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CHAPTER 1
INTRODUCTION
1.1 INTRODUCTION
The shape of the sailing hull has evolved over the years from the
first working boats to the present high performance racing yachts.
The hulls of the first working vessels were designed to perform their
working tasks, they were developed from a trial and error approach
which meant that in most circumstances the vessels would become
better each time a new vessel was produced.
Changes started to occur when people started feeling the need for
speed and competitiveness, and therefore racing was born. As soon as
racing started the rating rules or class design rules were formed to
try to introduce a form of fairness. It was these rules which then
determined the direction that the racing yacht hull development would
take. It is obvious that the rating rules can cause a negative effect
on the evolution of the hull shape of the racing yachts, as they may
inadvertently cause the development of the hull to move towards hull
forms which are either inefficient or altogether dangerous. This
situation was experienced by the IOR rule introduced in 1970 ref. {1}
which produced the unseaworthy yachts involved in the Fastnet
disaster. It must however be noted that this is not always the case
as some rating rules may force the designer to produce safer or more
efficient hull forms.
The hull form currently being used by modern racing yachts is the
skimmer dish hull. The idea behind the hull form is to take advantage
of the planing hull form and also increase the stability of the
vessel so more sail can be carried. Recently this hull form has been
exaggerated to vessels with very low length to beam ratios of around
1:3. It is possible to say that this line of development is correct
and should continue as some rules seem to be biased to this type of
hull form.
However there is a class of racing yachts which are using hull
forms at the other end of the spectrum, these yachts are model racing
yachts. The class design rules used for models are quite simple
compared to their full size counterparts which enable designers to
experiment more which is also helped by the minimal cost required to
build one of these models. The hull forms currently being used by
these yachts are super slender and are becoming more slender every
year. The length to beam ratio’s of these yachts are around 1:11 and
greater in some cases. If we trace the development of model yachts
back over the years you begin to see that they are about five to ten
years ahead of their full size counterparts. If we look at the hull
forms used by model yachts in the 1980’s it can be seen that skimmer
dish hull forms were used but they were soon replaced by slender
yacht hulls.
It should also be noted that full size yachts have used these
slender hull forms in the past. A good example of one of these yachts
was Evolution designed by E. H. Bentall in 1880, this boat had a LWL
of 50.75 ft and a beam of only 6.4 ft. This yacht was designed to a
rating rules which eventually developed into the current IMS rating
rule being used today. Slender yachts were abandoned due to the loss
of some yachts and their crews, and these types of yacht were deemed
unsafe. If you look at the rating rule which these yachts were
designed to you soon see that the sail area carried by the yacht was
neglected and no limits were set. This would produce yachts that were
overpowered i.e. producing very high heel and pitching moments, and
also dangerous resulting in them being classed as unsafe. However
rules have come along way since then and sail area is now considered
in both full size and model class design rating rules. A possible
reason for this current trend in full size yacht design could be put
down to the cost required to experiment with a new hull shape, but
another reason is possibly current trends. If a yacht with a wide
beam wins a race and is largely publicised, there is a good chance
that a large proportion of new hull designs will have a large beam.
The best example of this type of trend is the wing keel used by
Australia II which won the America’s Cup in 1983. This boat attracted
a large amount of media attention and the performance advantage over
the American entry was put down to the wing keel. A short time after
the race wing keels were put on a large proportion of the new
production yachts. Unfortunately the performance advantage that
Australia II exhibited was only partly down to the wing keel, C. A.
Marchaj ref. [1] shows that a large amount of the gain in performance
was down to a redesign of the hull of the yacht. This illustrates
that yacht design is not purely governed by rules and technical
knowledge of the subject but also fashions or trends in the industry
at the present time.
Models yachts have the potential to be an area where the full size
yacht designers can look for new ideas and possible new development
paths. Due to their relative low cost and short construction and
modification times, the model yacht designers can experiment with new
or radical ideas very quickly, compared to full size yacht designers.
The current trend in model yachts is that of super slender hull
forms, the yachts using these hull forms have shown superior
performance compared with the wider model hull forms, so it would
seem stupid to ignore this significant development. This project will
look at the feasibility of applying the current model yacht hull form
i.e. the super slender hull, and investigate the performance of full
size yachts if they were to use this hull form. The project will
identify critical design areas and examine the effects of the slender
hull form on these areas.
1.2 AIMS
The main aim of this project is to investigate the feasibility of
taking a model hull form and developing it to full size vessels. To
do this the project will look at
- Current trends in model hull forms
- The advantages and disadvantages that are associated with
Slender hull forms
- Ways to overcome any problems in the model hull forms
- Ways to link experiments with model hull forms to full size
racing yachts
- Applications for full size slender yachts
1.3 REVIEW ON MODEL YACHT DEVELOPMENT
Model Yacht development has followed a slightly different path
than full size yacht development. The first model yachts were scaled
down copies of the full size racing yachts, as soon as model yacht
racing started the models began to follow their own development path.
One of the first model racing classes was the 10-rater class that
dates back to 1878 in England, and adopted a simple rule relating
sail area to waterline length. As soon as more people became
interested in the models the development potential of model racing
yachts became apparent. The cost required to experiment with new
ideas was minuscule compared with that of a full size vessel. It soon
became apparent that the model yachts were over taking the full size
yachts in development of hull and appendage shape. The model yachts
were the first to adopt a separate keel and rudder and move away from
the traditional long keel hull form. As more people became interested
in model yachts new racing classes were developed such as the A class
and the Marblehead. With these new classes using rules which were
less restricting and had little relation to full size yacht design
rules, the model yachts were able to evolve into more efficient hull
forms considerably quicker than their full size counterparts. Unlike
full-size rating or class design rules these rarely change for
models, and when they do they are generally very small changes.
Unlike in full-size yacht development where the yachts have varied
when the rules have changed the models have generally been left to
develop or evolve in their open class designs. This has allowed the
designers to experiment with different aspects of the design such as
beam, displacement, draught and sail shapes.
Once the models started to develop away from the full-size yachts
they began to change their shape considerably. The most noticeable
shape change was the change to completely segregated rudders and
keels unlike the long keels used by full size yachts. Variations of
keel and hull shapes could be combined easily on the models, and hull
and keel design split to become separate areas. Model yacht designers
could then produce a single keel and rudder and use them on various
hull forms to assess the potential of the hulls without a
considerable increase in cost. Round bilge and chine hulls were
experimented with and extreme variations in some of the vessels
dimensions were tried to locate performance advantages.
One of the factors that limited the hull form used by the models
was the control method adopted. Most of the first racing classes used
vane control as radio control was still under developed and was only
available to the rich. Since the yachts were raced on straight line
courses the hulls were developed with directional stability in mind.
This produced hulls that were long by modern standards. This could be
seen clearly in the 10 Rater class which has a rule relating
waterline length to sail area, with the longer the waterline length
the smaller the amount of sail can be carried. This rule when linked
with the long hull forms being used produced yachts which had small
sail areas. It was not until the availability of cheaper radio sets
and the use of triangular racing courses, that the hull forms were
modified and the waterline length were reduced to allow better
tacking ability, and the sail areas were increased.
The main limiting factor which influenced the models was the
materials which were available for construction. The first hulls were
planked in wood and could be quite heavy and required a large
displacement if a reasonable ballast ratio was to be obtained. This
restriction tended to restrict the experiments with slender hull
forms and it was not until the 60’s when fibre glass became readily
available that the weight of the hull could be reduced. Model
designers were now able to reduce the displacements of the yachts,
but the wide beams were still kept as stability was still seen to be
a problem. As the use of fibre glass advanced and lighter cloth and
epoxy resin became available, the ballast ratio’s of the models
increased and the yachts were relying on the length of keel and
amount of ballast, more than the hull form to provide stability.
When the exotic FRP’s became available such as Carbon and Kevlar
model yacht design changed once again. Aluminium masts were now
replaced by Carbon Fibre which allowed increased height and higher
aspect ratio sails to be used. Keels could also be increased in
length by reinforcing them with Kevlar or Carbon, this linked with
the lower hull weights meant higher ballast ratios could be obtained
and therefore stability could be provided entirely by the keel and
less was required from the hull. Some model designers started to
reduce the beam of the yachts but others decided to reduce to canoe
body draught of the hull. This split could be seen in the late 1980’s
when designers were producing vessels with low beams ( although not
yet in the super slender region ) while others were producing yachts
with the skimmer dish hull forms to try to take advantage of the
planing properties of that hull form. A good example of one of the
skimmer dish hull forms is a yacht which won the world champion ships
designed by Janusz Walicki called Scalpel shown in Figure 1.1,
which has a length to beam ratio of approximately 1:4. This vessel
showed good performance in normal conditions but had problems in
higher wind conditions when interaction between the wind and the
large flat deck would cause steering problems. It was not until the
early 1990’s that yachts fitting into the super slender class were
first seen, the most noticeable was a yacht designed by Canadian Bob
Stern called Monocat which had a length to beam ratio of 1:10.
The performance of these slender hulled yachts was considerably
better than those before them and the slender hull was soon adopted
by most model racing yachts. At the present day June 1996 it is very
rare to see a model yacht with a wide beam at any top model racing
venue and vessels with length to beam ratios of 1:13 have been
produced.
Figure 1.1, Radio
Marblehead Scalpel
1.4 FULL SIZE YACHT DEVELOPMENT
Evolution of yachts specifically for the use of pleasure and
racing has only really occurred recently in the latter period of the
nineteenth century. The first vessels that were used for racing and
pleasure sailing were converted working boats. The reason for the use
of working boats was that they were tried and tested technology, and
had been developed over the years with the trial and error method as
very little was known about hydrodynamics and aerodynamics. The first
boats used were converted pilot boats which were designed for speed
as this was a requirement for the job they were used for.
Most of the first top racing yachts were massive schooners and
since there were no time allowances for smaller vessels these large
vessels owned by wealthy people would dominate race meetings. Yacht
development was following different paths around the world with the
most noticeable difference being between America and England. The
English design theory was to have a reasonably slender hull form and
a fixed keel, the Americans on the other hand were designing vessels
with lifting keels or centreboards and wider hulls with low draught.
The difference in performance of the two types of hull form were
illustrated when the America came to England to race against
the top English yachts. The hull forms were dramatically different,
and also the considerable size difference between the vessels also
played a part as the America was much larger than the biggest
English boat, and no time allowance was given to the smaller yachts,
needless to say the America won the race by a considerable
difference. This example illustrated the need for rules which would
compensate for differences in size or dimensions. It is fare to say
that it is not only the size of America that gave her the
advantage, as she also had superior sail and rig design. Even with
this decisive win to the American yacht the English still kept to
their same design and many of the America’s Cup losses can be put
down to this.
As rating rules began to influence the design of the yachts the
two countries went in two different directions. When a rating rule or
a class design rule is introduced it serves the purpose of trying to
prevent any of the boats in a certain race having a major advantage
over the other boats, and therefore means that the sailing skills of
the sailors are the main contribution to winning or losing. This
however is rarely the case as this is only possible if all the boats
are identical in every way i.e. a one design class. The reason for
this is that a designer is always trying to design the fastest boat
for a given rule. When this is taken to its extremes it is possible
to refer to it as a rule cheater. This does not mean that the rule is
broken, it means that the type of boat produced may have extreme
dimensions. The best example of this was the revised BOM formula in
1855. The idea behind the rule was for the length of the yachts was
to be limited by linking the length of the hull to its beam i.e. the
wider the beam the shorter the boat for the same rating. This rule
looks good on paper but in practice caused some interesting hull
forms to be produced. The designers were unwilling to reduce the
length of the boats and in some cases the length was increased, this
meant that the beam of the yachts was reduced considerably. To get
over the loss of stability due to the narrow beam the total weight
was increased so more lead could be used as ballast. A good example
of this rule is the Evolution designed by E. H. Bentall in
1880, this boat had a LWL of 15.47m and a beam of only 1.95m. These
types of boats with unlimited sail area and reduced beam were
considerably overpowered and the loss of the 5 tonner Oona in
1886 proved to be the indication that a change in the rules should be
made so that sail area could be taken into account. The Americans on
the other hand were moving to vessels with large beams and low
draught as they had found this gave good speed. These vessels were
very stable but their vanishing angle was very low and a number of
lives were lost when the yachts were caught in squalls ref. [2].
The two different types of yacht would meet in the America’s Cup
and it soon became apparent that the English yachts were better in
strong winds while the American designs were best suited to light
wind conditions. This difference is shown in Figure 1.2, with the top
vessel being of English origin called Irex while the lower
vessel is American called Gracie, both yachts are of the
1880’s period.
Figure 1.2, Madge
Also playing a part in the development of the yachts was the rig
design, designers were moving away from Schooners in 1880 to Cutter
rigs and smaller yachts. Previously the race winning yachts had been
massive at around 100ft but they had slowly been reducing in length.
One of the more successful English yachts was Madge who raced
against a number of American yachts. Madge was not at the
extreme end of English design and showed good performance by winning
all but one of the races that she was entered in. This shows that a
compromise between the two types of yacht would probably produce a
yacht with better performance. Unfortunately there is always a
resistance to change, and tradition tends to be accepted as being
correct no matter what the evidence shows.
Sail area was still unlimited and it was only in 1887 that the
rating rules started to consider sail area. The addition of sail area
and the elimination of restricting the length due to the beam changed
the hull form of the yachts to hulls with a wider beam, and therefore
a reduction in ballast. This moved the trend towards the other end of
the scale into the area of ultra-light displacement. The development
of the hulls moved to low displacement skimmer dish hull forms, and
formed a link with the vessels being used by the American’s. Linked
in at this time was the increase in technical knowledge which
illustrated the need to reduce wetted surface area to reduce
resistance, and also larger side areas would reduce leeway. The
extreme skimmer dish hull forms proved to be a problem in head seas
due to slamming and high rig loads. Figure 1.3 shows one of the most
extreme skimmer dish hull forms
Figure 1.3,
Independence
ever built the Independence, she was 42.9m and displaced
146.75 tons of which 80 tons was ballast in the keel, the most
staggering statistic was the amount of sail that could be set at any
one time which was 6228.0m². To illustrate the structural problems
that were being found she was built in January 1901 and she had to be
scrapped by September 1901. Eventually it was seen that a compromise
between the two ends of the spectrum would produce a more seaworthy
vessel without causing a major compromise with the speed of the
vessel. Also in 1891 some of the yachts were starting to experiment
with separate keel and rudder. The new rule formed in 1896 took both
beam and sail area into consideration and also a new measurement the
Girth of the vessel. This was introduced to penalise the skimmer dish
hull forms with their "hard" bilge’s, in an attempt to make the
vessels more seaworthy. In 1919 the rule was modified and beam was
removed and freeboard added, this rule then took on International
Status. The rule continue to evolve with a compromise between speed
and seaworthiness being its main aim.
It was in the transitional period between the 1960’s and the
1970’s that something seems to have happened with relation to the
thinking behind the rules. It seems that for some reason that is not
clear in the books related to this topic that the consideration of
seaworthiness began to be neglected and the only consideration was
speed. One possible reason for this is a sudden increase in material
technology and understanding of hydrodynamics. This would have
increased the speed capacity of new vessels and may have set up the
sudden "speed fever" as it is sometimes called which would have been
impaired by rules which took account of the seaworthiness of the
vessel. This may have forced the rules to help produce these faster
craft. The best possible example of this could be the IOR
International Offshore Rule introduced in 1970, this rule differed
from the ones before it in that it was based on evaluating the speed
of the vessel not its seaworthiness. This rule produced boats which
developed towards yachts that had light displacements, increased beam
and flat bottoms i.e. skimmer dish hull forms. It was this rule which
produced the vessels involved in the disastrous 1979 Fastnet Race
(England) and also the 1982 Double-handed Farallones Race (San
Francisco) were a number of lives and vessels were lost. Many of the
inquiries blamed the rule for producing unseaworthy boats, which
shows that the rating rule can force the designers to produce unsafe
vessels.
The latest rating rule is the IMS rule or International
Measurement System is an attempt to solve the problems of the old IOR
rule, to do this the rule take advantage of current computer
modelling techniques and uses a V.P.P to predict performance and also
considers the seaworthiness of the vessel to increase safety.
Although some current feelings are that it to is producing yachts
more dangerous than the older IOR boats. The main problem that seems
to have arisen is that stability is being penalised by the rules so
sailors are demanding yachts with low stability so that they do not
have to be handicapped by the rules. The whole area of low stability
being penalised seems to stem from the fact that a very stable yacht
is a lot faster in heavy weather conditions than a yacht with low
stability. The rules therefore penalise these yachts to make the
races fair. This however does induce designers to produce yachts with
low stability as they do not wish to be penalised.
This section does concentrate mainly on the yachts which stemmed
from the rating rules. The reason for this is that this covers the
main section of yachts, there have been however other more radical
designs created for races such as the Transatlantic Race, the rule
for this race used to say that any vessel could enter as long as it
was single handed i.e. sailed by one person. The types of yachts
being sailed in the races would vary considerably, both multi-hulls
and monohulls sailed against each other. One of the most radical
vessels ever seen was entered into the race in 1972. The vessel of
French origin was called Vendredi 13 and was 39m in length and
was a three-masted schooner, her length to beam ratio was around 8.5.
More recently the rules for this race have been changed and they have
limited the maximum length to 18.29m for single handed sailing for
safety reasons.
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