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An Interview with Multihull Designer John Shuttleworth

I am pleased to present this recent interview with noted designer John Shuttleworth of the UK. John actually lives not far from where I was born and often works with the Southampton University and local Woolston model test tank, where I studied some 50 years ago. —Mike Waters, Dec 2008

Background: John Shuttleworth has been involved in sailing and designing multihulls since the 1970's.
After getting his M.Sc. (majoring in Engineering) he also worked on computer analysis and tank test data to upgrade multihull design, as well as taking practical experience through some extensive cruising along the coasts of South Africa and South America, as well as an Atlantic crossing in a trimaran of his own design. Since starting his professional naval architectural business in 1976, John has designed over 50 multihulls including a number of highly successful ocean racers including among others, the 65' Brittany Ferries GB; 56' Fleury-Michon Vl (renamed Elle-et-Vire); 60' Great American and the 80' Novanet, that all broke records, some of which still stand today. Mr Shuttleworth has often presented technical papers on multihull design and performance at international symposiums and was a member of the 'Distinguished Panel of Designers' at World Multihull symposiums in both 1984 and 1988. John has raced multihulls in Britain, France, and the USA, as well as in a Trans-Atlantic record attempt with Chay Blyth. John also joined a New Zealand Americas Cup challenge team, to oversee the computer performance prediction and data gathering methods, conducting an extensive tank test program, with probably the most advanced computer performance prediction work to date on multihulls.

Of particular interest to smaller multihull enthusiasts, would be his high-performance 30' N'aia trimaran design and his cruiser-racer, Shuttleworth 31 catamarans.

For a more complete bio on John Shuttleworth, go to:

MW: What has been your experience of working with test models and are your designs better because of them?
JS: Very positive experience. With the help of materials like Plasticine that permit one to make rapid changes to models, just one full day at a test tank with some experienced help, can permit a designer to test 3 or 4 related but optional shapes and take home valuable data that can be examined at leisure. This is particularly valuable for the larger designs that will be costly to build and expensive to modify later.

MW: Do you know of other designers that use tank testing to predict and improve performance?
JS: Well, although a few of the most recent ORMA 60 boats have been tested, there have not been many. Tests that were done have not been made public, though some tank testing was done on a few very early designs being prepared for record-breaking attempts, even prior to my own efforts in the early 90's.

MW: How does the performance of your smallest multihull (30') compare to others out there?
JS: Well the 30' N'aia trimaran design was built to compete with 40 footers and as originally designed, could certainly outperform all other 30 footers of the day.
But to get that sort of extreme performance requires an extreme rig with a very experienced sailing crew and generally, one has to tame down at least the rig to make it more versatile for general sailing and less specialized sailors.

MW: For a smaller trimaran, do you have any preferred folding system that you'd recommend?
JS: Well the Farrier system is certainly one of the most effective and now that I understand the patent has expired, I would expect to see more variants of that concept and hopefully, at somewhat lower cost. The problem of fendering and hull-side fouling will still persist though, and that will always give a place for other solutions that keep the ama vertical.

MW: Is it possible that you'll soon be designing a smaller multihull that might interest our readers?
JS: It's pretty unlikely in the near to mid-term future, as I'm heavily involved in larger craft that can better justify the design time and cost involved. In fact, I am presently working on a large power trimaran project and using both tank tests as well as radio-controlled models to test a variety of options in varying conditions to optimize the final boat.

MW: What particular aspects are you testing for this project?
JS: Well for example, we are not only testing to find the preferred longitudinal position of the relative short amas but also experimenting with the effect of raising and lowering the amas relative to the main hull. This can be done on a model with RC control and we can then see the effect quite dynamically, through raising or lowering the amas in real time.

MW: Will you be reporting on any of this work later on?
JS: Once the boat is completed, I hope to write up the results of our testing and make it available for future designers.

MW: Do you have any preferences for construction materials for smaller multihulls?
JS: For ultimate performance, I presently prefer a Nomex* core with carbon-fibre skins. However, more than adequate performance can be achieved for most people using a less exotic foam-sandwich construction. I am not a great supporter of the very rigid foams—preferring the more flexible foams, such as Airex*, that have a certain spring-back that absorbs shock without delamination. However, newer foams such as Core-cell seem to be a good compromise and give a somewhat higher strength to the shell without being excessively rigid.
These foam materials do however require consistently high build quality and in some circumstances, one would be better to stay with wood that is more familiar [ie: wood can be used without vacuum bagging etc]. Wood core like of cedar, still gives an excellent performance but is inevitably somewhat heavier and there are applications where plywood is still effective too.

*[Editor's footnote: Nomex, is a honeycomb core of phenolic coated Aramid paper, made using fibres of Kevlar rather than cellulose. Although it has high mechanical properties, low density and good long-term stability, it's significantly more expensive than most other core materials. It also requires high technical skills to get a good bond without resin filling the open cells. Airex (now from Alcan) is a relatively flexible PVC-polyurethane foam that though harder to delaminate, has lower mechanical properties.]

MW: Finally, what is your latest thinking on ama buoyancy?
JS: It would depend on size and performance goals but I would personally favor a buoyancy of about 150% of the overall displacement. 125% would be ok but the 150% is to allow the weight to be increased without compromising stability and power to carry sail for a performance oriented boat.

MW: Thank you John for your time and interesting insights, and please stay in touch.
JS: My pleasure… nice to chat with you and I'll be following your website with interest. I certainly agreed with your reply to the question, 'do trimarans plane?'

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