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 ratioís 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 1980ís 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 Americaís 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



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 60ís 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 ratioís 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 FRPís 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 1980ís 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 1990ís 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


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 Americaís 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 Americaís 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 1880ís 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 Americanís. 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 6228.0m≤. 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" bilgeís, 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 1960ís and the 1970ís 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.




If you have any questions or comments please E-Mail:
© Copyright 1998 Anthony York