The Boat Design Spiral

Spiral_diagram

I had always thought, like most sailors I suspect, that a naval architect sat down with his or her computer and over a number of weeks a fully formed design emerged. But, in fact, nothing could be further from the truth.

Boat design is an iterative process with many steps that are well explained in great detail in the Principles of Yacht Design. See the diagram above. A simplified explanation goes like this:

First iteration:

  • Specify the boat broadly to meet the usage envisioned (in our case offshore voyaging) by defining numbers like sail area, size (more a matter of weight than length), beam, load carrying capacity, various important numbers like sail area displacement ratio and prismatic coefficient, as well as that all important number, target price.
  • Draw a hull to meet those specifications.
  • Evaluate the hull against the specifications.
  • Repeat till satisfied.

Second iteration:

  • Draw a general arrangement that includes sail plan, deck layout, and interior layout.
  • Scratch head a lot and think about all the different things we would like to get into the boat. Select the ones that contribute the most to the goals of the boat. Discard those that will detract without adding anything really important. This is hard stuff to get right and requires lots of offshore experience as well as laser-like focus on what really matters. It is also the point in the process where it is easiest to go off the rails and design a bad boat.
  • Re-draw general arrangement.
  • Do a rough costing to check that the boat can still be built for the target price.
  • Repeat till satisfied.

Third and subsequent iterations:

  • Engineer the boat including scantlings.
  • Specify and design the mechanical equipment, deck gear and rig. The last two require figuring out the static stability.
  • Figure out what all this is going to weigh.
  • See if the hull designed in the first iteration will float on its marks at that weight. (It almost certainly won’t.)
  • Change hull shape so it displaces exactly the same weight of water as the boat will weigh dry (an iterative process in and of itself).
  • Check that everything specified in the second iteration still fits in the new hull.
  • If it doesn’t, change hull and/or stuff that goes into it.
  • Make sure the boat has the correct static and dynamic stability to be safe in heavy weather offshore.
  • Check that the boat can still be built at the target price. Take out stuff or make boat smaller (probably means lighter) if the target price has been exceeded.
  • Repeat until done.

Last iteration

  • Run hydrostatic and velocity prediction programs on the design to make sure that the boat will be the best possible for the usage defined in the first iteration.
  • Make small tweaks resulting from computer model runs.
  • Have the design reviewed by another naval architect to make sure nothing has been missed.
  • Go back to the third iteration if things change a lot.
Principles of Yacht Design 4th edn-2

Thank you to Lars Larson, Rolf Eliasson and Michal Orych, authors of Principles of Yacht Design and their publishers A&C Black Publishers, an imprint of Bloomsbury Publishing Plc, for permission to use the design spiral diagram.

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John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 18 years, as co-editor/publisher of AAC.

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