Here is an interesting exchange that I had recently (April 2009) with the noted French multihull designer Erik Lerouge. As one of my personal favorites, I have long admired his work, being most notable for designs that both look great and sail efficiently. Windage is kept low, wing decks high; combined with overall high beam, floats of high volume and very efficient rigs, often with rotating wing masts. His "Azuli" and "Freydis" designs have been particularly successful.
MW: Can you please first give my readers an idea of your sailing background?
EL: I started sailing a TORNADO in 1970. Then bought an IROQUOIS with my father a year later and started designing multihulls in 1976, now having signed off on over 400 different sailing designs, the majority of which have been catamarans. After a few one-offs, I designed the PULSAR 26 which was a highly successful racing and cruising trimaran. A nice blend of comfort, seaworthiness and performance all in a trailerable package.
From this, the PULSAR range was developed.
Most interesting is the PULSAR 30 "TROU NOIR" which did very well racing in Finland. Also creating a fascinating history was the PULSAR 50 called "RAYON VERT" a true Class 50 that was homebuilt by a couple in their fifties in only 2 years. She was entirely laminated under vacuum over male plug moulds and had carbon beams, board, mast and boom. The couple then left for a troublefree 3-year Round the World cruise with no other multihull experience other than a couple of sails on a HOBIE 16!
I have become very interested in foil stabilized trimarans. This is something that continues to interest me and I am looking forward to the trials of DINABAR.
MW: Have you used any tank testing in the design of your hulls and if so, how are your designs better because of them?
EL: I did do some research on catamaran hulls. First it was to prove my point with regards to forward section dynamic lift and then to optimize stern sections. At that time, there was the debate between fine sterns and flat transoms. This has been extremely useful and I have used these results on all my multihull hulls.
MW: Do you know of other designers that use tank testing to predict and improve performance?
EL: This is usually only done for high profile racing programs, such as for'Groupama 2'. I personally do not feel there is a lot more to learn re basic hull shapes. The overall concept is more important and the challenge now is improving behavior in waves of ever faster multihulls. As this cannot yet be analyzed in tank tests, there is fortunately still room for intuition and experience!
MW: Based on any tank test results you may have seen for very slim hulls, what % of the total hull resistance for a typical multihull ama or hull, is frictional compared to wave making?
EL: There is no simple answer. It all depends of the range of speed you are considering. In any case, frictional drag is fundamental so there is absolutely no hydrodynamic reason to design wide flat-bottomed hulls. Only the ends might be adjusted for the speed potential or wave behavior. Fine sterns are good at low speed in calm waters but poor at high speed or to damp pitching.
MW: How does the performance of your smallest multihull (26') compare to others out there?
EL: Sorry, but it is illegal in France to make these kind of comparisons… our vision of morality!
But the main difference with the competition is that my boats have full length, high buoyancy floats. This gives outstanding rough weather ability and allows one to carry a powerful rig.
MW: For a smaller trimaran, do you have any preferred folding system that you'd recommend?
EL: It's plain nuts and bolts! This is light, stiff, reliable, cheap and easy to built and maintain. Although folding systems are well marketed, they are rarely used in Europe. But all systems have their drawbacks: Swing-wings exceeds towing width for a roomy 26 footer and increase overall length in marina. What's the point of such a complicated system if you still need to unbolt the floats for transport?
Or if you have to pay a size up for marina fees?
(Ed: this unbolting is no longer the case with the latest Dragonfly models. However, Erik points out that "in Europe, this can still be an issue as max. towing width is 2.55 m or 8'-4". Towing in Europe and America is not the same. Neither are the roads, cars or distances".)
On other systems, access from a pontoon, fouling on the topsides, stability in difficult conditions, height on the trailer are other problems. All folding systems work better with small floats so for conventional trimarans, this is bad news.
My PULSAR models get their performance and safety from their large and long floats, that are just not suitable for folding systems. In my opinion, folding systems require a total rethink of the trimaran concept, with really small floats, most likely foil assisted.
MW: Is it possible that you'll soon be designing a smaller multihull that might interest our readers?
EL: The PULSAR 23 is a new model. It is presently conventional but it may evolve into a foil-stabilized folding trimaran.
MW: Can you elaborate more on this new 23' model?
EL: Our program is not to do the fastest 23' tri around but a simple, practical, safe fast cruising trimaran. She will be towable with a simple French driving license. (A special license for towing is required in France if total weight of vehicle + trailer exceeds 3.5 tons.)
Originally, she was to be a smaller sister to the Pulsar 26. No more standing headroom, just seating. So that it could be lighter and less voluminous to transport. The 7 m length limit avoids certain taxes in France. Then to further reduce building costs and keep a nice interior, I decided to eliminate the dagger board in favor of float keels as already fitted on the Pulsar 33. These were inspired by my successful catamaran keels. See: www.lerouge-yachts.com/cat_TKS.htm
MW: What particular aspects are you testing for, re any new project and will you be reporting on any of this work later on?
EL: A problem for trimarans is that they are more expensive to build than catamarans. They require more moulds, have more hulls and more skin area—and also lots of stability [that demands a stronger rig etc]. I am working on this problem and something may happen if I find the correct boatyard willing to invest in this market.
MW: Balanced against cost, do you have any preferences for construction materials for smaller multihulls?
EL: Without any doubt, foam-sandwich is ideal for 90% of the cases. Strip planking is a good economic choice for a home-builder in countries where proper wood is readily available and foam prohibitively expensive to obtain.
MW: For foam sandwich, do you use large sheet layup over male stations, or vertical or horizontal strip? And do you use vacuum bagging or hand wet layout?
EL: Depends of shape, availability of materials and budget. For one-offs: assuming Airex foam is the choice, I use sheets on a battened male mould for the main hull. Many were built using Corecell foam strips however. Always longitudinally over stations, with the main hull over male sections while the floats were in half female sections, having a joint at the centerline. All this can be done in hand wet layout.
If the lightest weight is desired, then I use a male plug, foam sheets and vacuum bagging.
For production boats: all are done in female moulds and resin infusion has become the norm.
MW: What is your latest thinking on % of ama buoyancy?
EL: On conventional trimarans, such as my PULSAR, I go for nearly 200%, for both safety and power. On a foil stabilized trimaran, just 90% would be interesting to give a nice static stability curve up to 90° without the main hull lifting.
MW: What has been your experience using a significant knuckle in the main hull re making the boat drier and/or adding interior space or any other reason?
EL: I have used a significant knuckle on the PULSAR 26 to be able to fit a nice accommodation within the length limit of the old Micro-Multihull rule. The foredeck then became wider and more comfortable. But the knuckle generally increases building time as well as skin area and weight and can also catch waves. For the same cost of materials, I would prefer to design a simpler but longer hull with simple flare, such as on my PULSAR 31. In normal sub-gale conditions, the knuckle also adds to the dryness of the boat.
Comparing boats by their length has always been the big drama of yacht design. This is done for harbour fees, custom, racing rules or whatever. But what should be really important is to design 'the best boat for a given budget'. For that, a longer simpler boat is difficult to beat. It is undisputed fact that a longer, narrower main hull will be more seakindly than a shorter, wider one.
MW: One can note that many of your designs have wing masts. Do you design these yourself and if so, what is your approach? For example: Do you spread the boats maximum righting moment over the length of the mast - opposed by the two supporting points—the mast step and the hounds (where the shrouds are attached)?
EL: All my designs have rotating masts for obvious reasons! .. they are more efficient.
When available, aluminum masts are cost effective so I have been pushing mast manufacturers to invest in suitable extrusions for rotating masts.
The ideal is a carbon mast and there are several good manufacturers with suitable moulds and an autoclave who are able to make the lightest masts with high-modulus for a realistic price. Personally, this is the first place where I would put my money on a boat.
Some carbon masts are made by the boatyards themselves or even by homebuilders. I have designed and built a couple for myself but while the result is better than an aluminum tube, with low-modulus carbon and "only" vacuum techniques, it cannot compare in weight with the best carbon mast manufacturers.
In some cases, a suitable aluminum rotating mast does not exist for the size of the boat, or is too expensive to import and a carbon mast exceeds the available budget. In such a case, I have designed a plywood/epoxy/carbon wingmast which can be built in 200-300hrs with minimal material cost. The weight is close to that of aluminum but with a far greater chord. The mast is also stiffer and we can save a lot on the rigging. But I am not marketing these masts designs. They are done as an added service to my clients when it seems the best answer.
The engineering of these masts depends of the sailplan. Mainsail loads are spread, while the jib and spinnaker loads are concentrated. Worst cases of sailing are analyzed in various sail configurations, noting that some parts of the mast work in bending, others in compression.
In some cases, I could eliminate the spreaders.
MW: For your plywood/epoxy/carbon wingmasts, is all the load taken by the carbon or is the mast primarily built with fibreglass over the ply and only supplemented by carbon at high stress areas?
EL: Fiberglass is only there to protect the plywood and with its low modulus has a very small influence on mast thickness. Stress is largely taken by the ply skin and beams. Their scantlings are mostly dictated by the building process. Carbon is cost/weight effective to take the remainder of the load.
MW: Merci beaucoup Erik for your time and enthusiasm to answer these questions for my readers.
EL: You are most welcome. Au revoir et bonne chance!
We recommend that our readers also visit Erik's website. Go to www.lerouge-yachts.com/trimarans.htm and this links to many of his wonderful boats as well as good info on various building methods.
Interesting also for the pics (text in French) is this website from a Pulsar 26 enthusiast trimarandiomedea.free.fr/
Also for the pics (text in Finnish) is this site on the Pulsar 30 'Trou Noir': www.trounoir.fi/trimaraani
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