The Design Spiral, or where to start building a boat.

Discussion in 'Boat Design' started by apex1, Jul 28, 2009.

  1. apex1

    apex1 Guest

    Of course, Yachtdesign is´nt rocket science, would it be that simple we did not need Naval Architects or professional designers.
    Unfortunately it is not common knowledge that boat or ship design is a bit more than a simple layout sketch on a napkin. Due to the faulty use of the word "design" instead of the correct term "construction" that common point of view will never change.

    The main difficulty a novice boatbuilder or buyer has, is to understand his real demands and requirements and their influence in a design spiral leading to his taylor fit vessel. (not too few "designers" have a similar problem)

    Several recently opened threads have shown repeatedly that the novice comes with a well established "desire" or imagination of the perfect boat and changes his goal several times during the educating steps of participation in a discussion here.

    To give both, the novice and the prospective designer a brief overview what leads to a successful construction I dug out a several years old text describing the design spiral.

    I hope you enjoy it.


    The design spiral in modern Yacht building.

    The basic information needed is an outline of the intended vessel's size, layout, materials of construction, the intended use, range, speed, rig, general aesthetic, and most importantly, the budget that is available for the project.
    The first step in boat design is to define very clearly the main function or purpose of the boat. (we call this the book of requirements). Without a clear idea of how the boat will be used, you will not be able to adequately resolve the many conflicting choices that will confront you during the design process. Define the main function of the boat and use that vision to guide you through the various trade-offs which must be made to achieve the final result. Although a boat consists of a series of compromises, how you select between the trade-offs will determine the success or failure of the boat.
    Often times owners (and designers) have unrealistic ideas about the characteristics which can be combined into one boat. For instance, can you have a sailboat which is fast, weatherly, contains full accommodations, and has shallow draft? Maybe, but it depends on how you quantify these attributes. One person's idea of fast may not agree with another's. The answer to this question is ... perhaps, but not necessarily achieving your (or the owner's) expected goals. You must quantify the design goals and state which are the most important in order of decreasing importance. This can be done in a short book of requirements that can be used to keep you focused on the overall purpose of the boat. It is then the goal of the design process to help you design the boat and determine if all of the specified criteria can be met. If these goals are not possible to achieve, then their ordering will help you determine how to select an adequate compromise.
    Another common boat design approach is to prepare a design proposal in competition with other designers for a design contract. The design proposal is a result of a conceptual design process and will be discussed in a following section. Before you begin the conceptual design process, however, you still need some form of design or concept statement which describes the major attributes of the design. Preparing a competitive proposal for a client who can't seem to write down a clear book of requirements is a no-win situation, unless you're good at mind reading or you're involved with helping the client formulate the book of requirements.

    The overall boat design process can be described by the following steps:
    Step 1. The Book of requirements. - Define the purpose of the boat and quantify and list the major design attributes in decreasing order of importance. Include a measure of merit for the vessel, if needed.
    Step 2. The Conceptual Design Phase. - This step determines whether the boat described in the book of requirements is feasible and how you will have to modify the stated goals in the book of requirements to achieve a successful boat design. ................................

    see attachment to read on!

    Attached Files:

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  2. Alik
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    Alik Senior Member

    Isn't it a Westlawn text? I remember those illustrations.
  3. Ad Hoc
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    Ad Hoc Naval Architect

    The basic design spiral is quiet well explained above. In my field we call this SOR, or statement of requirements.

    The crux of this though, is down to what "design data" one has to hand to validate/verify what is in the SOR. Since drawing a nice general arrangement and writing a nice specification can indeed look very good and plausible. However, unless the designer has sufficient data to produce the design, all it is is just a more detailed version of the "idea", but still not really saying much.

    For example, a client comes along with an idea or a concept. Lets say they want 100 passengers, with room for 20tonne cargo, TEU's, to travel between 2 islands of say 50nm apart in seas that are always flat calm at say 30knots. This satisfies the daily passenger trade as well as the freight during winter times, as well as "feeding" the other island. How does one start?

    Well, you can talk about does he really want this or that, or don't you really mean 5 tonne of cargo as 20 is too much really (because you're not sure how to do it), until the cowes come home, or the most efficient way is to ahve an efficient hull form designed by program XX or whatever. Non of these answers the question, can it be done and what is the cost?

    What do you do?

    So, the way to producing a concept, by ‘concept’ I mean a design that you know will fulfill the SOR. I do not mean a pretty picture not drawn to a scale. A drawing not to scale is useless whether it is a sketch of a structural detail or a GA drawing. Because a not to scale sketch can hide gross impracticalities. The outline can be a simple sketch that can be drawn to scale to produce a GA.

    The sketch has to virtually incorporate every detail. It certainly has got to show all the items mentioned in the SOR that affect the arrangement. It’s got to be realistic and allow for thickness of bulkheads and the flare of the topsides in a monohull for example.

    So what about hull, what do you do there? There are usually three possibilities. A near match (existing design), a previous design that can be geosimed, or nothing. But whatever you do, you have got to start somewhere and that can either be a hull if the design is weight driven or a deck layout if the design is deck area driven.

    So, if you do not have an existing design that matches or one that can be geosimed, then it becomes more tricky. So you must know intimately the Froude numbers and length disp ratios where jets and props work well and don’t work well, which helps in the selection of a hull. So the selection of hull type/form will depend on Froude number, seakeeping (if any) and of course the SOR. You need a huge number of model tests in your file even for a restricted number of types of craft because you need to know trends. The effect of increasing beam, the effect of transom immersion, the effect of length/displ ratio. And nine times out of ten, parameters you need are outside the range of speed or displacement done in a model test. That’s why you need trends. This is why detailed "accurate" computer programs of of zero use when designing from scratch. One is relying too much on the data from a limited output source. Not to mention the computer is just a "guesstimate" and not a real boat that has been tested or can be validated.

    Quite aside of the basic naval architecture, you will need to invent/create any required special feature that the SOR demands and mould the GA around it. (e.g. deck space for the TEUs). This may require a basic knowledge of mechanics/ hydraulics/ electronics.

    Relationship with other disciplines. You will need to have basic understanding of mechanical and electrical engineering in order to be able to challenge the electrical engineer or subby when he says he needs a huge electrical room or 2 tonne of batteries. And the mechanical engineer/draughtsman when he says your GA shaft line is impossible, and of course construction. No point drawing up a detail that cannot be fabricated or at least quickly and cheaply!. At the end of the day the naval architect has to balance out the demands from each discipline and decide on the compromise. The best designs are usually the ones that are well balanced i.e. those where one aspect has not been able to hold sway over all others. On the other hand if the SOR only stresses one feature then the naval architects task is easy…

    In the whole procedure, the key to success and failure is weights. Being very realistic with the weights of the items on board, once the GA has some detail. This also comes from the SOR. Adding healthy margins and never be optimistic.

    Once this first cycle has been done, then the naval architect must revisit the original concept and answer the question, does it float, does it float upright and does it meet the SOR.

    Once these have been "ticked off"...then the "design" has been done. And any claims about it, can be supported by previous data or evidence of other "items", when non previously existed.

    But this takes time...not time in putting together, but time in learning this whole procedure and learning what is important to fulfill the concept. Having a computer program will not short circuit this, all it does is make your crap design quicker, not better. The design 'drivers'...things which affect weight, cost and hence performance take time to understand and place accordingly in the design spiral and how to produce a design that delivers the SOR on time, budget and performance....

    Those who have never done this, will never understand, since their view is simple and focused on simple things or one line issues. The role of the naval architect is to balance ALL the conflicting issues to ensure it works as one.

    Just putting out press releases and pretty pictures with nice renderings to say they have a "new design" but with no data to support it, is unprofessional, however, these charlatans are very easy to spot.
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  4. Alik
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    Alik Senior Member

    This is what I call 'CONCEPT DESIGN' in our practice, for 21m sailboat:


    # Item name Number

    1 Drawings
    101 Lines plan A2100.HL.01
    102 General arrangement - exterior A2100.LO.01
    103 General arrangement - exterior A2100.LO.02
    104 General arrangement - interior A2100.LO.03
    105 General arrangement - interior A2100.LO.04
    107 Sails and rig plan A2100.SP.01
    108 Sails and rig plan A2100.SP.02

    2 Renderings
    201 Exterior rendering
    202 Exterior rendering
    203 Exterior rendering
    204 Exterior rendering
    205 Drawing style rendering (4 exterior views combined)
    206 Interior rendering (X-Ray view)

    3 Preliminary calculations
    301 Main particulars (summary table of boat's dimensions and parameters)
    302 Table of weights
    303 Hydrostatics file
    304 Stability diagrams
    305 Stability assessment
    306 Structural report
    307 Sailing performance predictions (polar plots)
    308 Resistance curves
    309 Power/speed curves
    310 Propeller calculations

    Sometimes we also include preliminary list of equipment, taken and modified from similar design.

    Ad Hoc is right about the term 'concept' - sometimes stylists/designers produce pretty sketches and call them 'concepts', but those are useless without feasibility check.
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  5. Alik
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    Alik Senior Member

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  6. Ad Hoc
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    Ad Hoc Naval Architect

    No, i fully concur...just a pretty what? :)
  7. gonzo
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    gonzo Senior Member

    Experienced shipwrights, within their limits build with little or not design. It is still pretty common for a customer to ask for say a 54' boat, with a fair amount of flare and rather beamy. If this thread is only about dreamers with no practical experience, then there are several routes for them. They can study, apprentice, or both, which I prefer.
  8. Frosty

    Frosty Previous Member

    Alik when I drive on the Chonburi bangkok expressway ( second level) just before Suvanaboom airport turn off I smell fibreglass--is that you? near Ban Phi.

    I tried to get on the new ring road and over to Samut Prakarn using the new bridge instead of the Rama 9.

    Still cant get onto it.
  9. Alik
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    Alik Senior Member

    As You know we design, we do not build.
  10. Frosty

    Frosty Previous Member

    Oh!! didn't know,---do now,--.
  11. joz
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    joz Senior Member

    Similar Alik but not the same what been posted isn't in the text as I am going through it at the moment, unless its in the SG 2 notes
  12. apex1

    apex1 Guest

    Well, Gonzo I concur with your first sentence. But thats not part of the topic.
    The lack of understanding how a custom built develops is the topic. And of course the solution for that problem. Reading the full text makes that clear.


    Ad Hoc
    do´nt get stuck at the concept phase (phrase), the text goes another 60 pages further ahead.

  13. terhohalme
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    terhohalme BEng Boat Technology

    I think, the original text is from Stephen M. Hollister (New Wave Systems).
  14. apex1

    apex1 Guest

    I do´nt remember who sent me the text, but your estimation makes sense.

  15. Willallison
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    Willallison Senior Member

    As I recall, the Hollister text was very similar, and certainly the diagrams are very similar if not the same, but it wasn't as long
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