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The design of an offshore sailing catamaran

by Colin Leath

The purpose of this project was to learn about offshore sailing catamarans and their construction by designing one. This project involved finding a boat design software package that would make this task possible, and learning how to use it. It required researching of special features unique to large catamarans, and an understanding of why they are used. Finally, the boat software design file had to be converted to a CAD file to complete the design and animate it.

This project was partially successful. The basic design of the catamaran was completed, but it was not possible to complete it to the detail that would be necessary for actual construction of the boat. The accommodation space has only been minimally thought out, and the arrangement of the running rigging has not yet been designed. There was no time to experiment with the boat design software to see if it was capable of performing a stability analysis and other analyses on a catamaran. However, the main objective of the project was achieved, the author did learn about the computerized design of offshore catamarans, and he gained a more intimate knowledge of their special features.

This project began about two years ago when the author was exploring the world of high-performance sailing. This involved the study of hydrofoils, windsurfers, multihulls and a strange sailing vessel which instead of a sail used a large rotating cylinder. The author finally chose to design an offshore sailing catamaran because of the possibility that he might someday want to build one and explore the oceans in it or start a charter business that would sail windsurfers to exotic and windy locations. In addition, the catamaran is finally being recognized as the optimum hull design for almost all aquatic water craft by both the commercial and recreational boating communities.

A catamaran is a twin hulled boat. The two hulls are called by their traditional Polynesian name, "ama", and are connected by two or more beams. The catamaran was first developed in the south pacific by the natives who made their living from the sea, and wanted greater stability in the open ocean than was offered by a simple canoe. The catamaran design also made it possible to place a deck in between the two hulls that greatly increased the room on the boat. The early catamarans were used to transport the royalty of the natives. The king would sit in luxury on the platform between the hulls while his servants would sit in the hulls and paddle. When the Europeans first witnessed the catamaran, they were impressed by its great speed, but the design failed to catch on for several reasons.

The Europeans needed to transport huge amounts of cargo. It would have required revolutionary thinking and a large purse to finance the research necessary to reproduce the south pacific catamaran on a such a large scale. However, a few innovators did experiment with the catamaran for recreational use. One famous yacht designer, Herreschoff, designed and built a catamaran which was raced against the best monohulls back in 1876. The New York Yacht Club proceeded to ban the boat from racing because it was so much faster than its much larger and more expensive competitors.

Only in the 1960's, did catamarans begin to return to from exile. One of the earliest people to try modernizing this ancient design was Rudy Choy. He created the first cruising catamaran on the island of Hawaii. Not only could he sail much faster than anyone else, but he could steer his boat through the waves and right up to the beach. Finally, in the 1990's, catamarans are beginning to enjoy the acceptance they deserve. The record breaking exploits of catamarans that have been sailed around the world in less than 80 days (Commodore Explorer, Enza), have helped, as have the efforts of pioneers in the design of catamarans such as Jim Brown, Dick Newick, James Warrham, and Chris White. The old prejudices against the economic and speedy catamaran are finally being replaced by admiration for the capabilities of this design.

There are so many advantages to the twin hulled design, that it would be best to first relate its disadvantages, so they are not overlooked as you skim the next few pages. There is also some confusion as to the difference between a trimaran and a catamaran, and some disagreement as to which is better. Catamarans and trimarans are both multihulls, as opposed to the traditional monohulled sailboats. All multihulls have a basic advantage over monohulls: They do not need a weighted keel or centerboard to keep them from tipping over. The multihull derives its stability from its wide beam. In order to tip a multihull over, the force of the wind on the sails must overcome the upward force of the leeward ama and the downwind force of the upwind ama. To tip over a monohull the force of the wind on the sail only has to overcome the force of weight on the windward side of the hull, and the downward force of the weighted centerboard or keel. The multihulls advantages over monohulls are all derived from their wide beam and lack of a lead weight.

The disadvantages of a multihull are also derived from those characteristics. First of all, to create the wide beam of the multihull, it is necessary to design and build more than one hull. This tends to make the construction of a multihull more time consuming and more expensive than the construction of a monohull of similar size. In addition, because of the wide beam of a multihull, it has traditionally been more expensive to pay for their slip at the marina. Most slips that are wide enough for a catamaran were designed for a monohull of a much greater length. However, as public opinion changes, more and more slips will be designed for multihulls and their slip fees should decrease. There are several other characteristics of multihulls that may or may not be seen as a disadvantage. The great stability of a multihull also makes it very stable when it capsizes. The monohull, when it capsizes, if it doesn't sink, has a good chance of turning back over on its own, usually loosing its mast and sails in the process. All multihull designs, if they are done well, will make provisions for living in the boat upside down for an extended period of time, should a capsize occur. Finally, there is the question of aesthetics. There are still many sailors out there that can never get used to the idea of a multihulled boat. They consider all multihulls, in spite of their definite advantages to be ugly. Some of this is due to early efforts of backyard multihull builders to cram as much into and onto the multiple hulls as possible. The result was something like a houseboat with sails stuck on. Most designers now try to achieve a balance between exploiting the wide beam of multihulls and designing low profile, aesthetically pleasing deck houses. Indeed, that turned out to be one of the objectives of this project.

One of the greatest advantages of multihulls is their safety. Unless they are built out of steel or ferro­ concrete, multihulls will not sink entirely because of the inherent floatation of the materials they are made of. Monohulls loose that inherent flotation when they add their lead weight. If a monohull fills entirely with water, it will sink. Multihulls have a great comfort advantage over monohulls. While it is natural for monohulls to sail heeled over at angles of up to 30 degrees, it is not natural for catamarans to heel more than 5­10 degrees or for trimarans to heel more than 15 degrees. In addition, multihulls can either go much faster (more than twice as fast) as similarly sized monohulls, or have twice as much room as a same sized monohull. Multihulls also ride better at anchor because the anchor can be connected to bows of the amas and prevents sailing back and forth. The troublesome spinnaker pole is unnecessary because of the beam of the multihull, and multihulls have a much shallower draft, enabling them to select safer anchoring spots and sail closer to shore, even right on to the beach. The list goes on...

The trimaran can be considered a compromise point in the transition from monohull to multihull. Many sailors who have been on monohulls all their life will prefer to sail trimarans over catamarans. One disadvantage of catamarans is their poor maneuvering ability. It is much easier to turn a boat that only has one hull in the water than one that has two. The trimaran is a compromise. While it does have three hulls, the amas are raised relative to the center hull so that at the crucial part of the turn, ideally, there will be only one hull in the water. While maneuvering in port, it is also much easier to turn a trimaran than a catamaran. According to some trimaran enthusiasts, trimarans can point better (sail higher into the wind) than catamarans. This is probably the case when the catamarans do not have retractable daggerboards. It is also supposed to be nice to ride along on the windward ama of a trimaran, high and dry, speeding over the water below. The other advantage of trimarans is that all the accommodations can be placed in the center hull which can be designed much like a traditional sailboat. The amas can be used for storage of things that won't be needed while sailing. This means that the designer does not have the difficulty of designing a low profile bridgedeck on a smaller sized catamaran, nor does the builder have to build two equally sized hulls with accommodations in both.

In spite of all this the author chose to design a catamaran. Perhaps it is his inexperience with trimarans, but the author finds the catamaran hull form more simplistic and aesthetically pleasing than the trimaran hull form. In addition, the objective was specifically to design an offshore sailing catamaran. It is no coincidence that the two vessels to have sailed around the world in record time are catamarans. There is no doubt about the catamaran's superiority over the trimaran when going down wind. And much of offshore sailing consists of running before the wind. Also, the front netting of a half bridgedeck cat seems like a very appealing place to relax as the water rushes below at 20 knots as well as an excellent place to store windsurfing equipment.

Multihulls offer a definite advantage over the monohull because of their safety, stability and speed. The trimaran has its advantages, but for long distance offshore sailing, the catamaran is the best.

Another area that should be discussed is the purpose of using computer assisted design to design yachts. The advantages include the ability to fully define the hull, which was not possible when designing by hand. Also greater accuracy can be achieved in the design and construction of the yachts by using computers to assist in the manufacture of the materials used in the manufacture of the hull. In addition, a design can be quickly and completely analyzed by a computer that has access to the complete definition of the hull and rig. Finally, the design can be fully animated making it much easier to demonstrate to a potential buyer.

Procedure

Several processes were involved in the completion of this project. These include research, experimentation, solicitation for the donation of a boat design program, learning the new program, and the design of the catamaran. In some cases several procedures were being carried out at the same time.

The research process began the summer before the author's junior year, specifically June, 1992. This involved reading the books available in local libraries on catamarans and other sailing vessels. At this point, notes were taken on desirable features of the offshore sailing catamaran. The author began to familiarize himself with the terminology used by yacht designers and the equations used to estimate the performance of a given design. The author also spent a significant amount of time sailing on the Potomac River, learning first hand the sailor's vocabulary.

During this research period, a list of formulas, and the significance of their variables was included. These formulas included James Wharrham's method of determining stability of a catamaran, as well as the significance of various ratios, such as the Displacement to Length ratio, and the hull waterline length to beam ratio. The generalizations derived by these formulas allowed the author to roughly determine the characteristics that would be best for a speedy offshore cruising catamaran.

During the reading of books and articles about offshore catamarans, a list of characteristics which the author to be desirable in his design were kept. A significant objective of the project was to keep the design simple, with this in mind, the author considered design materials and features such as boomless sails, a remote outboard engine as opposed to an inboard engine and the use of the constant camber hull construction method.

At the beginning of the author's senior year, after having completed the prerequisite CAD class in the author's junior year, the author experimented designing a hull form with AutoCAD v.12. It soon became apparent that it would be too difficult to design an entire catamaran using AutoCAD.

After this realization, the author began corresponding via email with a friend of his, David O'Steen who had at one time worked for a yacht design company. David has extensive knowledge of the design process as well as desirable characteristics for the cruising multihull. David was able to send the author the addresses of several companies that designed and sold yacht design software. The author wrote to these companies requesting them to consider donating their software to the Thomas Jefferson CAD Lab (see Appendix). Several of the companies sent information on their software, and one even sent a demo disk. Several of the companies offered less expensive "student" versions of their software, but with fewer capabilities of the actual design software. The author had to study the brochures and make several phone calls to determine the extent of the capabilities of the student design packages. The author came to the realization that the software available to him (the software in the $100 to $200 range) did not have the capability of converting the surfaces in the yacht design program to a DXF file which could be read by AutoCAD to allow the author to animate his drawing. Fortunately the letter of a third company, New Wave Systems, arrived with the news that it would donate to the CAD lab its complete software package, the Nautilus System (market price $2,850), for the cost of the manuals.

Several weeks of snow days delayed the mailing of the purchase order, so it was not until the end of January that the software was received. After the software was received, it was installed, backup copies were made, and the author began to experiment with it and read through the tutorial and the manuals.

The design software allows several methods to be used for the design of the basic hull shape. These include station definition and surface definition. The station definition is convenient if you have a list of offsets (a list of the coordinates of the hull surface at specific intervals along the hull), which makes it easy to enter designs that were created on by hand into the computer.

The other process is surface definition. This is convenient if you are designing a boat from scratch, because the software includes a module that allows you to create a rough estimate of the desired hull shape by entering the type of hull (planing or monohull sailboat), the desired waterline length, LOA, tilt, stern style, and beam.

Because the initial surface definition module is only conducive to monohull design, the initial hullform definition of the cat was done using the station definition module. Using the function of this module that allows entering the stations with a mouse, a rough approximation of the hull shape was achieved. Only after inputting thirteen or so such stations was it possible to convert the station definition to a surface definition. There was some initial confusion as to why a complete surface defining both sides of the hull and the deck and stern could not be created from the station definition. It became clear that wherever there is a hard edge or corner, a new surface must be used.

A good amount of time was used in completing the second stage of the hull design. This involved the faring of the hull (making it appear smooth and hydrodynamic). During this time many of the capabilities of the surface editor were learned. One of the capabilities that was essential to designing the cat were the functions called x­mirror, y­ mirror, and z­mirror. The design program is set up so that only half the hull has to be designed. The other half is assumed by the program to be the mirror image. There is perhaps an asymmetric hull drawing mode, but I did not explore this option. In designing the catamaran it was decided to not make the hulls asymmetric (as are those of the popular Hobie Cat). This meant only one side of the hull needed to be faired, and the results of this fairing could then be copied to the other half using the mirror command.

During the design of the hull, it was important to keep in mind the limitations of the constant camber design process. This process uses precurved sheets of plywood to construct the hull form. This construction method is strong, simple, lightweight and inexpensive, however, plywood can only be curved so much. Using this construction process requires that the hulls be long, and fairly narrow.

The height of those who would be sailing the catamaran also had to be taken into consideration. It was decided that two meters would be sufficient for standing headroom for most people.

In order to gain experience with the metric system, the author converted what was initially a 40 foot boat to an approximately 13 meter boat using the Nautilus System's Hull Conversion module early on in the design process.

Once the hullform was complete and faired, it was necessary to match a deck surface to the hull. If these tasks had been done in this order, it would not have been so difficult. It would have been simple to create a surface, add matching columns and then use the Merge Point function to match them perfectly. However, it was attempted to modify both at the same time with out using Merge Point because for a time the author feared not being able to separate the points.

In attempting to fair the hull, a function called K­curve was used. This function draws a line which exaggerates the 'unfairness' of the curve. The curve can then be changed in small increments using the "ooch" and "reooch" commands. As time went on it became apparent that an enormous discrepancy in the K­curve near the bow of the boat could not be fixed. Something was wrong.

By looking at an example sailboat hull that was provided with the program, It finally became apparent that the bottom corner of the surface in the cat hull had been merged with the top corner. By this time the author was finally learning that surfaces have four edges. In order to use the surface correctly, the bow point had to be split using the function "split point". This produced another day of work because all of the rows that had been added to define the shape of this hull were displaces at the bow point. The bow had to be redone and the hull refaired.

After the hull definition had been completed for good, I had to add another surface to cover the stern of the cat. After some experimentation I was able to get it fairly well matched up with the deck and hull. One of the problems of the Nautilus System is that it is difficult to exactly match the edges of surfaces. The surfaces are only defined by a few points so unless the columns and rows are in exactly the same places as they are on the surface whose edges you hope to match (as they are on the deck and hull) an exact match is not possible.

The advantage of the Nautilus System over AutoCAD is the method it uses to define surfaces. Curved surfaces are defined by much fewer points in the Nautilus System than in AutoCAD. It is much easier and faster to manipulate surfaces in the Nautilus System because of this. The Nautilus System also makes it fairly easy to switch between plan, profile, and section views of the design. It is also easy to get a 3­d perspective on the design, although there is no "hide" function. The Surface Editor of the Nautilus system allows no keyboard commands except the occasional yes/no and when you need to specify exact numbers. This makes it fairly easy to get started in the nautilus system.

It soon became apparent that the hull was too narrow for comfortable habitation by humans, so another period was spent modifying the hull and deck to a distance of about 2 meters at its widest point.

The next problem was designing the method of connecting the hulls. At first a large number of small diameter flexible tubes were going to be used. However, after consulting with David O'Steen, it was decided that three large diameter aluminum tubes should be used.

The aluminum tubes were not easy to draw using the Nautilus System. Also, there is no copy command, so the tubes had to be drawn three times. Making copies of the tubes was not so difficult using the mirror commands, but moving the into position required entering the exact value of the desired location by hand using Set Point.

The next step was the design of the rudder. Considerable time was put into designing a practical and simple rudder system, but in the end, only the rudder foil itself could be designed. This was done using the NACA curve function which requires entering numbers to specify the angle of the plane of the curve of the foil, the thickness of the foil (as a percentage of the length of the foil), and several other factors. The result, depending on the numbers entered is a nice wing shaped foil. This same process was used to create the daggerboard, mast, and spreaders.

Although it would have made the project much easier to not have a bridgedeck, it was eventually decided that a bridgedeck would make life aboard the boat much easier. The benefits of a bridgedeck include: shelter from the elements while steering the boat and while going from one side of the boat to the other, and a nice central location for dining, navigating and steering the boat.

Most of the time from 4/10/94 to 5/5/94 was spent on the bridge deck. The problems encountered include:

The structure needed to be at least 2 meters high in places to allow for standing headroom.

Because performance is a factor the structure should be as aerodynamic and as low profile as possible.

While at anchor in hot weather, or while sailing in light air in hot weather, ventilation and shelter from the sun would be desirable. In addition, while sitting on deck, eating or relaxing, a nice view is desirable.

The person at the wheel should be able to easily see in all directions.

The structure should offer easy access to the sails in case of emergency, as well as to the stern of the boat.

The structure should leave room to walk comfortably and safely around all sides.

The doors to the structure should fasten flat against the structure when open.

Sufficient head room should be provided while climbing from the hulls into the deckhouse.

The surface of the structure closest to the water should be at least 1 meter from the waterline to reduce the chance of waves pounding against it. This surface should also be as aerodynamic as possible.

Much of the procedure in the design of this structure was playing around with the locations of certain features of the deckhouse, especially the stairway and the doorway. Again because of the Nautilus System's lack of commands such as INT and NEA, matching the floor of the deckhouse to the hull required displaying more rows and columns on the hull and then using the different views to determine an approximate location for the points defining the floor. Another disadvantage of the Nautilus System is that it does not allow editing in the three dimensional viewing mode.

No other serious difficulties were encountered during the design of the deck house, but there were several small problems.

Early on in the design of the deck house it became clear that too many surfaces were needed to complete the structure. The Nautilus System only allows 15 surfaces in each database. This meant that the components of the hull and crossbeams that would be needed in the design of the deck house would have to be copied to another database so the rest of the deck house could be drawn. This was done, but a great concern remained. Could both files be translated to *.DXF and would the *.DXF files overlay in AutoCAD just like they did in the nautilus system? Fortunately the answer is yes. Regardless of the size of the object on the monitor, provided that the scale for both drawings is the same, they will overlay perfectly in AutoCAD.

Also, in order to offer easy access to the stern of the boat, the aft crossbeam had to be moved forward and down. It is now incorporated as part of the aft wall of the deck house.

After the deck house was completed, the design of the rig was begun. By looking through the appendix of Chris White's book, The Cruising Multihull, I was able to get an approximate estimate of the sail area for a catamaran 13 meters in length. Using this information I calculated the mast height to fit this sail area using a full battened main and a genoa. The mast was designed using the NACA curve function. The spreaders were placed on the mast by comparing their position to the position of the spreaders on other rotating masts in Chris White's book. Finally a surface was added to the aft edge of the mast to approximate the mast track for the mainsail.

The next job was to design the sails, and this was easily accomplished using the NACA curve function.

Later, after moving the entire drawing to AutoCAD, it became clear that there was something amiss. The genoa went all the way up to the top of the mast (as it does in most sailboats) but because a rotating mast was being used the forestay had to attach at the same point as the backstays and shrouds, which in this case was twice the distance from the base of the mast to the spreaders. Fixing this mistake required going back to the Nautilus System, moving the spreaders, and redesigning the genoa. The effect of this was to decrease the sail area, which for safety's sake needed to be done anyway.

After the rig was completed, all had been completed on the design that would be practical to complete using the Nautilus System. The drawing had to be converted to AutoCAD *.DWG format. The Nautilus System makes this fairly easy to do, but it could be more efficient. For example, the surfaces in each database that you desire to convert to DXF format must be turned on while in the Surface Editor, this cannot be done while in the Convert Hull module as would be preferred. Also, the desired density of the mesh must be determined in the surface editor, and then this information must be retyped when using the convert hull module. In addition, two conversions are required to reach the DXF format.

Once the DXF file is imported into the AutoCAD drawing, amazingly, the scale is preserved. However, the DXF file must be dissected in a way to put surfaces in their desired layers. It is also difficult to determine beforehand what density of mesh will be practical in AutoCAD. The Nautilus System is much faster than AutoCAD in regenerating meshes, so some experimentation is required. After about two periods of work, the entire drawing had been moved to AutoCAD and separated into multicolored layers.

The remaining tasks were to draw the rigging, rotate the mast so the boat would look like it is sailing, draw the wheel and the seagull striker (to counteract the pull of the forestay on the fore beam).

While drawing the rigging, the aforementioned mistake was discovered and corrected. The seagull striker was drawn by rotating the UCS appropriately to draw the lines to allow a revsurf to be done. The wheel was drawn by selecting torus from the 3d objects menu and its supports were drawn using revsurf. A fairly difficult procedure was rotating the rotating mast and sail 30 degrees. This was done using the rotate 3d command. The difficuly was in selecting the axis to rotate the objects around because there is no central pivot point on the ends of the mast. Eventually it was decided to use the front edge of the mast as the pivot point.

After the drawing was completed, it had to be prepared for the various presentation formats desired. A different version had to be created for the animation, because the meshes on the original were too dense to make animation practical. In addition, to make the rigging standout, the 3d plines had to be exploded, and offset to allow revsurfs to be created.

Results

The result of this design project was the rough design of a 13 meter offshore cruising catamaran. The design is a compromise between performance, comfort, and cost. The design uses simple cost effective components, such as aluminum cross beams, and constant camber hulls. The design features a low profile deck house to improve the comfort of the captain and crew in the extreme weather conditions that can be encountered in the ocean, without overly hampering performance. The design also features a rotating mast to improve the efficiency of the sails, and has retractable daggerboards to improve performance to windward.

The author also gained experience using yacht design software, and then using AutoCAD to prepare his design for presentation, just as a few leading yacht designers do today.

The project accomplished all the objectives, although not to the extent that the author had originally hoped.

Discussion

My original goal was to design a performance oriented offshore catamaran that was designed to be almost entirely self sufficient and that would be easily sailed by one person. Ideally I would have been able to design the yacht right down to the method of water purification to be used on board. I would have been able to calculate the weight, the cost as well as evaluate the design using the many modules for that purpose in the Nautilus System. I would have been able to fully detail the running rigging. I would have been able to research the potential of using fixed wing sails on a cruising multihull. I would have been able to design the boat especially to make living solely off the sea practicable, and this would require planning a diet as well. I would have been able to submit my design to yacht designers for their critique, and so on.

Unfortunately time and effort are a factor. Time was not on my side after the realization of the difficulty of designing a hull in AutoCAD, and my efforts have gradually become more concentrated on windsurfing as opposed to offshore sailing. However, in spite of my increased concentration on windsurfing, I find the problems to be faced and the innovations to be experimented with by yacht designers intriguing, and I do wish that I had time to complete this project in more detail. I am happy however, that I have learned what little (compared to what there remains to learn) I have from this project, and it definitely has been one of the more worthwhile aspects of this school year.

I also have one complaint. It was not until I became aware that only by obtaining yacht design software would I be able to complete my project that I learned that it was acceptable to use school funds to purchase software needed for research. Perhaps this was kept from me for a reason (to save money), or perhaps I was not listening when it was discussed. Either way, I believe next year's CAD lab students would greatly benefit from a quick briefing on how this year's students accomplished their projects. While all of us will write recommendations in the back of our report, as previous students have probably done, did any of us read the recommendations of the students before us? I haven't. However, I did make use of the research of previous students. At any rate, I believe it would be a good exercise for next year's students to review a report on a project somewhat similar to theirs.

Conclusion

While the broad goal of the project certainly was attained, could the original goal ever be accomplished given the time and the resources available to a Thomas Jefferson student? It is doubtful, although it is certainly much more likely to be accomplished thanks to the pioneering efforts of the author. It is reasonable however to expect that an offshore catamaran could be designed and fully tested and then be sent out to be reviewed by yacht designers, modified and printed out in a set of plans that could actually be built. It would not be practical, even given the advantage of the nautilus software to attempt to design the yacht, research new technology, implement that technology in the design, then design one's own technology (designing the boat so that it would be practicable to live almost solely off the sea), implement that technology, and then submit the design to a complete analysis. There simply is not that much time to design the yacht in that detail, or to research in that much depth. This project, however, has been a step in that direction. Given seniors in the following years that decide to do similar projects, it is conceivable that a project approaching the depth and thoroughness of the one I had hoped to complete will be completed.

Recommendations

It is advisable to do one's senior tech lab project in an area you are very interested in. This will make your senior tech lab feel interesting and worth while. Also, if you are sufficiently clear as to what you want to do during your junior year, by all means go to the library and start reading up on your subject. It is not reasonable to expect anyone to begin work on their project during the summer, but if your project somehow ties in with your recreational pursuits, by all means, keep your project in mind, and perhaps keep a small notebook of things you want to remember. It is also important to briefly review a project similar to your own, preferably during your junior year, to get some idea of the problems you might encounter.

Correspondence

2241 Casemont Dr.
Falls Church, VA 22046
email: Cleath@tjhsst.vak12ed.edu

18 August, 1998

Mr. Stephen M. Hollister
NEW WAVE SYSTEMS, INC.
79 Naragansett Ave.
Jamestown, RI 02835

Dear Mr. Hollister:

I am currently a senior at Thomas Jefferson High School for Science and Technology in Alexandria, Virginia. As part of the curriculum, students have the opportunity to work on a project in one of the several technology labs. I am working in the Computer Assisted Design lab and I hope to design an offshore sailing catamaran in 3-D. Unfortunately, Autocad, which we use, does not have the features that would make this project practical. I have been working with an accomplished sailor and boat designer, David O'Steen. He suggested that a specialized boat design program would make this project feasible.

My goal is to learn more about sailboats and their design. I am writing to ask you to consider lending the basic hull design programs of "The Nautilus Systemtm" to the Thomas Jefferson CAD lab. If this is possible, I would be glad to work with you on any project you may have where you think I could be of help. Or, with your software and Mr. O'Steen's guidance, I could pursue my previous plan.

This project is important to me. Sailing has been one of the most naturally beautiful, fulfilling and exciting parts of my life. The ambition to design the perfect boat is not new, but can never be old, as the needs of the next person will always be different. I would like to realize this ambition for myself and for others someday as a boat designer. With your help, the opportunity I have at Jefferson can give me a start.

Could you also please send me a copy of your technical paper on hull shape definition using mathematical surfaces.

Thank you for your time.

Sincerely,

Colin Leath


2241 Casemont Drive
Falls Church, VA 22046
email: cleath@tjhsst.vak12ed.edu

18 August, 1998

Mr. Stephen M. Hollister
NEW WAVE SYSTEMS, INC.
79 Naragansett Avenue
Jamestown, RI 02835

Dear Mr. Hollister:

Thank you for sending the Nautilus software to the Thomas Jefferson CAD Lab. I am just now learning to use it, and I must admit that a few months ago I would have thought it a miracle to have access to such a complete and powerful boat design program as the Nautilus System. I am currently attempting to design a forty foot offshore sailing catamaran. I have completed a rough station definition of the hull and I have converted it to surface format to complete the rest of the design process. I imagine there must be a way to start a catamaran in the surface editor module, but it seemed easier to start with stations.

Thank you again for making boat design projects practical in the Thomas Jefferson CAD Lab. I believe that I and future students will put your software to good use.

Sincerely,

Colin Leath


From: DOSTEEN
Subject: Re: catamaran
To: cleath@tjhsst.vak12ed.edu
Date: Thu, 17 Mar 1994 17:46:39 -0500 (EST)

Hello, Colin!

I'm glad to hear you finally got some software, and can work on a boat, rather than a house. If your goal is strong and simple, I would think that fewer tubes would be a better approach. Most cats that use tubes have 3 or 4 larger diameter ones, rather than a lot of smaller ones. A typical 13m cat would have one at the bow, one under the mast, and one at the stern, with maybe a fourth between bow and mast one, or aft somewhere. These tubes would be about 30cm in diameter.

The larger the tube, the stiffer it is for a similar wall thickness, and larger thinner tubes are lighter than smaller, thicker ones. Also, attaching the tubes is an issue. Most cats simply have a cradle on deck and strap that bolts over the top of the tube, or a socket that it is bolted or epoxied into. That's not hard for three tubes, but making all those mounts for 20 would be extremely time-consuming.

With any tube, the key is to secure it firmly. It turns out that if you really honk down on a strap over a round tube, it stays pretty firm and is strong. I, however, prefer some sort of help, like at least an oval or ovalized tube, so that it won't rotate. (I once sailed on a MacGregor 36 cat, where the loads from the mast step on the center round tube caused it to rotate forward, to where in the middle of a race we shortened sail for fear it would spin all the way 'round and let the 50' mast loose!)

In carbon fiber, it's pretty easy to make a round tube with square ends, which would be a neat solution. Also, spreading the load to all those tube sockets may sound like a good idea, but it actually may make the boat too heavy. Wherever a large tube lands, there should be a bulkhead and some doublers on the hull skin, to help spread the tube loading into the hull structure. If you have many tubes, you have to do the same (on a smaller scale, since the load on each individual tube will be less) for each, which starts getting heavy. Finally, flexibility is *usually* not considered a virtue in cats.

If the hulls twist or move independently, it can cause the rig to sway (think of shrouds and stays loosening and tightening) and once you get motion started, it tends to keep on working, which can mean the whole craft flexes and groans its way through the water. There are designers who disagree--Malcolm Tennant designs his boats to have a certain limited amount of flex (but they're very carefully engineered) and James Wharram declares it a positive virtue, as a safety feature. Wharram's boats are slow, and he views safety as paramount, and thinks that in extreme conditions, when the cat is acting as a raft, that flexing allows the boat to move with the seas, rather than resist them. I think he's right, to a point; that is, most non-flexing cats can drift and survive extreme conditions just fine--if they were flexible, it might help, but it seems to me one ought to design not just for those rare extremes, but for moremoderate conditions, too.

I don't mean to toss cold water on your idea, but I don't see any particular virtue to having multiple tubes--was there something I missed about the advantage of many tubes?

Have fun!

David


From: DOSTEEN
Subject: Re: Howdy!
To: cleath@tjhsst.vak12ed.edu
Date: Mon, 09 May 1994 15:38:41 -0500 (EST)

Hi!

Nothing strikes me as particularly problematic in what you describe. From a performance standpoint, the higher and wider the deckhouse, the more windage. Now, when beating to windward, windage=drag, which slows you down. On any other point of sail, it is not much of a consideration, except in extreme conditions.

The other time when windage is important, is manuevering in tight quarters. If the bows have a lot of windage compared to the sterns, for example, the bows will tend to blow away from the wind, making tacking difficult, and turning up into the wind (say, near a dock) nearly impossible. The reverse is true, though this is actually a desirable trait. So, if your deckhouse is aft of center, it should not cause any handling problems, but one that high, 1.5-2m, will slow it down to weather. By how much? I don't know without doing some calculations. My guess is, not enought to offset the comfort factor. It is remarkable how much being warm and dry improves one's sailing ability! If you had lots of time, you could calculate the added area and drag, and add that to a velocity prediction program, but I wouldn't bother. There are plenty of cats out there with similar-sized houses. Any streamlining you can do without making it uncomfortable inside (and thus negating its usefulness) will help and is worthwhile.

Most cats do have two backstays, one to each stern. Many are either 3/4 rigged or use rotating spars, so the backstays actually land about 2/3 way aft on each hull. Only masthead rigs go all the way to the sterns. The main thing is set them out of the way of the mainsail and boom--either far enough forward so the boom won't contact them except when running dead downwind, or far enough aft so the boom and sail swing inside them. You can put the backstay attachment points wherever you want, but remember, they must be able to carry the full mass of the boat, so don't put them on top of the deckhouse or anything.

A main traveler is helpful, but not strictly necessary. It, too, will carry big loads. All sorts of sheeting arrangements are possible, from a big single block amidships to all sorts of fancy travelers and vangs and so on. A traveler is simple, gives good control, but takes up space and is expensive. A central block and then barber haulers and a good vang is an acceptable and much cheaper solution. If you need ideas, take a quick look at any library book on sailing rigs.

I'd love to see any slides or graphics or whatever you produce!

Yes, I'll be here all summer, teaching and doing other computer stuff, but I get to work a 4-day work week in the summer, which is quite nice.

David