Multihull Structure Thoughts

Yesterday we spoke of an owner who traded down from a Gunboat 80 to a Gunboat 68. The Gunboat 68 is a close design loop between Gunboat, VPLP, Patrick Le Quément, and Christophe Chedal Anglay. The Gunboat 68 is 68 x 29.9 foot with a weight 39,240 lbs and a maximum displacement of 52,470 lbs. The 82 foot mast fixed carbon mast on the standard version has a 1528 square foot mainsail, a 527 square foot small jib, a 969 square foot large jib, a 1410 square foot genoa and a 3500 square foot spinnaker. The Regatta rig has a 95 foot rotating carbon fibre wing mast has a 1,884 square foot mainsail, a 581 square foot small jib, a 1,184 square foot large jib, a 1,668 square foot genoa and a 4,660 square foot spinnaker. The draft varies from 3.8 to 9.8 foot over the C daggerboard foils. The underwing clearance is 3.3 foot.

If you compare the Gunboat 80 to the Gunboat 68 the GB 68 is 20,000 lighter in weight and still carries the same payload as the GB 80. The mast of the Regatta version of GB 68 is the same height of the GB 80. The power to weight ratio of the GB 68 is higher than the GB 80. This adds up to a GB 68 is a faster than wind speed boat above about 6 knots of windspeed, peak speeds above 25 knots and averages of 15 knots in good conditions.

The accommodation is semi custom but double berth cabins in the hulls and a main saloon with galley seating etc and a forward helm navigation station forward. There is a forward and aft cockpit for sail handling and external seating.

Now the interesting part, the build of these cats. The publicity says “We also had to balance the market’s price sensitivity and the realities of the huge number of hours it takes to build a complex, 40,000 lb machine! We meticulously evaluated the full range of options – from build methodology (male plug vs female tooling) to material choices (infused carbon and foam core vs all pre-preg carbon/Nomex), to how much to outsource (all built in-house vs 100% outsourced). The material choices could be pre-preg offering about a 10-15% weight saving on a composite structure. While it’s easy to get excited about that at face value, the devil is always in the details. On a typical Gunboat, the composite structure only makes up about 30% of the total displacement. A percentage of a percentage gets pretty small pretty quickly, to the point that building in full pre-preg carbon did not justify the additional costs and complexity involved.

The best option for the Gunboat 68 was to build the majority of the boat in-house, leveraging our expertise in infusion for the larger composite elements, then integrate advanced components commissioned from industry-leading partners. We outsourced flat panel parts of the structure that could be built in pre-preg on a vacuum table and easily shipped – reducing our risk and the need for in-house, high-temperature molds for prepreg carbon fiber – and gained the benefits of flat panel pre-preg structural elements.”

The reality is the GB 68 is built from materials that are suited for every application of the build: Epoxy resin, carbon, corecell foam, and nomex honeycomb etc. But let’s have a look at the “outsourced” flat panels that are built by FIBRE Mechanics in Lymington UK. The main crossbeams and many other bulkheads are built by this company. Why? Because they are experts in employing robotically-laid fiber to the structural components on the Gunboat 68 (i.e. the bulkheads and stringers). This is the same technology that produces aircraft tail fins and F1 car parts - structures with incredibly demanding engineering tolerances. The process is called North Thin Ply Technology™ (NTPT) - a cutting-edge technology using the highest-quality materials and yielding the most precise results. In NTPT, extremely thin layers of pre-preg carbon are laid by a machine, after which CNC-cut Corecell is fitted. Then, more layers of carbon are robotically applied and the whole panel is cured at 110°C in an autoclave. Thin ply pre-pregs allow greater optimization of modulus, strength and weight, and an overall improvement in the mechanical performance of the final part.


The automated layup machines at FIBRE Mechanics lay strips of glass or carbon from 30 gsm to a maximum of 300 gsm at a time over the preformed core material. The layups can be in any direction required to optimise the fibre layup for maximum strength of the part. In short instead of 1 layer of 1200 gsm triax glass you will have 6 layers of 200 gsm unidirectional glass laid in the correct orientation for maximum strength with minimum thickness and weight. The majority of the layup work is done by quickly by machine then the part is autoclaved and cooked for maximum strength.

This is how you get a 68 foot boat to weigh 40,000 lbs. It is not cheap but it provides an excellent base structure. The bulkheads etc are shipped to France where Gunboat builds the hulls and deck structures and the final assembly is completed.

The jpegs give the idea of some of the build aspects of the GB 68. This is a very impressive machine that will take a crew far and fast if wanted.

 

Sorry folks, my partner had a death in the family and I am not able to post for a few days. Stay well.

 

Sorry folks it will be another 4 days before posting again. Stay well.

 

I am sure I speak for many people when I say we very much appreciate and enjoy your posts, but life always takes priority. I hope things go as well as they might for you.

 

Thanks for the kind thoughts, the items will still be a bit intermittent for a week but then we should be back. 2 items, both from other locations about proa’s. The first is a PDF about a 56 foot pacific proa that has strong similarities to a Harryproa. The designer is a Brisbane guy who has technical knowledge about hydrofoils and foils in general and as a result has a wing rig. The jpegs in the PDF indicate a build is happening. DRAWN ONWARD is 56 x 30 foot with a displacement of 8,220 lbs. The wing sail has an area of 1345 square foot. The cabin hull is 37 foot long. The design is being built in foam glass. There is little other detail but Drawnonward has posted on other blogs. The web address is: https://guydelage.com/~guydelage/images/Praos/drawn-onward.pdf

The second article comes from the ProaFile and is from the guy who designed and built Sidecar (some jpegs below). SIDECAR is built in Tasmania and is a Pacific proa of 31.1 x 19 foot displacing 2,650 lbs carrying 505 square foot working sail and a maximum sail area of 678 square foot. The main hull length to beam is 14 to 1 with a prismatic coefficient of 0.53. He explains his rational for the design and has a very good table of relative performances of several proas of recent design versus a Seacart 30. You will be surprised as to how well some proas theoretically perform against a fast racer minimal cruiser tri. The web address is: Proa File | Comparative Analysis of Multihulls https://proafile.com/multihull-boats/article/comparative-analysis-of-multihulls

The jpegs are of Sidecar. There is also an article on page 77 of this thread. The PDF is Drawn Onward

 

Today is about a well designed dream. The naval architecture and interior was done by Du Toit Yacht Design in South Africa and the build was completed by Two Oceans Marine in 2020. “Liberty” is a 82HPC model. What is Liberty, a very fast cruiser occasional racer catamaran that was designed to be sleek and perform well with a moderate amount of accommodation. The 82HPC is 82.75 x 37.14 foot with a weight of 67,480 lbs. The 105 foot carbon fibre fixed mast carries 2,431 square foot mainsail and a 1,315 square foot fore triangle with a self tacking jib. Code 0 can be 2500 square foot and there are spinnaker options. Most sails are carbon fabric options. The length to beam on the hulls is about 14 to 1. The draft ranges from a 2.5 foot over the hull, 7 foot over the rudder and 11.8 foot over the daggerboards. The underwing clearance is 4.1 foot. The power is two Yanmar 4JH110 (110 HP).

The performance of this cat is unreported but 300 plus mile days and peaks of over 25 knots should be possible if driven hard. The power to weight of this cat is good. This cat would easily cruise at 10 knots with reduced sail in 15 knot winds. This would be an easy to sail cruiser with electric winches and the 2 permanent crew doing the work. The 6 guests would be watching the 60 inch TV and having martinis from the bar in moderate conditions. At 20 knot plus speeds the guests would be hanging on.

The accommodation has 3 luxury double berth cabins with ensuites, 2 crew cabins and galley in the hulls. The hulls at the gunnel are 9.2 foot wide. The main saloon and cockpit can be formed into one large entertainment area with seating for more than 10 people. The helming and sail control is on the aft corners of the cockpit.

The cat is built almost entirely from foam, epoxy, carbon fibre with the hull, deck, bulkheads, mast, boom, longeron and forward cross-beams all in carbon. Du Toit Yacht Design has a lot of multihull experience and has designed many large multihulls. This design shows there experience with the mast positioned nearly amidships, a relatively short high wing deck and a powerful rig combined with deep foils. Combine this with narrow hulls you will be surprised at how easily a cat like this can move through a seaway without throwing the crew around to much. Yes you will still pitch folks but even 1000 foot long ship pitch.

The jpegs give the idea. Are dreams, pity the real world of taxation etc is about to hit me.

 

This is a short one about a pacific proa designed and partially built by Joachim Herborn, a German who had intentions of sailing around the world on his proa. The Proa “Dolphin Sky” is 53 foot long and about 25 foot wide and originally had a 44 foot high by 2 foot wide wing mast and “sloop” rig. No other details are known.

In May 2005 a Dutch newspaper “De Telegraaf” reported “A family found the remains of Joachim Herborn's sailing boat near Oostkapelle. The German globetrotter saw his dream disappear when the boat for his trip to the South Seas proved not to be as seaworthy as he thought. The desperate man was barely saved from drowning when his boat was shipwrecked. The bearded eccentric wanted to sail around the world with his Proa, which was built after the Polynesian model. Both hulls were upright on the beach, no water ingress, the jib broken, the mast still standing, the sail flapping in the wind.”

So, lets go into a little history. In late 2004 after initial sailing of Dolphin Sky the wing mast rig did not work well and “was replaced by a delta sail in order to improve controllability and take pressure off the rudder system. Also, Joachim Herborn made comment he was slowly getting on top of the problems of the defective construction (for which he is not responsible).” The delta sail (crab claw) rig improved controllability but brought other issues which resulted the above newspaper article.

Dolphin Sky was built in foam glass and has 2 double berths and a reasonably roomy mid section with a galley and some seating. There is a cockpit between the hulls. There is a small bulb on the outside of the main hull to gain some internal space.

A Pacific proa, especially one for global cruising, is simple in principle but difficult to do in reality. The real problem is you are designing a very narrow hull with a small outrigger that controls the stability of the craft. To make a Pacific proa fast you want it light for its length so you can have a small rig that does not require a large or heavy outrigger to provide the stability. Pacific proas are easy to design if maximum windspeed is 15 knots which puts 1 lbs of pressure on a square foot of sail but in the real world of 35 knots of wind, 7.8 lbs per square foot of pressure is put on a sail. That means you need to deign a rig that can be efficiently reefed to match the limited stability available. You also need to be a very good sailor to anticipate the weather conditions to adjust the rig as required. There is one exception to the above, Russell Brown designed Jzerro etc had a large pod on the lee side of the main hull that provided additional stability in puffs which let Jzerro handle unexpected puffs before reefing but still was a light design and well built.

Get the proa design balance correct and you will have a fast simple cruiser. Get the design wrong and you will have all sorts of handling and stability issues. Pacific proa’s more than any other design type need designed based on experience gained over time.

Sorry about the limited jpegs.

 

A short one about performance. Garry Sagent of F-82 “Unleaded” wrote a very nice summary of F tris performance over a wind range versus monohull boats.

· Light air > 3 knots, boat speed < 4 knots, primary limiting factor skin friction, most multis race boat-for-boat with most monos

· Gentle breeze 3 to 8 knots, boat speed < 8 knots, primary limiting factor wave resistance, multis race boat-for-boat with sport monos

· Moderate breeze 8-15 knots, boat speed < 15 knots, primary limiting factor ama floatation / righting moment, multis faster than many monos, differences between multis also starting to appear

· Fresh breeze > 15 knots, boat speed > 15 knots, primary limiting factor SA / D in conjunction with ama floatation / righting moment, multis faster than most monos, significant differences between multis appear

Most Farrier/Corsair designs seem to have pretty decent performance across these gears except the lighter/floatier models (e.g. F-25C, F-82R, F-85SR) can leg out on the others in gears 3 and 4. There's a well-sailed F-31R that rates 0 in the PNW and they need >8 knots of breeze to be competitive versus monos.

The above is a general statement but a good summary. Cats operate slightly differently, but cat hull shapes have improved also.

For cats the hull shapes have evolved over the past 50 years from fine ended pot bellied symmetrical shapes to full ended asymmetrical shapes with a higher Prismatic coefficient. Translation. Lock Crowther used to design hulls with 12 to 1 length to beam and fine ends. Later in career he designed cats with 10 to 1 length to beam that would sail at the same average speed being better upwind with less pitching allowing better sail control etc. Richard Woods used to work with Wharram and his first cruising cats had similar hull shapes with similar sailing issues of pitching of Wharrams earlier designs. Richard evolved his hull shapes to fuller ended higher prismatic coefficient solutions that were faster and more comfortable across a wind range.

What is being said here is all multihulls have advantages over multihulls in certain wind ranges. Modern hull designs in both tris and cats have improved performance dramatically in the past 50 years. Thank the French spending a fortune on sponsored sailing and many of the pioneer designers who evolved their hull shapes to meet customer demand for more comfort and speed.

The only downside is many cats and tris are turning into accommodation cruisers that are trading of the performance gains of improved hull shapes for more displacement and windage. Result old designs appear to sail as well as new designs. Do a simple moderate accommodation design with modern hull shapes and you will sail faster than old or modern floating hotels.

Summary. Modern designs by a good designer with several years’ experience will be a good investment. The jpegs show hull shape evolution of Woods designs. The final is a Grainger design.

 

Sorry folks, nothing today I have a funeral to attend.

 

For a TRI cruising boat I think I'll stick with the classics... Many current designs are missing the motion comfort angle with hard riding amas and spray filled cockpits. I'm not sure you could twist the arm of a current "designer" to give you a wing deck but I want one for a number of cruisy reasons.

 

So sorry Oldmulti. It sounds like a rough time there.

 

Depending what size you were chasing Robin Chamberlin's cirrostratus is one of the nicest trimaran designs I've seen. As said elsewhere here it's a sort of updated Horstman. At least one was built in foam and I suspect he could be talked into selling a set of plans. 10m...

 

Thanks again for the kind thoughts. Once a 30 to 35 foot mono that could average 6 knots was the cruising boat of choice, now a 50 foot cat that can average 9 – 10 knots appears to be the preferred but for most unaffordable. Todays version is called the UNLIMITED C53 (there are some other names) the cat is 57 foot long over the bow sprit, 52 foot long on the waterline x 25.6 beam. The weight is 28,500 lbs with a full load displacement of 33,000 lbs. The 78 foot carbon mast carries a 915 square foot fat head mainsail, a 742 square foot self tacking jib, a 290 square foot staysail and a 1345 square foot Code 0 on a furler. The length to beam on the hulls is 11.75 to 1 with an underwing clearance of 4.3 foot. Th draft varies from 4.9 foot over the rudders to 8.7 foot over th dagger boards. There is a choice of asymmetric C foils or straight daggerboards depending on your performance aims. There are two 57 HP diesels with 35 mm shafts and folding props. The cat also has a single retractable bow thruster.

The accommodation is 4 double berth cabins with attached toilets in the hulls. The main saloon has a large galley, a interior steering an navigation area, seating and table. The wingdeck is short and as a result ther is not the “vast” saloon area of a French cat but what is there is sensible and very useable The cockpit through the large open doors can add to the saloon area. There is external steering wheels for helming outside.

The build is to quote “70% carbon”. The hulls are a hybrid of e-glass and carbon fiber over foam using multiaxial and unidirectional fabrics. The layup is infused with epoxy under pressure then post-cured. The outer skin is e-glass, the inner skin and bulkheads are carbon fibre. The bow crash sections have carbon fibre reinforcement. The rig and foils are full carbon fibre. This is a advanced structure that uses the materials as required.

Performance not mentioned but this is a 10 – 12 knot plus average boat with peaks that will approach 25 knots.

This is a good design but I do not want to know what it will cost. The limited jpegs will give the idea but the advertising PDF gives more insite.

 

Couldn't you call that a old design now Guzzi? I'm set with the Nicol anyway and it is probably faster in a number of conditions, plus..... I don't like glass/foam.
I actually have a number of plans from Newicks to Cross' but for a replacement cruising "truck" would probably build a Cross 10.5, his take on a center cockpit sheet ply tri from late in his career.

For me a cruising boat that is really going to travel into the corners should have economical construction and repair and a common enough rig to adapt the gear and sails from other craft if you need to replace quickly along the way. These boats are buses versus sports cars but with better windage compared to the current cats.

 

I'm waiting for a dissertation on "Gautier 1"