ARCHIEF
Technische Hogeschool
MODEL TESTING IN HOLLAND
Deift
by JANE BRETTA sophisticated programme for testing yacht models systematically has been going at the Technological University of Delft in the Netherlands for several
years. In the summer of 1979 I was privileged to take part in this work
as a 'student' for six weeks. The Ship Hydromechanics Laboratory at Delft
is well known for yacht model testing, but their fundamental work on the systematic series of yacht hulls deserves to be more widely known and apprec-iated by both designers and sailors interested in the shapes of their yachts. Only 5% of the department's time and money can be spent on yachts as opposed to 'real' ships, but they make maximum use of this under Professor Gerritsma,
himself a keen yachtsman. I was made very welcome during my visit and found
everyone in the department exceptionally helpful. The towing tank facilities
are very good; the smali of two tanks is used for yacht models and the
towing carriage is loaded with electronics to present the experimenter with
the appropriate data from each run.
Scale models are towed, upright and heeled, at various angles of leeway, and at several speeds to cover all practical sailing conditions. The resistance
to forward motion and the sideforce on the hull at two positions are meas-ured with sensitive straingauges. The data of these hull forces can be
combined
with a
knowledgeof
sailforces to simulate sailing conditions, e.g.to predict speed-made-good to windward and angle of heel under given wind
conditions; by computer, many combinations of hull forms and sailpians may
be concidered once the data are available. The sailforces are estimated
using coefficients derived from full-scale, and corresponding model, tests on the American yachts 'Gimcrack' and 'Baybea' and the Dutch 'Standfast', equating aerodynamic and hydrodynamic forces in the equilibrium sailing
If hull resistance data, and thence performance predictions assuming a standard sailplan, are obtained for a series of hull forms, the series having systematically varied design parameters, the relative effects of the
parameters and their interaction can be analysed. This is the aim behind
the 'Sysser' (SYStematic SERies) project in progress at Deift. The project
was started in conjuntion with MIT,whose interest was particularly in the application of the results to rating rules, and the results of the first nine models were presented at the HISWA Symposium of Yacht Architecture in 1977. The entire series of 22 models has now been tested in the tank ( I tested
model 'sysser 20' last August ) and when the results have been collated and analysed they will also be published "to provide the designer with basic
hydrodynamic design knowledge and performance estimation methods".
The parent model of the series, Sysser 1, closely resembles the 'Standfast 43',
a moderate and clean-lined form designed by Frans Maas in 1970. The series
is generated by systematic variation of the following design parameters: length/displacement ratio, prismatic coefficient and longitudinal position
of Lhe centre of buoyancy; waterline beam/hull depth and LWL/ waterline beam
ratios also have a reasonable spread. In some of the hulls, values of the
parameters are taken to extremes to give a suitable spread of results, and
some of the combinations produce queer-looking beasts. The lines drawings
are produced by the computer with impressive fairness and detail, and complete
with hydrostatic curves, the variations being accomplished by means of
geometric factors. The model with which I mainly worked, Sysser 20, had the
same displacement and beam as the parent, but was deeper-hulled, with finer
The models themselves have a waterline length of l.6m, which gives an
over-all length of about 7ft. This is a model size which, from experience at
Delft, gives an adequate guarantee of consistent results, and has been scaled up in the analysis to yachts of 1Dm waterline length, a linear scale of 6.25. Each model has exactly the same fin keel and rudder fitted. Turbulence
stimulators are applied to the hull and keel near the forward end, to
pro-duce the same sort of flow round the model as round the full-scale yacht; strips of carburundum particles are used for this.
Each model is weighed, suitably ballasted, and trimmed afloat before the tests
begin. Then the height of the centre of gravity of th model is found by
an 'inclining test', and the difference between this position and that of the full-scale yacht's centre of gravity (as calculated from a rule-of-thumb
formula depending on hull depth) must be compensated for by a movable weight
in the model. The model is then placed in the tank and attached to the
towing carriage. The speed of the carriage, and hence the model, is
care-fully controlled and most runs are done at speeds which scale up to about
4-8 knots.
Firstly the upright resistance of the model is measured over a full range of
speeds, to produce a graph like that in Fig.2. Then heel is induced by
turning the model slightly from the parallel to give her a leeway angle and
to produce the hydrodynarnic forces which heel her once she is moving.
Meas-urements of the sideforces, forward resistance and leeway angle are taken at
certain speeds, at heel angles of 100, 200 and 30°. Obviously not all, if
any, of the combinations of heel, speed and leeway correspond to real wind-ward sailing conditions, but the computer uses all the data to find the equilibrium sailing conditions at each angle of heel, and thence a curve of
analysis are standard for the series, the mast height depending on the stability
0
at 30 of each hull.
Thus, when the computer is fed with the resistance data from the tank, dimen-sions of the hull and sailpian, plus some necessary factors and coefficients, it will produce performance predictions for downwind and windward sailing in
smooth water for each of the Sysser hulls. Taking the results of Sysser 20
as an example, a comparison with the predictions for the parent hull shows that, downwind, Sysser 20 will perform less well at higher wind speeds (about
3% slower than Sysser 1). This difference corresponds to the difference in
resistance due to wavemaking of the hulls: Sysser 20, with her transition
from a deep, moderately beamy mid-section to fine ends, produced some
interest-ingly large waves at high speeds, and her upright resistance curve (Fig.2)
rises earlier and more steeply than that of Sysser 1. To windward, the
per-formance predictions for Syssers 1 and 20 give very similar curves for
speed-made-good, but probably for different reasons. Much deeper analysis of the
data and comparison with the other models is needed to relate these results
to the design differences.
The computer also generates stability curves for all the models, up to 900
of heel, assuming the yachts to be flush-decked. These curves have assumed
a new significance for designers in the light of the Fastnet Race 1979, in which many yachts were knocked down and several rolled over through 3600.
Between 90°and 180°, the superstructure plays a significant part in the
stab-ility, so a coachroof would change the whole picture; also, the stability
curve represents the situation in static conditions in smooth water and is of dubious relevance to the highly dynamic conditions in very large seas. How-ever, it is certainly better to be able to relate hull characteristics to
To the academic hydrodynamicist, the Sysser project comprises a neat,
analyt-ical necessity to the study of yacht design. Systematic studies of
resis-tance of merchant and naval ships' hulls have been well known and used since the Taylor Series was published in 1933, and the analysis of a similar series
for yachts is obviously a logical and overdue project. It is hoped that it
will be followed by similar studies conducted in waves, so that a start can be made on analysing those features which make a yacht perform well in rough water and make her sea-kindly.
But how is the Sysser project relevant to the practical designers' work?
It will provide "some basic knowledge", and they will be able to supplement
their intuitive feelings with figures. 'Sailing performance' of course
dep-ends on more than the hull's resistance and sideforce production curves
-the yacht's behaviour in a seaway, and -the combined effect of deck layout, rig, sailplan, gear and crew (all but the last of which are in part the designer's
responsibility) contribute vitally. It is the ideal combination of these
factors for which we are searching for optimum performance. Good
perform-ance is not the only priority. Every design seems to be the product of an
inspired juggling act as one strives to reconcile apparently conflicting needs into the "fortunate combination of proportions and detail" - Olin Stephens'
description of a successful design. Hull balance, stability under both normal
and extreme conditions, sea-kindliness, accommodation, cost of production and performance must all be weighed in the balance to the required propor'ions; to some extent, design features are traded off against each other to this end, by means of experienced comparison with other yachts and a certain amount of
'eye', 'feeling' or let us just call it 'Art'.
The Sysser results and performance predictions will add another dimension to the designers' methods by giving them some figures to support their
judgements. Designers will be able to use experience and intuition side by
side with scientific analysis as a matter of course, the latter confirming
and clarifying the former. This does not remove the Art; it just makes
it a little less mysterious.
It is hoped that the yacht owner and sailor will benefit from this kind of research by having more efficiently designed boats to sail. Perhaps more
importantly, as it becomes a realistic possibility that the relationship between a yacht's shape and her performance will be in part describable in terms of her main proportions, the prospective owner will be able to make a more optimistic assault on the often overwhelming task of clear thought
about the hull form of his ideal boat. There is undoubtedly a fascination
in the attempt to understand these complex relationships, a fascination
which will continue as long as people go sailing. The application of science
to the problem will only help if we ask the right questions, and use the answers with a full awareness of their limitations and a good pinch of
experience; it will never provide the entire solution, the 'perfect yacht',
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