Developing the Rocket Folding System

To get it right you have to juggle the size and cross section shape of the main hull and the float so they sit within the required width limit when folded, yet behave as you would wish in sailing trim. But there are other restraints.

The arcs of travel of the float (there are several) as it moves in the transverse plane are critical. You can’t have the float diving too deeply as it begins to fold - it would resist the folding action. And the beam arch has to be shaped so that it doesn’t extend past the required width when folded, which compromises the wave clearance when sailing if you don’t get it right.

And all of this has to be achieved by determining the ideal length of the two folding arms and positioning the axes of the four pivot pins so they can be conveniently attached to the boat without interfering with the beam structure or otherwise being overly complex to engineer and to build.

I spent a lot of time looking at all these variables when we were designing the Essential 8, and again with Trilogy in the 1990’s. Many hundreds of hours of work I can safely say. And simply adjusting all of the factors by trial and error.

Ian was generous enough to publish cross section drawings of his boats which included the folding arms and this provided a good starting point.

Was there some mathematical way of figuring out the possibilities and finding the optimum solution? Did Ian go through the same relentless process of trial and error, or did he have some innate understanding of the fundamentals of the geometry that enabled him to come up with the solution more readily?

Either way it’s a magnificent result to his credit. I hope we get to read about it one day.

In the case of Bare Essentials and Trilogy it seemed the compromises were just too great to consider incorporating the full folding system. In both cases sailing qualities were paramount. If it was necessary to use low buoyancy floats, have the beams too low to the water or limit overall beam then the folding system was out.

When I started the project to replace the C31 model with Corsair in 2011 I was confident I could improve on the sailing qualities of the boats, but somewhat resigned to the fact that there would be compromises in float shape, float buoyancy, beam clearance, maybe all three.


I attacked the problem again in earnest, duplicating and incrementally refining the geometry on layers in my drafting package. At one stage I was confident I had found the solution which was to rotate the lower strut past the vertical in the folded position. It could have been a disaster. But Paul Koch recognised that it wouldn’t work. We built a mock up of the geometry in a steel gantry to test it. Paul was right and it could have been an expensive mistake. Back to the drawing board.

To this day I can’t remember where the breakthrough finally came, but it was probably just the endless adjustment and refining of all the variables. The end result gave us a 30’ trimaran with 3000kg of float buoyancy (more than 200% if you can keep the boat under 1500kg), and excellent beam clearance, especially at the outboard end where it counts. Most importantly there was no compromise to the float shape which is surely the most critical element in trimaran design.


Trilogy at the Multihull Nationals Port Lincoln 2014. Trilogy's performance credentials are not in doubt with a long list of race victories over nearly 20 years and  including 6 times winner of the Australian National Championships

Comparison of Farrier F9 and Trilogy

The drawing above was created in the middle 1990's to compare the beam and float geometry of the Trilogy Design with the Farrier F9A. The comparison is not entirely fair because Trilogy is slightly longer than the F9A at 9.5m (31') but otherwise the volume of the main hull of both boats is very similar. However the difference in float shape, beam clearance, and overall beam is striking.


The image above is from just one of many drawings investigating the options for the folding geometry. This particular drawing has over 100 layers of drafting work with many of them turned off in this image.




Solving complex problems like determining the ideal axis location for a trimaran folding system is referred to as an optimization problem. There’s a Canadian company called D-Wave Systems building a computer designed to calculate solutions in a mathematical field known as Ramsey Theory. It’s about finding the optimum solution to a problem with far too many variables for the average human brain to deal with. 

A good example of this kind of work is determining optimum pricing, routing and scheduling for an airline with several hundred aircraft operating in various countries and with various seat payment tiers.  

Will we have software before too long that will enable us to input the hull shape we’d like to have, the beam clearance we prefer, the engineering constraints, and the path the float needs to take through the water as it folds, and have the answer to the geometry of the folding system pop up on screen in a flash?

Maybe so, maybe the capability is there now,  but I imagine the task of instructing the software what you’re trying to achieve and in what priority is a huge task in itself.



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