. Therefore, the Wolfson Unit developed an innovative experimental model testing approach that would be an improvement over existing methods, simulating the 6 degrees of freedom motions and accelerations. The unique un-restrained sailing test approach uses a mast mounted air screw device to simulate the aerodynamic propulsion from the sails allowing a scaled model of the yacht to be tested at a range of conditions, sea states and wave directions (from head seas to following waves). These tests can be used as a comparative tool to assess controllability and seakeeping characteristics of multiple configurations (e.g. hull shape, appendages, inertias) by quantifying induced motions and providing an estimate of added resistance in waves. Non-linear attitudes such as surfing can be investigated. Free-running model testing is a technique frequently used in the development of power vessels, but little adoption is made in the sailing yacht world. Furthermore dynamic and seakeeping studies are at present challenging for computational fluid dynamics based tools, encouraging an experimental based approach. This paper introduces an un-restrained model testing method on sailing yachts. Discussion will also be made on how this new method can be implemented in design decisions and add value during the design and performance evaluation process for sailing yachts.
A simplified Method to Assess Acceleration loads on Sailing Yacht Masts
The behaviour of sailing boats in open sea is strictly related to their hydro and aerodynamic performances and to the wide range of loads acting on the hull and rigging system. Their evaluation could be done only by a careful seakeeping analysis with particular attention to the acceleration loads caused by hull motions which can create severe problems to mast and rigging up to extreme consequences such as dismasting. The main reasons of dismasting are related both to human errors and to the lack of load knowledge; as a matter of fact Classification Societies' Rules are quite poor about this subject and the structural design if often committed to the designer experience. The aim of this work is to investigate on the hull dynamic responses which mainly influence the mast and rigging loads with particular attention focused on the pitching behaviour of the vessel. With this goal in mind the seakeeping behaviour of a number of sailing yachts, different each other in sizes and typology, has been investigated. Despite the small size of the database, the achieved results allowed to formulate a preliminary simplified method to estimate the pitch Ratio Amplitude Operator (RAO), based only on the boat length. From the pitch RAO knowledge a very rough and quick formulation to evaluate the longitudinal acceleration in the mast centre of gravity has been obtained.
Seakeeping and Added Resistance of IACC Yachts by a Three-Dimensional Panel Method
A three-dimensional panel method developed for the
prediction of the seakeeping properties of conventional
ships has been extended to predict the motions and
added resistance of IACC Yachts. The method solves
the three dimensional unsteady potential flow around
the yacht in monochromatic oblique waves. Predicted
quantities include the heave and pitch motion amplitudes
and phases and added resistance over a broad
range of wave frequencies yacht speeds. Computations
have been carried out for a series of IACC hull shapes
studied by PACT (Partnership for America's Cup Technology)
and correlations with experimental measurements
are found to be very satisfactory. The same
method was also used to study the added-resistance
properties of hull shapes supplied by the America3
Foundation. A sensitivity analysis was carried out of the
added resistance on the principal yacht hull shape parameters,
including the slenderness, displacement, LCBLCF
separation and pitch radius of gyration.
Waves cause undesirable forces and motions
on yachts which vary in severity from mildly uncomfortable to catastrophic, The problems
created by waves are receiving serious and
increasingly analytical attention from the naval
architects who design oceangoing ships, Many of
the physical phenomena and engineering principles
used to describe and analyze ship seakeeping
performance are applicable to the sailing yacht.
discussion and explanation of such phenomena and
principles are the subjects of this paper.