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Design spiral

Updated: Nov 22, 2023

Because of operating at the border of two different media - water and air - boats are one of the most complex technical products in the world from physics point of view. Having that said it is not a surprise that many of project variables cannot be defined and expressed simply, or in mathematical language, most variables are iterative. Each variable is influencing all the other variables, directly or indirectly.


With such a system first step is to estimate the first design variable (or few of them) and calculate all the other variables dependent on this first estimated variables. At the end, lastly calculated value will be new input for the first one (earlier estimated) in next turn on the design spiral. That ways we are closing a loop.

With several turns around this loop we are converging towards values that will give no change (or almost no change) in the next turn. Since we are narrowing the loop with every turn, we can say that we are working on spiral.


Design spiral used in TP Yacht Design studio


Not only because it makes the process very clean and organized, but particularly because it gives us systematical approach to every design, and ability to compare the design variable values at different stages of project with our database and expected values.

Very often we can hear discussion whether this concept is purely theoretical, or it brings value to design process in practice as well.

In our opinion it has very practical purpose. Not only because it makes the process very clean and organized, but particularly because it gives us systematical approach to every design, and ability to compare the design variable values at different stages of project with our database and expected values. This approach gives us possibility to have complete control over project at every moment of design process.

To completely understand importance of this iterative method, particularly in basic design stage, and design stage approach in general, it is worth to take a look at Cost-Influence Curve.


Influence - Cost Curve


Decisions made in early stage of project (left side of chart), impact the final result significantly while at the same time each decision has more possible solutions, and investigating their influence on design is not expensive if done in early stage

Cost - Influence Curve

Cost-influence curve presents the design approach where most of important and influencing decision are tended and desired to be made in early stage of design process. At this early stage (left corner of above diagram), many different possible solutions to choose between when making a decision are possible. Each decision at this stage has very big influence on the final result because it influences all future decisions, directly or indirectly.

Investigating different possible solutions or any change of decision is not expensive because we are still in the concept stage of project (re-modeling or re-drawing an idea is quick process comparing to making change in already built boat). All decisions made in this early stage should be freeze. Later changes of this decisions would be very expensive, simply because all decisions in-between should be re-assessed or updated.

In case this change comes in a stage of project where some parts of boat are already built or when the boat is complete (which is not very rare case at some projects on the market), huge interventions are required many times taking back to concept stage and rebuilding everything in between. Another option is to search for some other "handy" solution that will sacrifice other project features.


To show this in a more figurative way lets take a look at following simplified example:

We want to design a 15 meter motor yacht that will achieve max speed of 40 knots. For estimated weight adequate for boat of such size and intended use we design hull. With estimated weight, speed and hull geometry we calculate and design internal structures. To be able to achieve required speed we calculate resistance and assume to install engines and propulsion devices of adequate power.

Now we update weight calculation with new calculated data and find out that weight has increased for 20% comparing to estimated.

With new weight we have to again recalculate scantlings (because they are very dependent on weight and speed) and resistance - and pick new engines. Again we have to repeat the process until we manage to reduces the change from loop-to-loop to neglectable value.

In realistic design process, many variables are interfering, and process gets much more complex. In order to decrease number of iteration loops and to speed up the process, good first estimations based on similar projects are very important. Although important, they cannot replace the iterative process that give us more precise numbers, or at least confirm that estimations were correct.


Although many times neglected or misunderstood, design stage approach combined with design spiral method is one of most clean, organized and flexible approached if understood and used correctly. It gives us opportunity to control the design process all the way from concept to production, and it helps us achieve desired results.

Using this principle will significantly improve quality of your design and reduce stress on the way to final result.

If you have any additional questions regarding our design spiral concept, this article or you would like us to help you check and implement mentioned design methods into your next project, please do not hesitate to write us at


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